Facts on File DICTIONARY of ORGANIC CHEMISTRY Iranchembook.Ir/Edu Iranchembook.Ir/Edu

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The Facts On File DICTIONARY of ORGANIC CHEMISTRY iranchembook.ir/edu iranchembook.ir/edu

The Facts On File DICTIONARY of ORGANIC CHEMISTRY

Edited by John Daintith

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The Facts On File Dictionary of Organic Chemistry

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Library of Congress Cataloging-in-Publication Data

The Facts on File dictionary of organic chemistry / edited by John Daintith. p. cm. Includes bibliographical references. ISBN 0-8160-4928-9 (alk. paper). 1. Chemistry—Dictionaries. I. Title: Dictionary of organic chemistry. II. Daintith, John.

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CONTENTS

Preface vii

Entries A to Z 1

Appendixes I. Carboxylic Acids 233 II. Amino Acids 235 III. Sugars 238 IV. Nitrogenous Bases and Nucleosides 239 V. The Chemical Elements 241 VI. The Periodic Table 243 VII. The Greek Alphabet 244 VIII. Fundamental Constants 245 IX. Webpages 246

Bibliography 247 iranchembook.ir/edu iranchembook.ir/edu

PREFACE

This dictionary is one of a series covering the terminology and concepts used in important branches of science. The Facts on File Dictionary of Organic Chemistry has been designed as an additional source of information for students taking Advanced Placement (AP) Science courses in high schools. It will also be helpful to older students taking introductory college courses.

This volume covers organic chemistry and includes basic concepts, classes of compound, reaction mechanisms, and important named organic compounds. In addition, we have included a number of compounds that are important in biochemistry, as well as information on certain key biochemical pathways. The definitions are intended to be clear and informative and, where possible, we have illustrations of chemical structures. The book also has a selection of short biographical entries for people who have made important contributions to the field. There are a number of appendixes, including structural information on carboxylic acids, amino acids, sugars, and nitrogenous bases and nucleosides. There is also a list of all the chemical elements and a periodic table. The appendixes also include a short list of useful webpages and a bibliography.

The book will be a helpful additional source of information for anyone studying the AP Chemistry course, especially the section on Descriptive Chemistry. It will also be useful to students of AP Biology.

ACKNOWLEDGMENTS

Contributors

John O. E. Clark B.Sc. Richard Rennie B.Sc., Ph.D.

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ABA See abscisic acid. temperature intervals were called degrees absolute (°A) or degrees Kelvin (°K), and abscisic acid (ABA) A PLANT HORMONE were equal to the Celsius degree. It can be once thought to be responsible for the shown that the absolute temperature scale shedding (abscission) of flowers and fruit is identical to the currently used thermody- and for the onset of dormancy in buds namic temperature scale (on which the unit (hence its early name, dormin). The com- is the KELVIN). pound is associated with the closing of pores (stoma) in the leaves of plants de- absolute zero The zero value of ther- prived of water. modynamic temperature; 0 kelvin or –273.15°C. See absolute temperature. absolute alcohol Pure alcohol (ethanol). absorption 1. A process in which a gas absolute configuration A particular is taken up by a liquid or solid, or in which molecular configuration of a CHIRAL mol- a liquid is taken up by a solid. In absorp- ecule, as denoted by comparison with a ref- tion, the substance absorbed goes into the erence molecule or by some sequence rule. bulk of the material. Solids that absorb There are two systems for expressing ab- gases or liquids often have a porous struc- solute configuration in common use: the ture. The absorption of gases in solids is D–L convention and the R–S convention. sometimes called sorption. There is a dis- See optical activity. tinction between absorption (in which one substance is assimilated into the bulk of an- absolute temperature Symbol: T A other) and ADSORPTION (which involves at- temperature defined by the relationship: tachment to the surface). Sometimes it is T = θ + 273.15 not obvious which process is occurring. where θ is the Celsius temperature. The ab- For example, a porous solid, such as acti- solute scale of temperature was a funda- vated CHARCOAL may be said to absorb a mental scale based on Charles’ law, which large volume of gas, but the process may applies to an ideal gas: actually be adsorption on the high surface αθ V = V0(1 + ) area of internal pores in the material. θ where V is the volume at temperature , V0 2. The process in which electromagnetic the volume at 0, and α the thermal expan- radiation, particles, or sound waves lose sivity of the gas. At low pressures (where energy in passing through a medium. Ab- real gases show ideal behavior) α has the sorption involves conversion of one form value 1/273.15. Therefore, at θ = –273.15 of energy into another. the volume of the gas theoretically becomes zero. In practice substances become absorption spectrum See spectrum. solids at these temperatures; however, the extrapolation can be used for a scale of accelerator A substance that increases temperature on which –273.15°C cor- the rate of a chemical reaction. In this sense responds to 0° (absolute zero). The scale is the term is synonymous with CATALYST. It also known as the ideal-gas scale; on it is common to refer to catalysts as ‘acceler-

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acceptor

ators’ in certain industrial applications. R1COR2 + R3OH ˆ CR1R2(OH)(OR3) For example, accelerators are used in the The formation of a hemiacetal is an exam- VULCANIZATION of rubber and in the poly- ple of NUCLEOPHILIC ADDITION to the car- merization of adhesives. Also, in the pro- bonyl group of the aldehyde or ketone. The duction of composite materials using first step is attack of the lone pair on the O polyester resins a distinction is sometimes of the alcohol on the (positively charged) C made between the catalyst (which initiates of the carbonyl group. This is catalyzed by the polymerization reaction) and the accel- both acids and bases. Acid catalysis occurs erator (which is an additional substance by protonation of the O on the carbonyl, making the catalyst more effective). The making the C more negative and more sus- terms promoter and activator are used in a ceptible to nucleophilic attack. In base similar way. catalysis the OH– ions from the base affect the –OH group of the alcohol, making it a acceptor The atom or group to which a more effective nucleophile. pair of electrons is donated in forming a In general, hemiacetals exist only in so- COORDINATE BOND. lution and cannot be isolated because they easily decompose back to the component accessory pigment See photosynthetic alcohol and aldehyde or ketone. However, pigments. some cyclic hemiacetals are more stable. For example, cyclic forms of SUGAR molecules are hemiacetals. 12 Further reaction of hemiactals with another molecule of alcohol leads to a full acetal. For example: ˆ CH(OH)(CH3)(OC2H5) + C2H5OH CH(CH3)(OC2H5)2 The overall reaction of an aldehyde or ke- Acenaphthene tone with an alcohol to give an acetal can be written: R1COR2 + R3OH ˆ CR1R2(OR3) acenaphthene (C H ) A colorless crys- 2 12 10 It is also possible to have ‘mixed’ acetals talline derivative of naphthalene, used in 1 2 3 producing some dyes. with the general formula CR R (OR )- (OR4). Note that if the acetal is derived 1 2 acetal A type of compound formed by from an aldehyde, then R and/or R may reaction of an alcohol with either an al- be a hydrogen atom. The mechanism of dehyde or a ketone. The first step in for- formation of an acetal from a hemiacetal is mation of an acetal is the formation of acid catalyzed. It involves protonation of an intermediate, known as a hemiacetal. the –OH group of the hemiacetal followed For example, ethanal (acetaldehyde; by loss of water to form an oxonium ion, which is attacked by the alcohol molecule. CH3CHO) reacts with ethanol (C2H5OH) as follows: Formerly it was conventional to use the ˆ terms ‘hemiacetal’ and ‘acetal’ for com- CH3CHO + C2H5OH CH(OH)(CH3)(C2H5O) pounds formed by reaction between alde- The hemiacetal has a central carbon atom hydes and alcohols. Similar reactions (from the aldehyde) attached to a hydro- between ketones and alcohols gave rise to gen, a hydroxyl group, a hydrocarbon compounds called hemiketals and ketals. group (CH3), and an alkoxy group Current nomenclature uses ‘hemiacetal’ (C2H5O). If a ketone is used rather than an and ‘acetal’ for compounds derived from aldehyde, the resulting hemiacetal contains either an aldehyde or a ketone, but reserves two hydrocarbon groups. For example, re- ‘hemiketal’ and ‘ketal’ for those derived action of the ketone R1COR2 with the al- from ketones. In other words, the ketals cohol R3OH is: are a subclass of the acetals and the

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acid

hemiketals are a subclass of the hemiac- particularly in the oxidation of sugars, etals. fatty acids, and amino acids, and in certain biosynthetic pathways. It is formed by the acetaldehyde See ethanal. reaction between pyruvate (from GLYCOLY- SIS) and COENZYME A, catalyzed by the en- acetamide See ethanamide. zyme pyruvate dehydrogenase. The acetyl group of acetyl CoA is subsequently oxi- acetate See ethanoate. dized in the KREBS CYCLE, to yield reduced coenzymes and carbon dioxide. Acetyl acetic acid See ethanoic acid. CoA is also produced in the initial oxidation of fatty acids and some amino acids. acetone See propanone. Other key roles for acetyl CoA include the provision of acetyl groups in biosynthesis acetonitrile See methyl cyanide. of fatty acids, terpenoids, and other substances. acetophenone See phenyl methyl ketone. acetyl coenzyme A See acetyl CoA.

acetylation See acylation. acetylsalicylic acid See aspirin.

acetyl chloride See ethanoyl chloride. ACh See acetylcholine.

acetylcholine (ACh) A neurotransmit- achiral Describing a molecule that does ter found at the majority of synapses, not have chiral properties; i.e. one that which occur where one nerve cell meets an- does not exhibit OPTICAL ACTIVITY. other. acid A substance than contains hydro- acetylene See ethyne. gen and dissociates in solution to give hydrogen ions: acetyl group See ethanoyl group. HA ˆ H+ + A– More accurately, the hydrogen ion is sol- acetylide See carbide. vated (a hydroxonium ion): ˆ + – HA + H2O H3O + A acetyl CoA (acetyl coenzyme A) An im- Strong acids are completely dissociated in portant intermediate in cell metabolism, water. Examples are sulfuric acid and tri-

NH2 N N

H3C CH3 OO N N HO OPOPOCH2 O - OO - O NH O OH - P OO S CH3 - O N O H O Acetyl CoA

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acid anhydride

+ choloroethanoic acid. Weak acids are only water is acting as an acid (H3O is its con- partially dissociated. Most organic car- jugate base). Note that water can act as boxylic acids are weak acids. In distinction both an acid and a base depending on the to an acid, a base is a compound that pro- circumstances. It can accept a proton (from duces hydroxide ions in water. Bases are CH3COOH) and donate a proton (to either ionic hydroxides (e.g. NaOH) or R3N). Compounds of this type are de- compounds that form hydroxide ions in scribed as amphiprotic. water. These may be metal oxides, for ex- One important aspect of the ample: Lowry–Brønsted theory is that, because it → + – Na2O + H2O 2Na + 2OH involves proton transfers, it does not nec- Ammonia, amines, and other nitrogenous essarily have to involve water. It is possible compounds can also form OH– ions in to describe reactions in nonaqueous sol- water: vents, such as liquid ammonia, in terms of ˆ + – NH3 + H2O NH4 + OH acid–base reactions. As with acids, strong bases are completely A further generalization of the idea of dissociated; weak bases are partially disso- acids and bases was the Lewis theory put ciated. forward, also in 1923, by the US physical This idea of acids and bases is known as chemist Gilbert Newton Lewis (1875– the Arrhenius theory (named for the 1946). In this, an acid (a Lewis acid) is a Swedish physical chemist Svante August compound that can accept a pair of elec- Arrhenius (1859–1927)). trons and a base (a Lewis base) is one that In 1923 the Arrhenius idea of acids and donates a pair of electrons. In a traditional bases was extended by the British chemist acid–base reaction, such as: → Thomas Martin Lowry (1874–1936) and, HCl + NaOH NaCl + H2O independently, by the Danish physical the effective reaction is + – → chemist Johannes Nicolaus Brønsted H + OH H2O (1879–1947). In the Lowry–Brønsted The OH– (base) donates an electron pair to theory an acid is a compound that can do- the H+ (acid). However, in the Lewis nate a proton and a base is a compound theory acids and bases need not involve that can accept a proton. Proton donators protons at all. For example, ammonia are called Brønsted acids (or protic acids) (NH3) adds to boron trichloride (BCl3) to and proton acceptors are called Brønsted form an adduct: → bases. For example, in the reaction: NH3 + BCl3 H3NBCl3 ˆ – CH3COOH + H2O CH3COO + Here, ammonia is the Lewis base donating + H3O a LONE PAIR of electrons to boron trichlor- the CH3COOH is the acid, donating a pro- ide (the Lewis acid). ton H+ to the water molecule. The water is The concept of acid–base reactions is an the base because it accepts the proton. In important generalization in chemistry, and + the reverse reaction, the H3O ion is the the Lewis theory connects it to two other acid, donating a proton to the base general ideas. One is oxidation–reduction: – CH3COO . If two species are related by oxidation involves loss of electrons and re- loss or gain or a proton they are described duction involves gain of electrons. Also, in as conjugate. So, in this example, organic chemistry, it is connected with the – CH3COO is the conjugate base of the acid idea of electrophile–nucleophile reactions. CH3COOH and CH3COOH is the conju- Acids are ELECTROPHILES and bases are NU- – gate acid of the base CH3COO . CLEOPHILES. In organic chemistry a number In a reaction of an amine in water, for of inorganic halides, such as AlCl3 and example: TiCl4, are important Lewis acids, forming ˆ + – R3N + H2O R3NH + OH intermediates in such processes as the The amine R3N accepts a proton from FRIEDEL–CRAFTS REACTION. water and is therefore acting as a base. + R3NH is its conjugate acid. Water donates acid anhydride A type of organic com- the proton to the R3N and, in this case, pound containing the group –CO.O.CO–. 4 iranchembook.ir/edu

acrylic resin

Simple acid anhydrides have the general acid value A measure of the free acid formula RCOOCOR′, where R and R′ are present in fats, oils, resins, plasticizers, and alkyl or aryl groups. They can be regarded solvents, defined as the number of mil- as formed by removing a molecule of water ligrams of potassium hydroxide required from two molecules of carboxylic acid. For to neutralize the free acids in one gram of example, ethanoic anhydride comes from the substance. ethanoic acid: → 2CH3COOH – H2O CH3CO.O.COCH3 A long-chain dicarboxylic acid may also N form a cyclic acid anhydride, in which the –CO.O.CO– group forms part of a ring. Acid anhydrides can be prepared by reaction of an acyl halide with the sodium salt of a carboxylic acid, e.g.: Acridine RCOCl + R′COO–Na+ → RCOOCOR′ + NaCl Like the acyl halides, they are very reactive acridine (C12H9N) A colorless crys- acylating agents. They hydrolyze readily to talline heterocyclic compound with three carboxylic acids: fused rings. Derivatives of acridine are ′ → used as dyes and biological stains. RCOOCOR + H2O RCOOH + R′COOH Acrilan (Trademark) A synthetic fiber See also acylation; anhydride. that consists of a copolymer of 1- cyanoethene (acrylonitrile; vinyl cyanide) acid dyes The sodium salts of organic and ethenyl ethanoate (vinyl acetate). See acids used in the dyeing of silk and wool. acrylic resin. They are so called because they are applied from a bath acidified with dilute sulfuric or acrolein See propenal. ethanoic acid. acrylic acid See propenoic acid. acid halide See acyl halide. acrylic resin A synthetic resin made by acidic Having a tendency to release a polymerizing an amide, nitrile, or ester de- proton or to accept an electron pair from a rivative of 2-propenoic acid (acrylic acid). donor. In aqueous solutions the pH is a Acrylic resins (known as ‘acrylics’) are measure of the acidity, i.e. an acidic solu- used in a variety of ways. A common ex- tion is one in which the concentration of ample is poly(methylmethacrylate), which + H3O exceeds that in pure water at the is produced by polymerizing methyl same temperature; i.e. the pH is lower than methacrylate, CH2:CH(CH3)COOCH3. 7. A pH of 7 indicates a neutral solution. This is the clear material sold as Plexiglas. Another example is the compound methyl A hydrogen atom in a acidic hydrogen 2-cyanoacrylate, CH2:CH(CN)COOCH3. molecule that enters into a dissociation This polymerizes very readily in air and is equilibrium when the molecule is dissolved the active constituent of ‘superglue’. In in a solvent. For example, in ethanoic acid both these cases there is a double C=C (CH3COOH) the acidic hydrogen is the bond conjugated with the carbonyl C=O one on the carboxyl group, –COOH: bond and the polymerization has a free- ˆ CH3COOH + H2O radical mechanism. The free election is on – + CH3COO + H3O . the carbon atom next to the carbonyl group, which stabilizes the radical. An- acidity constant See dissociation con- other example of an acrylic polymer is stant. formed by free-radical polymerization of

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acrylonitrile

acrylonitrile (CH2:CHCN) to give poly- solid catalyst at which catalytic activity oc- (acrylonitrile). This is used in synthetic curs or at which the catalyst is particularly fibers (such as Acrilan). In this case the un- effective. paired electron is on the carbon next to the 2. The region of an ENZYME molecule that –CN group. Acrylic resins are also used in combines with and acts on the substrate. It paints. consists of catalytic amino acids arranged in a configuration specific to a particular See propenonitrile. acrylonitrile substrate or type of substrate. The ones that are in direct combination are the con- actinic radiation Radiation that can tact amino acids. Other amino acids may cause a chemical reaction; for example, ultraviolet radiation is actinic. be further away but still play a role in the action of the enzyme. These are auxilliary actinomycin Any of a number of antibi- amino acids. Binding of a regulatory com- otics produced by certain bacteria. The pound to a separate site, known as the AL- main one, actinomycin D (or dactino- LOSTERIC SITE, on the enzyme molecule may mycin), can bind between neighbouring change this configuration and hence the ef- base pairs in DNA, preventing RNA syn- ficiency of the enzyme activity. thesis. It is used in the treatment of some cancers. activity 1. Symbol: a Certain thermodynamic properties of a solvated substance action spectrum A graph showing the are dependent on its concentration (e.g. its effect of different wavelengths of radia- tendency to react with other substances). tion, usually light, on a given process. It Real substances show departures from is often similar to the ABSORPTION SPEC- ideal behavior and a corrective concentra- TRUM of the substance that absorbs the ration term – the activity – has to be intro- diation and can therefore be helpful in duced into equations describing real identifying that substance. For example, solvated systems. the action spectrum of photosynthesis is similar to the absorption spectrum of 2. Symbol: A The average number of atoms chlorophyll. disintegrating per unit time in a radioactive substance. activated charcoal See charcoal. activity coefficient Symbol: f A meas- activated complex The partially bonded ure of the degree of deviation from ideality system of atoms in the TRANSITION STATE of of a dissolved substance, defined as: a chemical reaction. a = fc

activation energy Symbol: Ea The minimum energy a system must acquire before A ... B ... C a chemical reaction can occur, regardless of whether the reaction is exothermic or endothermic. Activation energy is often represented as an energy barrier that has to energy be overcome if a reaction is to take place. See also Arrhenius equation; transition AB ؉ C state. H activator See accelerator. AB ؉ C

reaction coordinate active mass See mass action.

active site 1. A site on the surface of a Activation energy

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adenosine

→ where a is the activity and c the concentra- CH3COCl + H2O CH3COOH + HCl tion. For an ideal solute f = 1; for real sys- With an alcohol (e.g. ethanol) it gives an tems f can be less or greater than unity. ester (ethyl ethanoate): → CH3COCl + C2H5OH acyclic Describing a compound that is CH3COOC2H5 + HCl not cyclic (i.e. a compound that does not With ammonia it gives an amide contain a ring in its molecules). (ethanamide; acetamide): → CH3COCl + NH3 acyl anhydride See acid anhydride. CH3CONH2 + HCl With an amine (e.g. methylamine) it gives acylating agent See acylation. an N-substituted amine (N-methyl ethanamide) → acylation Any reaction that introduces CH3COCl + CH3NH2 an acyl group (RCO–) into a compound. CH3CONH(CH3) Acylating agents are compounds such as See also acylation. acyl halides (RCOX) and acid anhydrides (RCOOCOR), which react with such nu- addition polymerization See polymerization. cleophiles as H2O, ROH, NH3, and RNH2. In these reactions a hydrogen atom of a hydroxyl or amine group is replaced addition reaction A reaction in which by the RCO– group. In acetylation the additional atoms or groups of atoms are in- acetyl group (CH CO–) is used. In benzoy- troduced into an unsaturated compound, 3 such as an alkene, alkyne, aldehyde, or ke- lation the benzoyl group (C6H5CO–) is used. Acylation is used to prepare crys- tone. A simple example is the addition of bromine across the double bond in ethene: talline derivatives of organic compounds to H C:CH + Br → BrH CCH Br identify them (e.g. by melting point) and 2 2 2 2 2 Addition reactions can occur by addition also to protect –OH groups in synthetic re- of electrophiles or nucleophiles. See elec- actions. trophilic addition; nucleophilic addition. acyl group The group of atoms RCO–. adduct See coordinate bond. acyl halide (acid halide) A type of organic compound of the general formula NH2 RCOX, where X is a halogen (acyl chlo- N N 6 ride, acyl bromide, etc.). 1 7 Acyl halides can be prepared by the re- 3 9 N N action of a carboxylic acid with a halo- H genating agent. Commonly, phosphorus Adenine halides are used (e.g. PCl5) or a sulfur dihalide oxide (e.g. SOCl2): adenine A nitrogenous base found in → RCOOH + PCl5 RCOCl + DNA and RNA. It is also a constituent of POCl3 + HCl certain coenzymes, and when combined → RCOOH + SOCl2 RCOCl + with the sugar ribose it forms the nucleo- SO2 + HCl side adenosine found in AMP, ADP, and The acyl halides have irritating vapors ATP. Adenine has a purine ring structure. and fume in moist air. They are very reac- See also DNA. tive to the hydrogen atom of compounds containing hydroxyl (–OH) or amine adenosine (adenine nucleoside) A NU- (–NH2) groups. For example, the acyl CLEOSIDE formed from adenine linked to D- halide ethanoyl chloride (acetyl chloride; ribose with a β-glycosidic bond. It is widely CH3COCl) reacts with water to give a car- found in all types of cell, either as the free boxylic acid (ethanoic acid): nucleoside or in combination in nucleic

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adenosine diphosphate NH 2 on which adsorption takes place. See ad- N N sorption.

N N adsorption A process in which a layer of atoms or molecules of one substance HOCH O 2 forms on the surface of a solid or liquid. All solid surfaces take up layers of gas from the surrounding atmosphere. The adsorbed OH OH layer may be held by chemical bonds (chemisorption) or by weaker van der Adenosine Waals forces (physisorption). Compare absorption. acids. Phosphate esters of adenosine, such as ATP, are important carriers of energy in aerobic Describing a biochemical process biochemical reactions. that takes place only in the presence of free oxygen. Compare anaerobic. adenosine diphosphate See ADP. aerobic respiration (oxidative metabo- adenosine monophosphate See AMP. lism) Respiration in which free oxygen is used to oxidize organic substrates to car- adenosine triphosphate See ATP. bon dioxide and water, with a high yield of energy. Carbohydrates, fatty acids, and ex- adiabatic change A change for which cess amino acids are broken down yielding no energy enters or leaves the system. In an acetyl CoA and the reduced coenzymes adiabatic expansion of a gas, mechanical NADH and FADH . The acetyl coenzyme work is done by the gas as its volume in- 2 A enters a cyclic series of reactions, the creases and the gas temperature falls. For KREBS CYCLE, with the production of car- an ideal gas undergoing a reversible adia- bon dioxide and further molecules of batic change it can be shown that γ NADH and FADH2. NADH and FADH2 pV = K1 γ 1–γ are passed to the ELECTRON-TRANSPORT T p = K2 γ–1 CHAIN (involving cytochromes and flavo- and TV = K3 γ proteins), where they combine with atoms where K1, K2, and K3 are constants and is the ratio of the principal specific heat ca- of free oxygen to form water. Energy re- pacities. Compare isothermal change. leased at each stage of the chain is used to form ATP during a coupling process. The adipic acid See hexanedioic acid. substrate is completely oxidized and there is a high energy yield. There is a net pro- adjacent Designating atoms or bonds duction of 38 ATPs per molecule of glucose that are next to each other in a molecule. during aerobic respiration, a yield of about 19 times that of anaerobic respiration. ADP (adenosine diphosphate) A nu- Aerobic respiration is therefore the pre- cleotide consisting of adenine and ribose ferred mechanism of the majority of organ- with two phosphate groups attached. See isms. See also oxidative phosphorylation; also ATP. respiration.

adrenalin See epinephrine. aerosol See sol.

adsorbate A substance that is adsorbed affinity The extent to which one sub- on a surface. See adsorption. stance reacts with another in a chemical change. adsorbent Having a tendency to adsorb. As a noun the adsorbent is the substance afterdamp See firedamp.

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alcohol glucose

glycolysis 2 ATP

pyruvate 38 ATP

2 ATP acetyl CoA 34 ATP

H O Krebs 2 cycle H respiratory chain ½O2

CO2

Aerobic respiration

agent orange A herbicide consisting of a mixture of two weedkillers (2,4-D and H OH 2,4,5-T). It was designed for use in chemical warfare to defoliate trees in areas where C an enemy may be hiding or to destroy enemy crops. Agent orange, so-called from C H the orange-colored canisters in which it H3 was supplied, was first used by US forces primary (ethanol) during the Vietnam war. It contains traces of the highly toxic chemical DIOXIN, which causes cancers and birth defects.

H3C OH air gas See producer gas. C alanine See amino acid. H3C H albumen The white of an egg, which consists mainly of the protein ALBUMIN. secondary (propan-2-ol)

albumin A soluble protein that occurs in many animal fluids, such as blood serum and egg white. H3C OH

alcohol A type of organic compound of C the general formula ROH, where R is a hydrocarbon group. Examples of simple alco- H3C CH3 hols are methanol (CH3OH) and ethanol (C2H5OH). Alcohols have the –OH group tertiary (2-methylpropan-2-ol) attached to a carbon atom that is part of an alkyl group. If the carbon atom is part of Alcohol

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aldaric acid

° an aromatic ring, as in PHENOL, C6H5OH, 3. Dehydration over hot pumice (400 C) the compound does not have the character- to alkenes: → istic properties of alcohols. Phenyl- RCH2CH2OH – H2O RCH:CH2 methanol (C6H5CH2OH) does have the 4. Reaction with sulfuric acid. Two types characteristic properties of alcohols (in this of reaction are possible. With excess case the carbon atom to which the –OH is acid at 160°C dehdyration occurs to attached is not part of the aromatic ring). give an alkene: → Alcohols can have more than one –OH RCH2CH2OH + H2SO4 group; those containing two, three, or H2O + RCH2CH2.HSO4 → more such groups are described as dihy- RCH2CH2.HSO4 dric, trihydric, and polyhydric respectively RCH:CH2 + H2SO4 (as opposed to alcohols containing one 4. With excess alcohol at 140°C an ether is –OH group, which are monohydric). For formed: → example, ethane-1,2-diol (ethylene glycol; 2ROH ROR + H2O (HOCH2CH2OH) is a dihydric alcohol See also acetal; acylation; Grignard and propane-1,2,3-triol (glycerol; reagent. HOCH2CH(OH)CH2OH) is a trihydric alcohol. Dihydric alcohols are known as aldaric acid See sugar acid. diols; trihydric alcohols as triols, etc. In general, alcohols are named by using the suffix -ol with the name of the parent hydrocarbon. O Alcohols are further classified accord- aldehyde group ing to the environment of the –C–OH R C grouping. If the carbon atom is attached to two hydrogen atoms, the compound is a H primary alcohol. If the carbon atom is attached to one hydrogen atom and two Aldehyde other groups, it is a secondary alcohol. If the carbon atom is attached to three other aldehyde A type of organic compound groups, it is a tertiary alcohol. Alcohols can with the general formula RCHO, where be prepared by: the –CHO group (the aldehyde group) con- 1. Hydrolysis of haloalkanes using aque- sists of a carbonyl group attached to a hy- ous potassium hydroxide: drogen atom. Simple examples of RI + OH– → ROH + I– aldehydes are methanal (formaldehyde; 2. Reduction of aldehydes by nascent hy- HCHO) and ethanal (acetaldehyde; CH3- drogen (e.g. from sodium amalgam in CHO). water): Aldehydes are formed by oxidizing a → RCHO +2[H] RCH2OH primary alcohol; in the laboratory potas- The main reactions of alcohols are: sium dichromate(VI) is used in sulfuric 1. Oxidation by potassium dichromate(VI) acid. They can be further oxidized to car- in sulfuric acid. Primary alcohols give boxylic acids. Reduction (using a catalyst aldehydes, which are further oxidized to or nascent hydrogen from sodium amal- carboxylic acids: gam in water) produces the parent alcohol. → → RCH2OH RCHO RCOOH For example, oxidation of ethanol 1. Secondary alcohols are oxidized to ke- (C2H5OH) gives ethanal (acetaldehyde; tones. CH3CHO): 1 2 → 1 2 → R R CHOH R R CO C2H5OH + [O] CH3CHO + H2O 2. Formation of esters with acids. The re- Further oxidation gives ethanoic acid action, which is reversible, is catalyzed (acetic acid; CH3COOH): + → by H ions: CH3CHO + [O] CH3COOH ROH + R′COOH ˆ The systematic method of naming alde- ′ R COOR + H2O hydes is to use the suffix -al with the 10 iranchembook.ir/edu

alginic acid

name of the parent hydrocarbon. For ex- known since the 1900s but Alder and Otto ample: methane (CH4) is the parent hydro- Diels recognized that this mechanism is carbon of the alcohol methanol (CH3OH), very common. They first reported their re- the aldehyde methanal (HCHO), and sults in 1928. Alder and Diels shared the the carboxylic acid methanoic acid 1950 Nobel Prize for chemistry for this (HCOOH); ethane (C2H6) is the parent work. hydrocarbon of the alcohol ethanol (C2H5- OH), the aldehyde ethanal (CH3CHO), alditol See sugar alcohol. and the carboxylic acid ethanoic acid (CH COOH); etc. An older method of 3 aldohexose An aldose SUGAR with six naming aldehydes is based on the name of carbon atoms. the related acid. For example, methanoic acid (HCOOH) has the traditional name aldol A compound that contains both an ‘formic acid’ and the related aldehyde aldehyde group (–CHO) and an alcohol (HCHO) is traditionally called ‘formalde- group (–OH). See aldol reaction. hyde’. Similarly, ethanoic acid (CH3OOH) is commonly known as ‘acetic acid’ and the aldol reaction A reaction in which two aldehyde CH3CHO is known as ‘acetaldehyde’. molecules of aldehyde combine to give an Reactions of aldehydes are: aldol – i.e. a compound containing both 1. Aldehydes are reducing agents, being aldehyde and alcohol functional groups. oxidized to carboxylic acids in the The reaction is base-catalyzed; the reaction process. These reactions are used as tests of ethanal (acetaldehyde) refluxed with for aldehydes using such reagents as sodium hydroxide gives: → FEHLING’S SOLUTION and TOLLEN’S 2CH3CHO CH3CH(OH)CH2CHO REAGENT (silver-mirror test). The mechanism is similar to that of the 2. They form addition compounds with CLAISEN CONDENSATION: the first step is re- hydrogen cyanide to give cyanohydrins. moval of a proton to give a carbanion, For example, propanal gives 2-hydroxy- which subsequently attacks the carbon of butanonitrile: the carbonyl group on the other molecule: C H CHO + HCN → – → – 2 5 CH3CHO + OH CH2CHO + H2O C H CH(OH)CN – → 2 5 CH3CHO + CH2CHO 3. They form bisulfite addition compounds CH3CH(OH)CH2CHO. with the hydrogensulfite(IV) ion (bisul- – fite; HSO3 ): aldonic acid See sugar acid. – → RCHO + HSO3 RCH(OH)(HSO3) 4. They undergo condensation reactions aldopentose An aldose SUGAR with five with such compounds as hydrazine, hy- carbon atoms. droxylamine, and their derivatives. 5. With alcohols they form hemiacetals aldose A SUGAR containing an aldehyde and ACETALS. group (CHO) or a potential aldehyde 6. Simple aldehydes polymerize readily. Polymethanal or methanal trimer can be group. formed from METHANAL depending on the conditions. ETHANAL gives ethanal algin See alginic acid. trimer or ethanal tetramer. See also Cannizzaro reaction; condensa- alginic acid (algin; (C6H8O6)n) A yel- tion reaction; ketone. low-white organic solid that is found in brown algae. It is a complex polysaccha- Alder, Kurt (1902–1958) German or- ride and produces, in even very dilute solu- ganic chemist who is noted for the process tions, a viscous liquid. Alginic acid has known as the DIELS–ALDER REACTION. Par- various uses, especially in the food industry ticular cases of the reaction had been as a stabilizer and texture agent.

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alicyclic compound

– + → alicyclic compound An aliphatic cyclic RCOO Na + NaOH RH + Na2CO3 compound, such as cyclohexane or cyclo- 2. By reduction of a haloalkane with propane. nascent hydrogen from the action of ethanol on a zinc–copper couple: aliphatic compound An organic com- RX + 2[H] → RH + HX pound with properties similar to those of 3. By the WURTZ REACTION – i.e. sodium in the alkanes, alkenes, and alkynes and their dry ether on a haloalkane: derivatives. Most aliphatic compounds 2RX + 2Na → 2NaX + RR have an open chain structure but some, 4. By the KOLBÉ ELECTROLYTIC METHOD: such as cyclohexane and sucrose, have RCOO– → RR rings (these are described as alicyclic). 5. By refluxing a haloalkane with magne- The term is used in distinction to ARO- sium in dry ether to form a GRIGNARD MATIC COMPOUNDS, which are similar to REAGENT: benzene. RI + Mg → RMgI 5. With acid this gives the alkane: alizarin (1,2-dihydroxyanthraquinone) RMgI + H → RH An important orange-red organic com- The main source of lower molecular pound used in the dyestuffs industry to weight alkanes is natural gas (for methane) produce red lakes. It occurs naturally in the and crude oil. root of the plant madder and may also be synthesized from anthraquinone. CH3 CH2 CH2 CH2 CH = CH2 A water-soluble strong base. alkali hex-1-ene Strictly the term refers to the hydroxides of the alkali metals (group 1) only, but in common usage it refers to any soluble base. Thus borax solution may be described as CH3 CH2 CH2 CH= CH CH3 mildly alkaline. hex-2-ene alkaloid One of a group of natural organic compounds found in plants. They contain oxygen and nitrogen atoms; most CH CH CHCH= CH CH are poisonous. However, they include a 3 2 2 3 number of important drugs with characteristic physiological effects, e.g. morphine, hex-3-ene codeine, caffeine, cocaine, and nicotine. Alkene

alkane A type of hydrocarbon with general formula CnH2n+2. Alkanes are satu- alkene A type of aliphatic hydrocarbon rated compounds, containing no double or containing one or more double bonds in triple bonds. Systematic names end in -ane: the molecule. Alkenes with one double methane (CH4) and ethane (C2H6) are typ- bond have the general formula CnH2n. The ical examples. The alkanes are fairly unre- alkenes are unsaturated compounds. They active (their former name, the paraffins, can be obtained from crude oil by cracking means ‘small affinity’). In ultraviolet radia- alkanes. Systematic names end in -ene: ex- tion they react with halogens to give a mix- amples are ethene (C2H4) and propene ture of substitution products. This involves (C3H6), both of which are used in plastics a free-radical chain reaction and is impor- production and as starting materials for tant as a first step in producing other com- the manufacture of many other organic pounds from alkanes. There are a number chemicals. The former general name for an of ways of preparing specific alkanes: alkene was olefin. 1. From a sodium salt of a carboxylic acid The methods of synthesizing alkenes treated with sodium hydroxide: are:

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alkyne

1. The elimination of HBr from a the reaction of metallic sodium on an alco- haloalkane using an alcoholic solution hol. For example, ethanol reacts with of potassium hydroxide: sodium to give sodium ethoxide: → → – + RCH2CH2Br + KOH KBr + H2O + 2C2H5OH + 2Na 2C2H5O Na ) RCH:CH2 + H2 2. The dehydration of an alcohol by pass- Alkoxides are ionic compounds containing ing the vapor over hot pumice (400°C): an alkoxide ion (RO–). They are named ac- → RCH2CH2OH RCH:CH2 + H2O cording to the parent alcohol. Thus, The reactions of simple alkenes include: methanol (CH3OH) gives methoxides – 1. Hydrogenation using a catalyst (usually CH3O , ethanol (C2H5OH) gives ethoxides ° – nickel at about 150 C): C2H5O , etc. → RCH:CH2 + H2 RCH2CH3 2. Addition reactions with halogen acids to alkoxyalkane (diethyl ether) See ether. give haloalkanes: → RCH:CH2 + HX RCH2CH2X alkylbenzene A type of organic hydro- 2. The addition follows MARKOVNIKOFF’S carbon containing one or more alkyl RULE. groups substituted onto a benzene ring. 3. Addition reactions with halogens, e.g. Methylbenzene (toluene; C6H5CH3) is the → RCH:CH2 + Br2 RCHBrCH2Br simplest example. Alkylbenzenes can be 4. Hydration using concentrated sulfuric made by a FRIEDEL–CRAFTS REACTION or by acid, followed by dilution and warming: the WURTZ REACTION. Industrially, large → RCH:CH2 + H2O RCH(OH)CH3 quantities of methylbenzene are made from 5. Oxidation by cold potassium perman- crude oil. ganate solutions to give diols: Substitution of alkylbenzenes can occur → RCH:CH2 + H2O + [O] at the benzene ring; the alkyl group directs RCH(OH)CH2OH the substituent into the 2- or 4-position. 6. Oxidation to form cyclic epoxides (oxi- Substitution of hydrogen atoms on the ranes). Ethene can be oxidized in air alkyl group can also occur. using a silver catalyst to the cyclic compound epoxyethane (C2H4O). More alkyl group A group obtained by re- generally peroxy carboxylic acids are moving a hydrogen atom from an alkane used as the oxidizing agent. or other aliphatic hydrocarbon. For exam- 7. Polymerization to polyethene (by the ple, the methyl group (CH3–) is derived ZIEGLER PROCESS or PHILLIPS PROCESS). from methane (CH4). See also oxo process; ozonolysis. In general, addition to simple alkenes is alkyl halide See haloalkane. ELECTROPHILIC ADDITION. Attack is by an electrophile on the pi orbital of the alkene. alkyl sulfide A THIOETHER with the gen- In the case of attack by a halogen acid (e.g. eral formula RSR′, where R and R′ are HBr), the initial reaction is by the (positive) alkyl groups. hydrogen giving a positively charged intermediate ion (carbocation) and a Br– ion. alkyne A type of hydrocarbon contain- The Br– ion then attacks the intermediate ing one or more triple carbon–carbon carbocation. In the case of a halogen (e.g. bonds in its molecule. Alkynes with one Br2) the bromine acts as an electrophile to triple bond have the general formula form an initial cyclic positively charged CnH2n–2. The alkynes are unsaturated com- bromonium ion and a negative Br– ion. The pounds. The simplest member of the series – Br ion further attacks the bromonium ion is ethyne (acetylene; C2H2), which can be to give the substituted product. prepared by the action of water on calcium dicarbide. → alkoxide An organic compound con- CaC2 + 2H2O Ca(OH)2 + C2H2 taining an ion of the type RO–, where R is The alkynes were formerly called the an alkyl group. Alkoxides can be made by acetylenes.

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allosteric site

In general, alkynes can be made by the cracking of alkanes or by the action of a O hot alcoholic solution of potassium hy- H N C droxide on a dibromoalkane, for example: C H → N • BrCH2CH2Br + KOH • A • KBr + CH2:CHBr + H2O H O → • H CH2:CHBr + KOH • C CHCH + KBr + H2O O C The main reactions of the alkynes are: N C A 1. Hydrogenation with a catalyst (usually H ° O • nickel at about 150 C): • → C2H2 + H2 C2H4 O → H N C2H4 + H2 C2H6 C • • • hydrogen bond 2. Addition reactions with halogen acids: C → H A amino-acid side C2H2 + HI H2C:CHI → A H2C:CHI + HI CH3CHI2 3. Addition of halogens; for example, with Alpha helix bromine in tetrachloromethane: → C2H2 + Br2 BrHC:CHBr → BrHC:CHBr + Br2 Br2HCCHBr2 which peptide chains are coiled to form a 4. With dilute sulfuric acid at 60–80°C and spiral. Each turn of the spiral contains ap- mercury(II) catalyst, ethyne forms proximately 3.6 amino-acid residues. The ethanal (acetaldehyde): R group of these amino-acids extends out- → C2H2 + H2O H2C:C(OH)H ward from the helix and the helix is held This enol form converts to the alde- together by hydrogen bonding between hyde: successive coils. If the alpha helix is CH3COH stretched the hydrogen bonds are broken 5. Ethyne polymerizes if passed through a but reform on relaxation. The alpha helix hot tube to produce some benzene: is found in muscle protein and keratin. It is → 3C2H2 C6H6 one of the two basic secondary structures 6. Ethyne forms unstable dicarbides of PROTEINS. (acetylides) with ammoniacal solutions of copper(I) and silver(I) chlorides. alpha-naphthol test (Molisch’s test) A Addition to simple alkynes is ELECTRO- standard test for carbohydrates in solution. PHILIC ADDITION, as with ALKENES. Molisch’s reagent, alpha-naphthol in alcohol, is mixed with the test solution. Con- allosteric site A part of an enzyme sep- centrated sulfuric acid is added and a violet arate from the active site to which a spe- ring at the junction of the two liquids indi- cific effector or modulator can be attached. cates the presence of carbohydrates. This attachment is reversible and alters the activity of the enzyme. Allosteric enzymes alternating copolymer See polymer- possess an allosteric site in addition to their ization. ACTIVE SITE. This site is as specific in its relationship to modulators as active sites are aluminum trimethyl See trimethylalu- to substrates. See active site. Some iron- minum. enzymatic proteins e.g. hemoglobin also undergo allosteric effects. amalgam An alloy of mercury with one or more other metals. Amalgams may be allyl group See propenyl group. liquid or solid. An amalgam of sodium (Na/Hg) with water is used as a source of alpha amino acid See amino acid. nascent hydrogen.

alpha helix A highly stable structure in amatol A high explosive that consists of

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amine

a mixture of ammonium nitrate and TNT H (trinitrotoluene). C2H5 N

amide 1. A type of organic compound of H general formulae RCONH2 (primary), (RCO)2NH (secondary), and (RCO)3N primary (ethylamine) (tertiary). Amides are crystalline solids and are basic in nature, some being soluble in water. They can be formed by reaction of ammonia with acid anhydrides: → (RCO)2O + 2NH3 C2H5 – + RCONH2 + RCOO NH4 They can also be made by reacting ammo- C2H5 N nia with an acyl chloride: → RCOCl + 2NH3 RCONH2 + NH4Cl H Reactions of amides include: 1. Reaction with hot acids to give car- secondary (diethylamine) boxylic acids: → RCONH2 + HCl + H2O RCOOH + NH4Cl 2. Reaction with nitrous acid to give carboxylic acids and nitrogen: C2H5 → RCONH2 + HNO2 C H N RCOOH + N2 + H2O 2 5 3. Dehydration by phosphorus(V) oxide to C H give a nitrile: 2 5 RCONH – H O → RCN 2 2 tertiary (triethylamine) See also Hofmann degradation. – 2. An inorganic salt containing the NH2 Amine ion. Ionic amides are formed by the reaction of ammonia with certain reactive metals (such as sodium and potassium). ondary; three, tertiary. Since amines are Sodamide, NaNH2, is a common example. basic they can form the quaternary ion, + R3NH . All three types, plus a quaternium amination The introduction of an salt, can be produced by the HOFMANN amino group (–NH2) into an organic com- DEGRADATION (which occurs in a sealed pound. An example is the conversion of an vessel at 100°C): aldehyde or ketone into an amide by reac- → + – RX + NH3 RNH3 X tion with hydrogen and ammonia in the + – ˆ RNH3 X + NH3 RNH2 + NH4X presence of a catalyst: → + – → RNH2 + RX R2NH2 X RCHO + NH3 + H2 RCH2NH2 + + – ˆ R2NH2 X + NH3 R2NH + NH4X H2O → + – R2NH + RX R3NH X R NH+ X– + NH ˆ R N + NH X amine A compound containing a nitro- 3 3 3 4 → + – gen atom bound to hydrogen atoms or hy- R3N + RX R4N X drocarbon groups. Amines have the Reactions of amines include: 1. Reaction with acids to form salts: general formula R3N, where R can be hy- → + – drogen or an alkyl or aryl group. They can R3N + HX R3NH X be prepared by reduction of amides or 2. Reaction with acyl halides to give N- nitro compounds. substituted amides (primary and sec- Amines are classified according to the ondary amines only): ′ → ′ number of organic groups bonded to the RNH2 + R COCl R CONHR + HX nitrogen atom: one, primary; two, sec- See also amine salt.

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amine salt The amino acids most commonly found in proteins AMINO ACIDS MOST COMMONLY FOUND IN PROTEINS alanine glycine proline** arginine histidine* serine asparagine isoleucine* threonine* aspartic acid leucine* tryptophan* cysteine lysine* tyrosine* glutamic acid methionine* valine* glutamine phenylalanine*

* essential amino acids in animal diets ** an imino acid derived from pyrollidine

Amino acid: the amino acids in proteins are R C COOH alpha amino acids. The –COOH group and –NH2 group are on the same carbon atom

NH2

amine salt A salt similar to an ammo- (but not in alcohol), and, with the sole ex- nium salt, but with organic groups at- ception of the simplest member, all are op- tached to the nitrogen atom. For example, tically active. triethylamine ((C2H5)3N) will react with In the body the various proteins are as- hydrogen chloride to give triethylammo- sembled from the necessary amino acids nium chloride: and it is important therefore that all the → + – (C2H5)3N + HCl (C2H3)3NH Cl amino acids should be present in sufficient Salts of this type may have four groups on quantities. In adult humans, twelve of the the nitrogen atom. For example, with twenty amino acids can be synthesized by chloroethane, tetraethylammonium chlo- the body itself. Since these are not required ride can be formed: in the diet they are known as nonessential → + – (C2H5)3N + C2H5Cl (C2H5)4N Cl amino acids. The remaining eight cannot Sometimes amine salts are named using the be synthesized by the body and have to be suffix ‘-ium’. For instance, aniline supplied in the diet. They are known as es- (C6H5NH2) forms anilinium chloride sential amino acids. + – C6H5NH3 Cl . Often insoluble alkaloids are used in medicine in the form of their aminobenzene See aniline. amine salt (sometimes referred to as the ‘hydrochloride’). aminoethane See ethylamine.

amino acid A derivative of a carboxylic amino group The group –NH2. acid in which a hydrogen atom in an aliphatic acid has been replaced by an amino sugar A sugar in which a hy- amino group. Thus, from ethanoic acid, droxyl group (OH) has been replaced by an the amino acid 2-aminoethanoic acid amino group (NH2). Glucosamine (from (glycine) is formed. The amino acids of spe- glucose) occurs in many polysaccharides of cial interest are those that occur as con- vertebrates and is a major component of stituents of naturally occurring PEPTIDES chitin. Galactosamine or chondrosamine and PROTEINS. These all have the –NH2 and (from galactose) is a major component of –COOH groups attached to the same car- cartilage and glycolipids. Amino sugars are bon atom; i.e. they are alpha amino acids. important components of bacterial cell All are white, crystalline, soluble in water walls.

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analysis

aminotoluine See toluidine. volatile liquid organic compound; a nitrous acid ester of 3-methylbutanol (iso- ammonia (NH3) A colorless gas with a amyl alcohol). It is used in medicine as an characteristic pungent odor. On cooling and inhalant to dilate the blood vessels (and compression it forms a colorless liquid, thereby prevent pain) in patients with which becomes a white solid on further angina pectoris. cooling. Ammonia is very soluble in water (a saturated solution at 0°C contains 36.9% amylopectin The water-insoluble frac- of ammonia); the aqueous solution is alka- tion of STARCH. line and contains a proportion of free ammonia. Ammonia is also soluble in ethanol. amylose A polymer of GLUCOSE; a poly- It reacts with acids to form ammonium saccharide sugar that is found in STARCH. salts; for example, it reacts with hydrogen chloride to form ammonium chloride: anabolic steroid Any STEROID hormone → NH3(g) + HCl(g) NH4Cl(g) or synthetic steroid that promotes growth See also amine salt. and formation of new tissue. Anabolic steroids are used in the treatment of wast- ammoniacal Describing a solution in ing diseases. They are also sometimes used aqueous ammonia. in agriculture to boost livestock production. People also use them to build up mus- amount of substance Symbol: n A cles, although this is now generally out- measure of the number of entities present lawed in sporting activities. in a substance. See mole. anabolism All the metabolic reactions AMP (adenosine monophosphate) A that synthesize complex molecules from nucleotide consisting of adenine, ribose, more simple molecules. See also metabo- and phosphate. See ATP. lism.

amphiprotic Able to act as both an ACID anaerobic Describing a biochemical and a base. For example, the amino acids process that takes place in the absence of are amphiprotic because they contain both free oxygen. Compare aerobic. acidic (–COOH) and basic (–NH2) groups. See also amphoteric; solvent. anaerobic respiration Respiration in which oxygen is not involved. It is found in ampholyte ion See zwitterion. yeasts, bacteria, and occasionally in muscle tissue. In this type of respiration the or- amphoteric A material that can display ganic substrate is not completely oxidized both acidic and basic properties. The term and the energy yield is low. In the absence is most commonly applied to the oxides of oxygen in animal muscle tissue, glucose and hydroxides of metals that can form is degraded to pyruvate by GLYCOLYSIS, both cations and complex anions. For ex- with the production of a small amount of ample, zinc oxide dissolves in acids to form energy and also lactic acid, which may be zinc salts and also dissolves in alkalis to oxidized later when oxygen becomes avail- 2– form zincates, [Zn(OH)4] . Compounds able (see oxygen debt). FERMENTATION is an such as the amino acids can also be de- example of anaerobic respiration, in which scribed as amphoteric, although it is more certain yeasts produce ethanol and carbon usual to use the term AMPHIPROTIC. dioxide as end products. Only two molecules of ATP are produced by this process. amu See atomic mass unit. Compare aerobic respiration.

amyl group See pentyl group. analysis The process of determining the constituents or components of a sample. amyl nitrite (C5H11ONO) A pale brown There are two broad major classes of 17 iranchembook.ir/edu

ångstrom

H3C H3C OC OC HO O HO OC OC H3C H3C

Anhydride

analysis, qualitative analysis – essentially dride of H2SO4. Organic anhydrides are answering the question ‘what is it?’ – and formed by removing H2O from two car- quantitative analysis – answering the ques- boxylic acid groups, giving compounds tion ‘how much of such and such a compo- with the functional group –CO.O.CO–. nent is present?’ There is a large number of These form a class of organic compounds analytical methods that can be applied, de- called ACID ANHYDRIDES. pending on the nature of the sample and the purpose of the analysis. These include anhydrous Describing a substance that gravimetric, volumetric, and systematic lacks moisture, or a salt with no water of qualitative analysis (classical wet meth- crystallization. ods); and instrumental methods, such as chromatographic, spectroscopic, nuclear, aniline (aminobenzene; phenylamine; fluorescence, and polarographic tech- C H NH ) A colorless oily substance niques. 6 5 2 made by reducing nitrobenzene (C6H5- ångstrom Symbol Å A unit of length de- NO2). Aniline is used for making dyes, fined as 10–10 meter. The ångstrom was pharmaceuticals, and other organic com- used for expressing wavelengths of light or pounds. ultraviolet radiation or for the sizes of molecules; the nanometer is now preferred. animal starch See glycogen.

anhydride A compound formed by re- anion A negatively charged ion, formed moving water from an acid or, less com- by addition of electrons to atoms or mol- monly, a base. Many nonmetal oxides are ecules. In electrolysis anions are attracted anhydrides of acids: for example CO2 is the to the positive electrode (the anode). Com- anhydride of H2CO3 and SO3 is the anhy- pare cation.

O O

C C H2C OH H2C O

H2COH H2C C C

O O

maleic acid maleic anhydride

Anhydride: a cyclic anhydride

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anomer

H H

H H HH H H H H H H H

H H H HH H H H HH [14]-annulene H H H H

H H H H H H HH H H H H H H H H HH H H H HH HHH HH H [30]-annulene HH

HH [18]-annulene

Annulene

anionic detergent See detergent. annulene larger than benzene. It is not an AROMATIC COMPOUND because it is not pla- anionic resin An ION-EXCHANGE ma- nar and does not obey the Hückel rule. – terial that can exchange anions, such as Cl C8H8 is called cyclo-octatetraene. Higher and OH–, for anions in the surrounding annulenes are designated by the number of medium. Such resins are used for a wide carbon atoms in the ring. [10]-annulene range of analytical and purification pur- obeys the Hückel rule but is not aromatic poses. because it is not planar as a result of inter- They are often produced by addition of actions of the hydrogen atoms inside the + a quaternary ammonium group (N(CH3)4 ) ring. There is evidence that [18]-annulene, or a phenolic group (–OH–) to a stable which is a stable red solid, has aromatic polyphenylethene resin. A typical exchange properties. reaction is: + – ˆ resin–N(CH3)4 Cl + KOH anode In electrolysis, the electrode that + – resin–N(CH3)4 OH + KCl is at a positive potential with respect to the Anionic resins can be used to separate mix- cathode. In any electrical system, such as a tures of halide ions. Such mixtures can be discharge tube or electronic device, the attached to the resin and recovered sepa- anode is the terminal at which electrons rately by elution. flow out of the system.

annulene A ring compound containing anomer Either of two isomeric forms of alternating double and single C–C bonds. a cyclic sugar that differ in the disposition The compound C8H8, having an eight- of the –OH group on the carbon next to membered ring of carbon atoms, is the next the O atom of the ring (the anomeric

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anomeric carbon

carbon). Anomers are diastereoisomers. anticlinal conformation See confor- They are designated α– or β– according mation. to whether the –OH is below or above the ring respectively. See illustration at antiknock agent A substance added to sugar. gasoline to inhibit preignition or ‘knocking’. A common example is lead tetraethyl, anomeric carbon See anomer. although use of this is discouraged in many countries for environmental reasons. anthocyanin One of a group of water- soluble pigments found dissolved in higher antioxidant A substance that inhibits plant cell vacuoles. Anthocyanins are red, oxidation. Antioxidants are added to such purple, and blue and are widely distrib- products as foods, paints, plastics, and uted, particularly in flowers and fruits, rubber to delay their oxidation by atmos- where they are important in attracting in- pheric oxygen. Some work by forming sects, birds, etc. They also occur in chelates with metal ions, thus neutralizing buds and sometimes contribute to the au- the catalytic effect of the ions in the oxida- tumn colors of leaves. They are natural tion process. Other types remove interme- pH indicators, often changing from red diate oxygen free radicals. Naturally to blue as pH increases, i.e. acidity de- occurring antioxidants can limit tissue or creases. Color may also be modified by cell damage in the body. These include traces of iron and other metal salts and or- vitamin E and β-carotene. ganic substances, for example cyanin is red in roses but blue in the cornflower. See See con- flavonoid. antiperiplanar conformation formation.

apoenzyme The protein part of a conjugate enzyme. It is an enzyme whose cofactor has been removed (e.g. via dialysis) rendering it catalytically inactive. When combined with its PROSTHETIC GROUP or Anthracene coenzyme it forms a complete enzyme (HOLOENZYME).

anthracene (C14H10) A white crystalline solid used extensively in the manufacture aprotic See solvent. of dyes. Anthracene is found in the heavy- and green-oil fractions of crude oil and is aqueous Describing a solution in water. obtained by fractional crystallization. Its structure is benzene-like, having three six- arene An organic compound containing membered rings fused together. The reac- a benzene ring; i.e. an aromatic hydrocar- tions are characteristic of AROMATIC bon or a derivative of an aromatic hydro- COMPOUNDS. carbon.

anthracite The highest grade of coal, with a carbon content of between 92% and O 98%. It burns with a hot blue flame, gives 81 off little smoke and leaves hardly any ash. 7 2

anthraquinone (C6H4(CO)2C6H4)A 6 3 colorless crystalline quinone used in pro- 5 4 ducing dyestuffs such as alizarin. O

antibonding orbital See orbital. Anthraquinone

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aspirin

arginine See amino acid. of atoms with alternate double and single bonds, but do not obey the rule (e.g. cyclo- aromatic compound An organic com- octatetraene, which has a nonplanar ring pound with characteristic chemical reac- of alternating double and single bonds) are tions, usually containing BENZENE rings in called pseudoaromatics. its structure. Aromatic compounds, such as Compare aliphatic compound. See also benzene, have a planar ring of atoms linked annulene. by alternate single and double bonds. The characteristic of aromatic compounds is aromaticity See aromatic compound. that their chemical properties are not those expected for an unsaturated compound; Arrhenius equation An equation relat- they tend to undergo nucleophilic substitu- ing the rate constant of a chemical reaction tion of hydrogen (or other groups) on the and the temperature at which the reaction ring, and addition reactions only occur takes place: under special circumstances. k = Aexp(–Ea/RT) The explanation of this behavior is that where A is a constant, k the rate constant, the electrons in the double bonds are delo- T the thermodynamic temperature in calized over the ring, so that the six bonds kelvins, R the gas constant, and Ea the ac- are actually all identical and intermediate tivation energy of the reaction. between single bonds and double bonds. Reactions proceed at different rates at The pi electrons are thus spread in a mo- different temperatures, i.e. the magnitude lecular orbital above and below the ring. of the rate constant is temperature depend- The evidence for this delocalization in ben- ent. The Arrhenius equation is often writ- zene is that the bond lengths between car- ten in a logarithmic form, i.e. bon atoms in benzene are all equal and log k = log A – E/2.3RT intermediate in size between single and e e This equation enables the activation en- double bond lengths. Also, if two hydrogen ergy for a reaction to be determined. It is atoms attached to adjacent carbon atoms named for the Swedish chemist Svante Au- are substituted by other groups, the compound has only one structure. If the bonds gust Arrhenius (1859–1927). were different two isomers would exist. Benzene has a stabilization energy of 150 Arrhenius theory See acid. kJ mol–1 over the Kekulé structure. It is possible to characterize aromatic behavior aryl group An organic group derived by by detecting a ring current in NMR. Cer- removing a hydrogen atom from an aro- tain heterocyclic molecules, such as PYRI- matic hydrocarbon or derivative. The DINE, also have aromatic properties. phenyl group, C6H5–, is the simplest exam- The delocalization of the electrons in ple. the pi orbitals of benzene accounts for the properties of benzene and its derivatives, ascorbic acid See vitamin C. which differ from the properties of alkenes and other aliphatic compounds. The phe- asparagine See amino acid. nomenon is called aromaticity. A definition of aromaticity is that it occurs in aspartic acid See amino acid. compounds that obey the Hückel rule: i.e. that there should be a planar ring with a aspirin (acetylsalicylic acid; C9H8O4)A total of (4n + 2) pi electrons (where n is any colorless crystalline compound made by integer). Using this rule as a criterion cer- treating salicylic acid with ethanoyl hy- tain nonbenzene rings show aromaticity. dride. It is used as an analgesic and anti- Such compounds are called nonbenzenoid pyretic drug, and small doses are aromatics. Examples are the cyclopentadi- prescribed for adult patients at risk of heart – enyl ion C5H5 and the tropyllium ion attack or stroke. It should not be given to + C7H7 . Other compounds that have a ring children. 21 iranchembook.ir/edu

association

association The combination of mol- with different values of l (0, 1, and 2). ecules of a substance with those of another Sub-shells with angular momentum 0, 1, to form more complex species. An example 2, and 3 are designated by letters s, p, d, is a mixture of water and ethanol (which and f. are termed associated liquids), the mol- 3. The magnetic quantum number (m). ecules of which combine via hydrogen This can have values –l, –(l – 1) … 0 … bonding. + (l – l), + l. It determines the orientation of the electron orbital in a magnetic asymmetric atom See chirality; iso- field. merism; optical activity. 4. The spin quantum number (ms), which specifies the intrinsic angular momen- atactic polymer See polymerization. tum of the electron. It can have values +½ and –½. atmosphere A unit of pressure defined Each electron in the atom has four quan- as 101 325 pascals (atmospheric pressure). tum numbers and, according to the Pauli The atmosphere is used in chemistry only exclusion principle, no two electrons can for rough values of pressure; in particular, have the same set of quantum numbers. for stating the pressures used in high- This explains the electronic structure of pressure industrial processes. atoms.

atom The smallest part of an element atomicity The number of atoms per that can exist as a stable entity. Atoms con- molecule of an element. Helium, for exam- sist of a small dense positively charged nu- ple, has an atomicity of one, nitrogen two, cleus, made up of neutrons and protons, and ozone three. with electrons in a cloud around this nucleus. The chemical reactions of an element atomic mass unit (amu) Symbol: u A are determined by the number of electrons unit of mass used for atoms and molecules, (which is equal to the number of protons in equal to 1/12 of the mass of an atom of the nucleus). All atoms of a given element carbon-12. It is equal to 1.660 33 × 10–27 have the same number of protons (the pro- kg. ton number). A given element may have two or more isotopes, which differ in the atomic number See proton number. number of neutrons in the nucleus. The electrons surrounding the nucleus atomic orbital See orbital. are grouped into shells – i.e. main orbits around the nucleus. Within these main or- atomic weight See relative atomic mass bits there may be subshells. These corre- (r.a.m.). spond to atomic orbitals. An electron in an atom is specified by four quantum num- ATP (adenosine triphosphate) The uni- bers: versal energy carrier of living cells. Energy 1. The principal quantum number (n), from respiration or, in photosynthesis, which specifies the main energy levels. n from sunlight is used to make ATP from can have values 1, 2, etc. The corre- ADP. It is then reconverted to ADP in var- sponding shells are denoted by letters K, ious parts of the cell by enzymes known as L, M, etc., the K shell (n = 1) being the ATPases, the energy released being used to nearest to the nucleus. The maximum drive three main cellular processes: me- number of electrons in a given shell is chanical work (muscle contraction and cel- 2n2. lular movement); the active transport of 2. The orbital quantum number (l), which molecules and ions; and the biosynthesis of specifies the angular momentum. For a other molecules. It can also be converted to given value of n, l can have possible val- light, electricity, and heat. ues of n–1, n–2, … 2, 1, 0. For instance, ATP is a nucleotide consisting of ade- the M shell (n = 3) has three subshells nine and ribose with three phosphate

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azeotropic mixture

NH 2 acid, also formerly used as herbicides but N N now widely restricted.

OOO N N Avogadro constant (Avogrado number) - Symbol: NA The number of particles in one O POPOPOCH2 O mole of a substance. Its value is 6.022 52 × - - OOO - 1023 mol–1.

Avogadro number See Avogadro con- OHOH stant. ATP Avogadro’s law The principle that equal volumes of all gases at the same tem- groups attached. Hydrolysis of the termi- perature and pressure contain equal num- nal phosphate bond releases energy bers of molecules. It is often called –1 (30.6 kJ mol ) and is coupled to an Avogadro’s hypothesis. It is strictly true energy-requiring process. Further hydroly- only for ideal gases. sis of ADP to AMP sometimes occurs, releasing more energy. The pool of ATP is axial conformation See cyclohexane. small, but the faster it is used, the faster it is replenished. ATP is not transported azeotrope (azeotropic mixture) A mix- around the body, but is synthesized where ture of liquids for which the vapor phase it is needed. has the same composition as the liquid phase. It therefore boils without change in Symbol: a A prefix denoting 10–18. atto- composition and, consequently, without For example, 1 attometer (am) = 10–18 progressive change in boiling point. meter (m). The composition and boiling points of azeotropes vary with pressure, indicating autocatalysis See catalyst. that they are not chemical compounds. Azeotropes may be broken by distillation autoclave An apparatus consisting of an in the presence of a third liquid, by chemi- airtight container whose contents are heated by high-pressure steam; the con- cal reactions, adsorption, or fractional tents may also be agitated. Autoclaves are crystallization. See constant-boiling mix- used for reactions between gases under ture. pressure in industrial processing and for sterilizing objects. azeotropic distillation A method used to separate mixtures of liquids that cannot auxin Any of a group of plant hor- be separated by simple distillation. Such a mones, the most common naturally occur- mixture is called an azeotrope. A solvent is ring one being indole acetic acid, IAA. added to form a new azeotrope with one of Auxins are made continually in growing the components, and this is then removed shoot and root tips. Synthetic auxins, and subsequently separated in a second cheaper and more stable than IAA, are em- column. An example of the use of azeo- ployed in agriculture, horticulture, and tropic distillation is the dehydration of research. These include indoles and naph- 96% ethanol to absolute ethanol. thyls: e.g. NAA (naphthalene acetic acid) Azeotropic distillation is not widely used used mainly as a rooting and fruit setting because of the difficulty of finding inex- hormone; phenoxyacetic acids, e.g. 2,4-D pensive nontoxic noncorrosive solvents (2,4-dichlorophenoxyacetic acid) used as that can easily be removed from the new weed-killers and modifiers of fruit develop- azeotrope. ment; and more toxic and persistent benzoic auxins, e.g. 2,4,5-trichlorobenzoic azeotropic mixture See azeotrope.

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azide

azide 1. An organic compound of gen- in acid dyes for wool and cotton. The dyes eral formula RN3. are azo compounds; usually sodium salts 2. An inorganic compound containing the of sulfonic acids. – ion N3 . azo group See azo compound. azine An organic heterocyclic compound that has a hexagonal ring contain- azulene (C10H8) A blue crystalline coming carbon and nitrogen atoms. Pyridine pound having a seven-membered ring (C5H5N) is the simplest example. fused to a five-membered ring. It converts to naphthalene on heating. azo compound A type of organic com- ′ pound of the general formula RN:NR , 8 where R and R′ are aromatic groups. Azo 1 7 compounds can be formed by coupling a DIAZONIUM COMPOUND with an aromatic 2 6 phenol or amine. Most are colored because of the presence of the azo group –N:N–. 3 5 4

azo dye An important type of dye used Azulene

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backbiting A process that can occur in rect (except for the stereochemistry of the certain free-radical POLYMERIZATION reac- double bond, which was subsequently tions, in which a radical with an unpaired shown by x-ray crystallography to be electron on the end of the chain converts trans). Baeyer discovered a number of sub- into one in which the unpaired election is stances including barbituric acid. His later not at the end of the chain. For example, investigations on ring compounds and the radical polyacetylenes led him to consider the sta- RCH2CH2CH2CH2CH2 CH2• bility of carbon–carbon bonds in cyclic may convert into compounds. This resulted in the Baeyer RCH2CH•CH2CH2 CH2CH3 strain theory. Baeyer was awarded the Effectively, this involves a transfer of a 1905 Nobel Prize for chemistry for his hydrogen atom within the molecule. Typi- work on indigo and aromatic compounds. cally, the free electron moves from the end of the chain to atom five, counting from Baeyer–Villiger reaction A type of re- the end. This is because the process in- action in which a ketone reacts with a per- volves a transition state with a six-mem- oxy acid, with resulting production of an bered ring. The new free radical is more ester. For example, stable than the original one. Further poly- R–CO–R → R–CO–O–R. merization occurs at the new unpaired The reaction involves ‘insertion’ of an oxy- electron leading to the production of poly- gen atom next to the carbonyl (CO) group. mers with butyl (CH3CH2CH2CH2–) side Typical peroxy acids used are trifluoro- chains. perethanoic acid (CF3.CO.O.OH) and meta-chloroperbenzoic acid (m-CPBA; Baeyer, Johann Friedrich Wilhelm ClC6H4.CO.O.OH). The reaction was dis- Adolph von (1835–1917) German or- covered in 1899 by the German chemists ganic chemist. Baeyer worked mainly in or- A. Baeyer and V. Villiger, and is commonly ganic synthesis and is noted for his study of used in organic synthesis. In certain cases the dye indigo. He started his work on in- hydrogen peroxide (H2O2) can be used as digo in 1865 and continued for 20 years; the oxidizing agent. This is sometimes he determined the structure of indigo in known as the Dakin reaction. The Baeyer– 1883. The structure he postulated was cor- Villiger reaction is a type of rearrange-

H H H H C 2 C 2 R CH C CH R CH2 C CH2 2 2

CH H CH2 2 C CH H2 3

Backbiting

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Bakelite

OH OH

CH CH2 2

CH2 CH2

OH OH

CH2

Bakelite

ment. For a peroxy acid X.CO.O.OH, tion of the balls in the drum. Compare there is an intermediate cation formed hammer mill. + R2C (OH)(O.CO.X). The mechanism involves migration of a group R onto the banana bond (bent bond) In strained- oxygen of the peroxy acid group. ring compounds the bond angles that would be produced by hybridization of or- Bakelite (Trademark) A common ther- bitals are not equal to the angles obtained mosetting synthetic polymer formed by the by joining the atomic centers. In such cases it is sometimes assumed that the bonding condensation of phenol (C6H5OH) and methanal (formaldehyde, HCOH). It is an orbital is bent or banana-like in shape. For example of a phenolic resin (or phenol– example, in cyclopropane the three carbon formaldehyde resin), and was one of the atoms are arranged in an equilateral trian- gle, and the bond angle is 60°. The sp3 hy- first useful synthetic polymers. The reac- bridization gives an angle of about 104° tion between phenol and methanal occurs between the orbitals. Consequently, the or- under acid conditions and involves elec- bitals overlap at an angle, giving a banana trophilic substitution on the benzene ring bond. The term ‘banana bond’ is also used to give a three-dimensional polymeric in a quite separate sense for a multicenter structure. Bakelite is named for the Bel- bond of the type present in electron- gian-born US chemist Leo Hendrik Baeke- deficient compounds such as diborane land (1863–1944), who discovered it in (B2H6). 1909. band spectrum A SPECTRUM that ap- ball mill A device commonly used in the pears as a number of bands of emitted or chemical industry for grinding solid ma- absorbed radiation. Band spectra are char- terial. Ball mills usually have slowly rotat- acteristic of molecules. Often each band ing steel-lined drums containing steel balls. can be resolved into a number of closely The material is crushed by the tumbling ac- spaced lines. The different bands corre-

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benzaldehyde

spond to changes of electron orbit in the basic Acting as a base; having a ten- molecules and the closely spaced lines in dency to release hydroxide ions (OH–) in each band, seen under higher resolution, aqueous solution. A basic solution has an are the result of different vibrational states excess of OH– ions over H+ ions; i.e. a pH of the molecule. greater than 7.

barrel A measurement of volume often batch process A manufacturing process used in the oil and chemical industries. One in which the reactants are fed into the barrel is equal to 159 liters (about 29 US process in fixed quantities (batches), rather gallons). than in a continuous flow. At any particular instant all the material, from its prepa- Barton, Sir Derek Harold Richard ration to the final product, has reached a (1918–98) British organic chemist noted definite stage in the process. Such processes for his work on the stereochemistry of or- present problems of automation and in- ganic molecules, particularly natural prod- strumentation and tend to be wasteful of ucts. In a major paper published in 1950 he energy. For this reason, batch processing is suggested that the rates of reactions in iso- used on an industrial scale only when small mers are strongly influenced by the spatial quantities of valuable or strategic materials orientations of their functional groups. are required, e.g. specialist chemicals or This paper initiated the branch of organic pharmaceuticals. Compare continuous chemistry known as conformational analy- process. sis. Barton studied many natural products, including phenols. In 1959 he developed a Beckmann rearrangement A type of simple synthesis for the hormone aldos- reaction in which the OXIME of a ketone is terone. He shared the 1969 Nobel Prize for converted into an amide using a sulfuric chemistry with Norwegian chemist Odd acid catalyst. First discovered by the Ger- Hassell. man chemist Ernst Beckmann (1853– 1923), it is used in the manufacture of base See acid. polyamides (see nylon). base analog An unnatural purine or pyrimidine that can be incorporated into Beckmann thermometer A type of DNA, causing altered base pairing. Some mercury thermometer designed to measure base analogs are used therapeutically as small differences in temperature rather anticancer drugs. than scale degrees. Beckmann thermome- ters have a larger bulb than common ther- base-catalyzed reaction A reaction mometers and a stem with a small internal ° catalyzed by bases. Typical base-catalyzed diameter, so that a range of 5 C covers reactions are the CLAISEN CONDENSATION about 30 centimeters in the stem. The mer- and the ALDOL REACTION, in which the first cury bulb is connected to the stem in such step is abstraction of a proton to give a a way that the bulk of the mercury can be carbanion. separated from the stem once a particular 5° range has been attained. The thermome- base pairing The linking together of the ter can thus be set for any particular range. two helical strands of DNA by bonds be- The Beckmann thermometer has com- tween complementary bases, adenine pair- monly been used for measuring such quan- ing with thymine and guanine pairing with tities as depression of freezing point and cytosine. The specific nature of base pair- elevation of boiling point. ing enables accurate replication of the chromosomes and thus maintains the con- bent bond See banana bond. stant composition of the genetic material. In pairing between DNA and RNA the benzaldehyde See benzenecarbalde- uracil of RNA pairs with adenine. hyde.

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benzene

(a) (b) (c) (d)

(e) (f) (g)

Benzene: early structures suggested for benzene: (a) Kekulé (1865); (b) Claus (1867); (c) Dewar (1867); (d) Ladenburg (1869); (e) Kekulé (1865); (f) Armstrong–Baeyr (1887); (g) Thiele (1899)

→ benzene (C6H6) A colorless liquid hy- C6H14 C6H6 + 4H2 drocarbon with a characteristic odor. Ben- Benzene is the simplest aromatic hydro- zene is a highly toxic compound and carbon. See aromatic compound. The continued inhalation of the vapor is harm- structure of benzene was the subject of ful. It was originally isolated from coal tar considerable speculation in the 19th cen- and for many years this was the principal tury. The basic problem – known as the source of the compound. Contemporary benzene problem – was that of reconciling manufacture is from hexane; petroleum the formula of benzene, C6H6, with its vapor is passed over platinum at 500°C chemical reactions. The empirical formula and at a pressure of 10 atmospheres: is the same as that of acetylene, C2H2, and

H C C H C C HH

Benzene: in benzene, 6 p orbitals can combine in different ways to give delocalized molecular orbitals. The one of lowest energy has two donut-shaped areas above and below the ring of carbon atoms.

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benzenecarbonyl chloride Cl Cl Cl

Cl 1,1-dichlorobenzene 1,2-dichlorobenzene 1,3-dichlorobenzene (o-dichlorobenzene) (m-dichlorobenzene) (p-dichlorobenzene)

Benzene: disubstituted derivates of benzene

it might be expected that benzene would over the ring. This is the reason benzene undergo similar reactions. However, ben- has all its C-C bonds of the same length zene does not show the usual behavior of a and undergoes ELECTROPHILIC SUBSTITUTION compound containing double or triple reactions. bonds. For example, acetylene adds bromine to yield CHBr:CHBr and eventually CHBr2- O CHBr2. Benzene, with an iron bromide cat- H alyst, suffers displacement of one of its C CH HC hydrogen atoms to yield C6H5Br. This type of activity in which substitution reactions occur indicates that benzene might be satu- HC CH rated. C Benzene, however, does not always act H as a saturated compound. In sunlight Benzenecarbaldehyde (benzaldehyde) bromine is added to give C6H6Cl6 and hydrogen can also be added with a nickel catalyst to yield cyclohexane, C6H12. A benzenecarbaldehyde (benzaldehyde; number of different formulae were put for- C6H5CHO) A yellow oily ALDEHYDE ward to try to explain the properties. In with a distinct almondlike odor (the com- 1865 the German chemist August Kekulé pound occurs in almond kernels). Ben- (1829–96) suggested a structure with alter- zenecarbaldehyde may be synthesized in nate double and single bonds in a hexago- the laboratory by the usual methods of nal ring. To account for the fact that aldehyde synthesis. It is used as a food fla- benzene has only three disubstitution prod- voring and in the manufacture of dyes and ucts, he further proposed that the positions antibiotics, and can be readily manufac- of the bonds oscillate so that two molecules tured by the chlorination of methylbenzene are in equilibrium. This structure – the (toluene) on the methyl group and the Kekulé formula – is the one often used in subsequent hydrolysis of dichloromethyl- formulae of compounds containing ben- benzene: → zene rings. C6H5CH3 + Cl2 C6H5CHCl2 → The modern idea of aromaticity is C6H5CHCl2 + 2H2O C6H5CH(OH)2 based not on equilibrium between Kekulé + 2HCl → structures but on RESONANCE between C6H5CH(OH)2 C6H5CHO + H2O them. The bonds in benzene have charac- ters between double and single bonds: the benzenecarbonyl chloride (benzoyl carbon atoms are held together by six sin- chloride; C6H5COCl) A liquid acyl chlo- gle bonds and the remaining six electrons, ride used as a benzoylating agent. See acy- from the double bonds, are delocalized lation.

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benzenecarbonyl group 3 benzenecarbonyl group (benzoyl 4 2 group) The group C6H5CO–. 5 1 O 6 O H Benzfuran (coumarone) C C HC OH

benzfuran (coumarone; C8H6O) A crys- HC CH talline compound having a benzene ring C fused to a furan ring. H

Benzenecarboxylic acid (benzoic acid) benzilic acid rearrangement A reaction in which benzil (1,2-diphenylethan- 1,2-dione) is treated with hydroxide and benzenecarboxylic acid (benzoic acid; then with acid to give benzilic acid (2-hy- C6H5COOH) A white crystalline car- droxy-2,2-diphenylethanoic acid): → boxylic acid found naturally in some C6H5.CO.CO.C6H5 plants. It is used as a food preservative. The (C6H5)2C(OH).COOH carboxyl group (–COOH) directs further The reaction, which involves migration of substitution onto the benzene ring in the 3 a phenyl group (C6H5–) from one carbon position. atom to another, was the first rearrangement reaction to be described (by German benzene-1,2-dicarboxylic acid (ph- chemist Justus von Liebig in 1828). thalic acid; C6H4(COOH)2) A white crystalline aromatic acid. On heating it benzoic acid See benzenecarboxylic loses water to form phthalic anhydride, acid. which is used to make dyestuffs and polymers. benzole A mixture of mainly aromatic hydrocarbons obtained from coal. benzene-1,4-dicarboxylic acid (terephthalic acid; C6H4(COOH)2) A colorless benzopyrene See benzpyrene. crystalline organic acid used to produce Dacron and other polyesters. benzoquinone See quinone.

benzene-1,3-diol (resorcinol; C6H4(OH)2) benzoylation The introduction of a A white crystalline phenol used in the man- benzoyl group (benzenecarbonyl group) ufacture of dyestuffs and celluloid. into a compound. See acylation.

benzene-1,4-diol (hydroquinone; quinol; benzoyl chloride See benzenecarbonyl C6H4(OH)2) A white crystalline phenol chloride. used in making dyestuffs. See also quinone. benzoyl group See benzenecarbonyl benzene ring The cyclic hexagonal group. arrangement of six carbon atoms that are characteristic of benzene and its deriva- benzpyrene (benzopyrene; C20H12)A tives. See aromatic compound; benzene. cyclic aromatic hydrocarbon with a structure consisting of five fused benzene rings. benzenesulfonic acid (C6H5SO2OH) A It occurs in coal tar and is produced by in- white crystalline sulfonic acid made by sul- complete combustion of some organic fonation of benzene. Any further substitu- compounds. Benzpyrene, which is present tion onto the benzene ring is directed into in tobacco smoke, has marked carcino- the 3 position. genic properties.

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binary compound

beta-pleated sheet A type of PROTEIN structure in which polypeptide chains run close to each other and are held together by hydrogen bonds at right angles to the main chain. The structure is folded in regular ‘pleats’. Fibres having this type of structure are usually composed of amino acids with Benzpyrene short side chains. The chains may run in the same direction (parallel) or opposite directions (antiparallel). It is one of the two benzpyrrole See indole. basic secondary structures of proteins.

benzyl alcohol See phenylmethanol. bi- Prefix meaning ‘two’. For example, biphenyls are compounds that have two benzyl group The group C6H5CH2–. phenyl groups joined together, as in C6H5–C6H5. The prefix is also commonly used in naming inorganic compounds to indicate the presence of hydrogen; for instance, sodium bisulfate (NaHSO4) is sodium hydrogensulfate, etc.

bicarbonate See hydrogencarbonate. Benzyne bimolecular Describing a reaction or a benzyne (C6H4) A short-lived interme- step in a reaction that involves two mol- diate present in some reactions. The ring of ecules, ions, etc. For example the decom- six carbon atoms contains two double position of hydrogen iodide, → bonds and one triple bond (the systematic 2HI H2 + I2 name is 1,2-didehydrobenzene). takes place between two molecules and is therefore a bimolecular reaction. All bi- Bergius process A process formerly molecular reactions are second order, but used for making hydrocarbon fuels from some second-order reactions are not bi- coal. A mixture of powdered coal, heavy molecular. See also order. oil, and a catalyst was heated with hydrogen at high pressure. binary compound A chemical com-

A A A

A A A A A A A A A A

A A A

Beta-pleated sheet

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bioassay

pound formed from only two elements. biotin A water-soluble vitamin generally Water (H2O) and sodium chloride (NaCl) found, together with vitamins in the B are examples. group, in the VITAMIN B COMPLEX. It is widely distributed in natural foods, egg bioassay An experimental technique for yolk, kidney, liver, and yeast being good measuring quantitatively the strength of a sources. Biotin is required as a coenzyme biologically active chemical by its effect on for carboxylation reactions in cellular me- a living organism. For example, the vita- tabolism. min activity of certain substances can be measured using bacterial cultures. The in- biphenyl (C6H5C6H5) An organic com- crease in bacterial numbers is compared pound having a structure in which two against that achieved with known stan- phenyl groups are joined by a C–C bond. dards for vitamins. See also polychlorinated biphenyl.

biochemical oxygen demand (BOD) bipyridyl See dipyridyl. The amount of oxygen taken from natural water by microorganisms that decompose bisulfite addition compound See alde- organic waste matter in the water. It is hyde. therefore a measure of the quantity of organic pollutants present. The biochemical bitumen See tar. oxygen demand is determined by measuring the amount of oxygen in a sample of biuret (H2NCONHCONH2) A colorless crystalline organic compound made by water, storing the sample, and then making heating urea (carbamide). It is used in a the measurement again five days later. chemical test for proteins. See also protein; urea. biochemistry The study of chemical compounds and reactions occurring in liv- bivalent (divalent) Having a valence of ing organisms. two. biodegradable See pollution. block copolymer See polymerization.

biosynthesis The series of reactions by boat conformation See cyclohexane. which organisms obtain the various compounds needed for life. BOD See biochemical oxygen demand.

biotechnology The application of tech- boiling The process by which a liquid is nology to biological processes for indus- converted into a gas or vapor by heating at trial, agricultural, and medical purposes. its boiling point. At this temperature the For example, bacteria such as Penicillium vapor pressure of the liquid is equal to the and Streptomycin are used to produce anti- external pressure, and bubbles of vapor biotics and fermenting yeasts produce alco- can form within the liquid. The boiling hol in beer and wine manufacture. Recent point is always the same for a particular developments in genetic engineering have liquid at a given pressure (for reference enabled the large-scale production of hor- purposes usually taken as standard pres- mones, blood serum proteins, and other sure). See also elevation of boiling point. medically important products. Genetic modification of farm crops, and even live- boiling point-composition diagram A stock, offers the prospect of improved pro- diagram for a two-component liquid sys- tection against pests, or products with tem representing both the variation of the novel characteristics, such as new flavors boiling point and the composition of the or extended storage properties. See also en- vapor phase as the liquid-phase composi- zyme technology. tion is varied.

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bromine

Boltzmann constant Symbol: k The Boyle’s law At a constant temperature, constant 1.380 54 J K–1, equal to the gas the pressure of a fixed mass of a gas is in- constant (R) divided by the Avogadro con- versely proportional to its volume: i.e. stant (NA). pV = K where K is a constant. The value of K de- bomb calorimeter A device for measur- pends on the temperature and on the na- ing the energy released during the combus- ture of the gas. The law holds strictly only tion of substances (e.g. foods and fuels). It for ideal gases. Real gases follow Boyle’s consists of a strong sealed insulated con- law at low pressures and high tempera- tainer in which a known amount of the tures. See gas laws. substance is ignited in an atmosphere of pure oxygen. The substance undergoes Brady’s reagent See 2,4-dinitrophenyl- complete combustion at constant volume hydrazine. and the resultant rise in temperature can be used to calculate the energy released by the Bragg equation An equation used to reaction. Such energy values (calorific val- deduce the crystal structure of a material ues) are often quoted in joules per kilogram using data obtained from x-rays directed at (J kg–1), formerly in calories. its surface. The conditions under which a crystal will reflect a beam of x-rays with bond See chemical bond. maximum intensity is: nλ = 2dsinθ θ bond energy The energy involved in where is the angle of incidence and re- flection (the Bragg angle) that the x-rays forming a chemical bond. For methane, for make with the crystal planes, n is a small instance, the energy of the C–H bond is one integer, λ is the wavelength of the x-rays, quarter of the energy involved for the and d is the distance between the crystal process → planes. C + 4H CH4 It is thus one quarter of the heat of atom- branched chain See chain. ization. The bond dissociation energy is a dif- bromine A deep red, moderately reac- ferent quantity to the bond energy. It is the tive element (symbol Br) belonging to the energy required to break a particular bond halogens; i.e. group 17 (formerly VIIA) of in a compound, e.g.: the periodic table. Bromine is a liquid at → CH4 CH3 + H room temperature (mercury is the only More formally, the bond enthalpy can other element with this property). It occurs be used. in small amounts in seawater, salt lakes, and salt deposits but is much less abundant bonding orbital See orbital. than chlorine. A number of organo- bromine compounds are important com- bond length The length of a chemical mercially. At one time the main use of bond; the distance between the centers of bromine was as 1,2-dibromoethane. This the nuclei of two atoms joined by a chemi- was added to gasoline to combine with the cal bond. Bond lengths may be measured lead produced by decomposition of the by electron or x-ray diffraction. antiknock agent lead tetraethyl. This use has declined with the reduction in use of Bosch process The reaction leaded gasoline for environmental reasons. → CO + H2O CO2 + H2 Quantities of bromine are used in poly- using WATER GAS passed over a hot catalyst. brominated diphenyl ethers (PBDEs), It was used by Carl Bosch (1874–1940) to which are effective flame retardents in produce hydrogen for the Haber process plastics. A number of bromine HALONS are for making ammonia. also important.

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bromoethane

bromoethane (ethyl bromide; C2H5Br) bucky tube See buckminsterfullerene. A colorless volatile compound, used as a refrigerant. It can be made from ethene and buffer A solution in which the pH re- hydrogen bromide. mains reasonably constant when acids or alkalis are added to it; i.e. it acts as a buffer bromoform See tribromomethane. against (small) changes in pH. Buffer solutions generally contain a weak acid and bromomethane (methyl bromide; one of its salts derived from a strong base; CH3Br) A colorless volatile compound e.g. a solution of ethanoic acid and sodium used as a solvent. It can be made from ethanoate. If an acid is added, the H+ reacts methane and bromine. with the ethanoate ion (from dissociated sodium ethanoate) to form undissociated Brønsted acid See acid. ethanoic acid; if a base is added the OH– reacts with the ethanoic acid to form water Brønsted base See acid. and the ethanoate ion. The effectiveness of the buffering action is determined by the buckminsterfullerene An allotrope of concentrations of the acid–anion pair: + – carbon containing clusters of 60 carbon K = [H ][CH3COO ]/[CH3COOH] atoms bound in a highly symmetric poly- where K is the dissociation constant. Phosphate, oxalate, tartrate, borate, hedral structure. The C60 polyhedron has a combination of pentagonal and hexagonal and carbonate systems can also be used for faces similar to the panels on a soccer ball. buffer solutions. The molecule was named for the American architect Richard Buckminster Fuller bumping Violent boiling of a liquid (1895–1983) because its structure resem- caused when bubbles form at a pressure above atmospheric pressure. bles a geodesic dome (invented by Fuller). The C polyhedra are informally called 60 Bunsen burner A gas burner consisting bucky balls. The original method of of a vertical metal tube with an adjustable making the allotrope was to fire a high- air-inlet hole at the bottom. Gas is allowed power laser at a graphite target. This into the bottom of the tube and the gas–air also produces less stable carbon clusters, mixture is burnt at the top. With too little such as C 70. It can be produced more air the flame is yellow and sooty. Correctly conveniently using an electric arc between adjusted, the burner gives a flame with a graphite electrodes in an inert gas. The pale blue inner cone of incompletely burnt allotrope is soluble in benzene, from gas, and an almost invisible outer flame which it can be crystallized to give yellow where the gas is fully oxidized and reaches crystals. This solid form is known as ful- a temperature of about 1500°C. lerite. The discovery of buckminsterfullerene burette A piece of apparatus used for led to a considerable amount of research the addition of variable volumes of liquid into its properties and compounds. Partic- in a controlled and measurable way. The ular interest has been shown in trapping burette is a long cylindrical graduated tube metal ions inside the carbon cage to of uniform bore fitted with a stopcock and form enclosure compounds. Buckminster- a small-bore exit jet, enabling a drop of liq- fullerene itself is often simply called uid at a time to be added to a reaction ves- fullerene. The term also applies to deriva- sel. Similar devices are used to introduce tives of buckminsterfullerene and to simi- measured volumes of gas at regulated pres- lar cluster (e.g. C70). Carbon structures sure in the investigation of gas reactions. similar to that in C60 can also form small tubes, known as bucky tubes. buta-1,3-diene (butadiene; CH2:CH- CH:CH2) A colorless gas made by cat- bucky ball See buckminsterfullerene. alytic dehydrogenation of butane. It is used

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butyl rubber

in the manufacture of synthetic rubber. It can be converted into the trans isomer by Buta-1,3-diene is conjugated and to some heating at 120°C. The cis form, when extent the pi electrons are delocalized over strongly heated, loses water to give a cyclic the whole of the molecule. The molecule acid anhydride (maleic anhydride). can exist in cis and trans forms. See also Diels–Alder reaction. H CO.OH butadiene See buta-1,3-diene. C

butanal (butyraldehyde; C H CHO) A 3 7 C colorless liquid aldehyde. H CO.OH cis (maleic) butane (C4H10) A gaseous alkane obtained either from the gaseous fraction of crude oil or by the ‘cracking’ of heavier HO.OC H fractions. It is the fourth member of the ho- C mologous series of alkanes. Butane is easily liquefied under pressure and its main use is as a portable supply of fuel (bottle gas). It C is also used in the industrial production of H CO.OH buta-1,3-diene. The isomeric hydrocarbon trans (fumaric) CH3CH(CH3)CH3 (2-methylpropane) is known as isobutane. Butenedioic acid butanedioic acid (succinic acid) A crystalline carboxylic acid, HOOC(CH2)2- COOH, that occurs in amber and certain Butlerov, Aleksandr Mikhailovich plants. It forms during the fermentation of (1828–86) Russian organic chemist who sugar (sucrose). was one of the main pioneers of the concept of structure for compounds. He first butanoic acid (butyric acid; C3H7- put forward his ideas in 1861. Butlerov COOH) A colorless liquid carboxylic predicted that tertiary alcohols exist and acid. Esters of butanoic acid are present in synthesized tertiary butanol. In 1876 he butter. proposed the basics of the concept of tautomerism. He also studied formaldehyde butanol (butyl alcohol; C4H9OH) Ei- (methanal) and its polymerization to vari- ther of two alcohols that are derived from ous sugars. butane: the primary alcohol butan-1-ol (CH3(CH2)2CH2OH) and the secondary butyl alcohol See butanol. alcohol butan-2-ol (CH3CH(OH)CH2- CH3). Both are colorless volatile liquids butyl group The straight-chain alkyl used as solvents. group CH3(CH2)2CH2–.

butanone (methyl ethyl ketone; CH3- butyl rubber A type of synthetic rubber COC2H5) A colorless volatile liquid ke- made by copolymerizing isobutylene (2- tone. It is manufactured by the catalytic methylpropene, CH3:C(CH3)2) with small oxidation of butane and used as a solvent. amounts of isoprene (methylbuta-1,3- diene, CH3:C(CH3)CH:CH2). butenedioic acid Either of two isomers. Before the introduction of tubeless tires Transbutenedioic acid (fumaric acid) is a butyl rubber was used for inner tubes crystalline compound found in certain because it is impervious to air. Subse- plants. Cisbutenedioic acid (maleic acid) is quently halogenated butyl rubbers were used in the manufacture of synthetic resins. developed (halobutyls), which could be

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butyraldehyde

cured at higher temperature and vulcan- butyric acid See butanoic acid. ized with other rubbers. Both chlorobutyls and bromobutyls are manufactured. These by-product A substance obtained dur- types of rubber are used in tubeless tires ing the manufacture of a main chemical bonded to the inner surface of the tire. product. For example, propanone was for- Other uses are in sealants, hoses, and pond merly manufactured from propan-1-ol, but liners. is now obtained as a by-product in the manufacture of phenol by the CUMENE butyraldehyde See butanal. PROCESS.

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cadaverine An amine, H2N[CH2]5NH2, The mean or thermochemical calorie produced from lysine in decaying meat and (calTH) is defined as 4.184 joules. The in- fish. ternational table calorie (calIT) is defined as 4.1868 joules. Formerly the mean calorie caffeine An alkaloid found in certain was defined as one hundredth of the heat plants, especially tea and coffee. It is a needed to raise one gram of water from stimulant of the central nervous system 0°C to 100°C, and the 15°C calorie as the and a diuretic. The systematic name is heat needed to raise it from 14.5°C to 1,3,7-trimethylxanthine. 15.5°C.

cage compounds See clathrate. calorific value The energy content of a substance, defined as the energy released in Cahn–Ingold–Prelog system See CIP burning unit mass. Calorific values are system. measured using a BOMB CALORIMETER. They are used to express the efficiency of fuels calciferol See vitamin D. (in megajoules per kilogram). Calorific values are also applied to foods (in kilojoules calcium acetylide See calcium carbide. per gram or in calories). Here they measure the energy produced when the food is oxi- calcium carbide (calcium acetylide; cal- dized in metabolism. cium dicarbide; Ca C2) A white solid that can be produced by heating coke with cal- calorimeter A device or apparatus for cium oxide at high temperature (2000°C). measuring thermal properties such as spe- It contains the dicarbide ion –C≡C– and re- cific heat capacity, calorific value, etc. See acts with water to give ethyne (acetylene; bomb calorimeter. C2H2): → CaC2 + 2H2O Ca(OH)2 + C2H2 Calvin, Melvin (1911–97) American Formerly it was an important source of chemist. Calvin worked out the biological ethyne. mechanisms that occur in photosynthesis. He used techniques such as chromatogra- calcium dicarbide See calcium carbide. phy and radioisotopes to study the reactions of photosynthesis that do not require calixarene See host–guest chemistry. light. He found that there is a series of reactions, now known as the Calvin cycle. calorie Symbol: cal A unit of energy ap- Calvin summarized his findings in proximately equal to 4.2 joules. It was for- a work entitled The Path of Carbon in merly defined as the energy needed to raise Photosynthesis (1957). Calvin won the the temperature of one gram of water by 1961 Nobel Prize for chemistry for his one degree Celsius. Because the specific work on photosynthesis. He continued to thermal capacity of water changes with work on various problems associated with temperature, this definition is not precise. photosynthesis.

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Calvin cycle

– Calvin cycle (reductive pentose-phos- ethanal to form CH2CHO (see aldol reac- phate cycle) See photosynthesis. tion). They can also be formed from organometallic compounds in which the cAMP See cyclic AMP. carbon atom is bonded to an electropositive metal. camphor (C10H16O) A naturally-occurring white organic compound with a characteristic penetrating odor. It is a cyclic compound and a ketone, formerly obtained from the wood of the camphor tree but now made synthetically. Camphor is N used as a platicizer for celluloid and as an H insecticide against clothes moths. Carbazole cane sugar See sucrose. carbazole (C12H9N) A white crystalline Cannizzaro reaction The reaction of compound used in the manufacture of aldehydes to give alcohols and acid anions dyestuffs. in the presence of strong bases. The aldehydes taking part in the Cannizzaro reac- carbene A transient species of the form ′ tion do not have hydrogen atoms on the RR C:, with two valence electrons that do carbon attached to the aldehyde group. For not form bonds. The simplest example is methylene, H C:. Some complex carbenes instance, in the presence of hot aqueous 2 can be isolated but most are short-lived in- sodium hydroxide: termediates in reactions. In a carbene the NaOH + 2C H CHO → 6 5 carbon atom has two valence electrons that C H CH OH + C H COO–Na+ 6 5 2 6 5 are not involved in bonding, and carbene This reaction is a disproportionation, in- species are highly electrophilic. Typically volving both oxidation (to acid) and reduc- they can attack carbon–carbon double tion (to alcohol). Another example is the bonds to produce cyclopropane deriva- reaction of methanal to give methanol and tives. Also they can attack single bonds in methanoate ions. insertion reactions. For example, they can → NaOH + 2HCHO CH3OH + insert into O–H bonds: HCOO–Na+ → R–O–H + R2C: R–O–C(R2)–H The reaction was first described by the They can also insert into C–H bonds: Italian chemist Stanislao Cannizzaro → R–H + R2C: R–C(R2)–H (1826– 1910) in 1853. Reactions like these make carbenes important ‘reagents’ in organic synthesis and var- canonical form See resonance. ious methods have been developed for generating them in the reaction medium. caproic acid See hexanoic acid. All carbenes can exist in two possible states. The carbon atom has one s orbital caprolactam (C6H11NO) A white crystalline substance used in the manufacture O of NYLON. H 2 CC carbamide See urea. H H2C N carbanion An intermediate in an organic reaction in which one carbon atom C C H2 C H2 carries a negative charge. Carbanions may H2 be formed by abstracting a hydrogen ion from a C–H bond using a base, e.g. from Caprolactam

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carbocation

and three p orbitals available for bonding. carbide A compound of carbon with a In carbenes sp2 hybridization occurs and more electropositive element. The carbides the carbon atom has three trigonal sp2 hy- of the elements are classified into: brid orbitals in a plane, with one p orbital 1. Ionic carbides, which contain the car- at right angles to the plane. Two of the bide ion C4–. An example is aluminum 2 hybrid sp orbitals each contain one elec- carbide, Al4C3. Compounds of this type tron, and form sigma bonds with other react with water to give methane (they carbon atoms. The two remaining valence were formerly also called methanides). electrons may be distributed in one of two The dicarbides are ionic carbon com- possible ways. One, the triplet state, has pounds that contain the dicarbide ion one electron in a sp2 orbital and the other –C:C–. The best-known example is cal- in the p orbital. The other, the singlet state, cium dicarbide, CaC2, also known as has both electrons in the sp2 orbital, with CALCIUM CARBIDE, or simply carbide. an empty p orbital. A triplet-state carbene Compounds of this type give ethyne can be detected by electron spin resonance with water. They were formerly called (ESR) because there are two unpaired acetylides or ethynides. Ionic carbides electrons. Carbenes in the singlet state are formed with very electropositive do not have unpaired electrons and are not metals. They are crystalline. detectable by ESR. Moreover, in a car- 2. Covalent carbides, which have giant- molecular structures, as in silicon carbene R2C:, the R–C–R bond angle will be larger if the carbene is in the triplet state bide (SiC) and boron carbide (B4C3). than if it is in the singlet state. In the singlet These are hard high-melting solids. Other covalent compounds of carbon state the nonbonding electrons form a (CO , CS , CH , etc.) have covalent LONE PAIR, which occupies more space than 2 2 4 molecules. a single electron. In general, the triplet state 3. Interstitial carbides, which are intersti- (with two unpaired electrons) is more sta- tial compounds of carbon with transi- ble than the singlet state. However, there is tion metals. Titanium carbide (TiC) is an not a large energy difference between the example. These compounds are all hard two states and, depending on the groups high-melting solids, with metallic prop- attached to the carbon, the singlet state erties. Some carbides (e.g. nickel carbide may be the more stable for certain car- Ni3C) have properties intermediate be- benes. Also, the state that is actually tween those of interstitial and ionic car- formed as an intermediate may depend on bides. the mechanism of production. The less stable form of the carbene may be produced carbocation An ion with a positive and its chemical behavior depends on how charge in which the charge is mostly local- quickly it reacts or converts to the more ized on a carbon atom. There are two stable state. types. Carbonium ions have five bonds to The electronic state of the carbene – the carbon atom and a complete outer shell whether it is singlet or triplet – may have of 8 electrons. The simplest example would important consequences for how it reacts. + be the carbonium ion CH5 , which could For example, a singlet carbene adding to a be regarded as formed by adding H+ to double bond to produce a cyclopropane methane, CH4, in the same way that the + ring does so in one stage, so any stereo- ammonium ion, NH4 , is formed from am- chemistry of the double bond is preserved monia, NH3. There is, however, a differ- in the product. If the carbene has a triplet ence between ammonia and methane in state it acts as a diradical, and the reaction that ammonia has a lone pair of electrons, + proceeds in two stages. Any stereochem- which it can donate in forming the NH4 + istry about the double bond will not be ion. The carbonium ion CH5 (and similar preserved in the final product ions) is a transient species, produced in the gas phase by electron bombardment of or- carbenium ion See carbocation. ganic compounds and detected in a mass

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carbocyclic compound

spectrum. Its shape is that of a carbon sources; i.e., carbonates (chalk and lime- atom with three hydrogens in a plane and stone) and fossil fuels (coal, oil, and gas). It one hydrogen above and one below (a trig- also occurs in the mineral dolomite. The el- onal bipyramid). ement forms only 0.032% by mass of the Carbenium ions have three bonds to the Earth’s crust. Minute quantities of elemen- central carbon and are planar, with the tal carbon also occur as the allotropes bonds directed toward the corners of a tri- graphite and diamond. A third allotrope, 2 angle (sp hybridization). They have six BUCKMINSTERFULLERENE (C60), also exists. electrons in the outer shell of carbon and a Naturally occurring carbon has the iso- vacant p orbital. Carbenium ions are im- topic composition 12C (98.89%), 13C portant intermediates in a number of (1.11%) and 14C (minute traces in the organic reactions, notably the SN1 mecha- upper atmosphere produced by slow neu- nism of NUCLEOPHILIC SUBSTITUTION. It is tron capture by 14N atoms). 14C is used for possible to produce stable carbenium ions radiocarbon dating because of its long + – in salts of the type (C6H5)3C Cl , which are half-life of 5730 years. orange-red solids. In these the triphenyl- methyl cation is stabilized by delocaliza- carbonate A salt of carbonic acid (con- 2– tion over the three phenyl groups. It is also taining the ion CO3 ). possible to produce carbenium ions using SUPERACIDS. carbonation 1. The solution of carbon dioxide in a liquid under pressure, as in carbocyclic compound A compound, carbonated soft drinks. such as benzene or cyclohexane, that con- 2. The addition of carbon dioxide to com- tains a ring of carbon atoms in its struc- pounds, e.g. the insertion of carbon diox- ture. ide into Grignard reagents.

carbohydrate Any of a class of com- carbon black A finely divided form of pounds occurring widely in nature and carbon produced by the incomplete com- having the general formula Cx(H2O)y. bustion of such hydrocarbon fuels as nat- (Note that although the name suggests a ural gas or petroleum oil. It is used as a hydrate of carbon these compounds are in black pigment in inks and as a filler for no way hydrates and have no similarities to rubber in tire manufacture. classes of hydrates.) Carbohydrates are generally divided into two main classes: carbon cycle The circulation of carbon SUGARS and POLYSACCHARIDES. compounds in the environment, one of the Carbohydrates are both stores of en- major natural cycles of an element. Carbon ergy and structural elements in living sys- dioxide in the air is used by green plants in tems; plants having typically 15% photosynthesis (in which it is combined carbohydrate and animals about 1% car- with water to form sugars and starches). bohydrate. The body is able to build up Plants are eaten by animals which exhale polysaccharides from simple units (an- carbon dioxide, or when plants and ani- abolism) or break the larger units down to mals die their remains decompose with the more simple units for releasing energy (ca- production of carbon dioxide. Some plants tabolism). are burned or converted to fossil fuels which are burned, again with the forma- carbolic acid A former name for phenol tion of carbon dioxide. (hydroxybenzene; C6H5OH). carbon dating (radiocarbon dating) A carbon The first element of group 14 method of dating – measuring the age of (formerly IVA) of the periodic table. Car- (usually archaeological) materials that con- bon is a universal constituent of living mat- tain matter of living origin. It is based on ter and the principal deposits of carbon the fact that 14C, a beta emitter of half-life compounds are derived from living approximately 5730 years, is being formed

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carbonize

continuously in the atmosphere as a result bustion of fossil fuels are thought to con- of cosmic-ray action. The 14C becomes in- tribute to the greenhouse effect. corporated into living organisms. After Carbon dioxide is the anhydride of the death of the organism the amount of radio- weak acid carbonic acid, which is formed active carbon decreases exponentially by in water: 12 14 ˆ radioactive decay. The ratio of C to C CO2 + H2O H2CO3 is thus a measure of the time elapsed since the death of the organic material. The carbon disulfide (CS2) A colorless poi- method is most valuable for specimens of sonous flammable liquid made from up to 20 000 years old, though it has been methane (natural gas) and sulfur. The pure modified to measure ages up to 70 000 compound is virtually odorless, but CS2 years. For ages of up to about 8000 years usually has a revolting smell because of the the carbon time scale has been calibrated presence of other sulfur compounds. It is by dendrochronology; i.e. by measuring used as a solvent and in the production of the 12C:14C ratio in tree rings of known xanthates in making viscose rayon. age. carbon fibers Fibers of graphite, which carbon dioxide (CO2) A colorless are used, for instance, to strengthen poly- odorless nonflammable gas formed when mers. They are made by heating stretched carbon burns in excess oxygen. It is also textile fibers and have an orientated crystal produced by respiration. Carbon dioxide is structure. present in the atmosphere (0.03% by volume) and is converted in plants to car- carbonic acid (H2CO3) A dibasic acid bohydrates by photosynthesis. In the la- formed in small amounts in solution when boratory it is made by the action of dilute carbon dioxide dissolves in water: ˆ acid on metal carbonates. Industrially, it is CO2 + H2O H2CO2 obtained as a by-product in certain pro- It forms two series of salts: hydrogencar- – 2– cesses, such as fermentation or the manu- bonates (HCO3 ) and carbonates (CO3 ). facture of lime. The main uses are as a The pure acid cannot be isolated. refrigerant (solid carbon dioxide, called dry ice) and in fire extinguishers and car- carbonium ion See carbocation. bonated drinks. Increased levels of carbon dioxide in the atmosphere from the com- carbonize (carburize) To convert an or-

CO2 free carbon respiration dioxide in the and decay atmosphere

organic combustion compounds respiration in animals photosynthesis and decay

fossil fuels feeding (coal and peat) organic compounds in green plants

Carbon cycle

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carbon monoxide _ O O

R C R C _ O O

O _ R C

Carboxylate ion

ganic compound into carbon by incom- occurs in aldehydes (RCO.H), ketones plete oxidation at high temperature. (RR′CO), carboxylic acids (RCO.OH), and in carbonyl complexes of transition carbon monoxide (CO) A colorless metals. The group is polar, with negative flammable toxic gas formed by the incom- charge on the oxygen. plete combustion of carbon. In the laboratory it can be made by dehydrating carboxyhemoglobin A complex formed methanoic acid with concentrated sulfuric when carbon monoxide coordinates to the acid: iron atom in hemoglobin molecules. The → HCOOH – H2O CO product is very stable and hemoglobin has Industrially, it is produced by the oxida- a much greater affinity for carbon monox- tion of carbon or of natural gas, or by the ide than for oxygen. The toxic effect of car- water-gas reaction. It is a powerful reduc- bon monoxide is due to its ability to block ing agent and is used in metallurgy. hemoglobin as an oxygen carrier. Carbon monoxide is neutral and only sparingly soluble in water. It is not the an- carboxylate ion The ion –COO–, pro- hydride of methanoic acid, although under duced by ionization of a carboxyl group. In extreme conditions it can react with a carboxylate ion the negative charge is sodium hydroxide to form sodium generally delocalized over the O–C–O methanoate. It forms metal carbonyls with grouping and the two C–O bonds have the transition metals, and its toxicity is due to same length, intermediate between that of its ability to form a complex with hemo- a double C=O and a single C–O. globin (in preference to oxygen). carboxyl group The organic group carbon tetrachloride See tetrachloro- –CO.OH, present in carboxylic acids. methane. carboxylic acid A type of organic com- carbonyl A complex in which carbon pound containing the CARBOXYL GROUP. monoxide ligands are coordinated to a Simple carboxylic acids have the general metal atom. A common example is tetra- formula RCOOH. Many carboxylic acids carbonyl nickel(0), Ni(CO)4. occur naturally in plants and (in the form of esters) in fats and oils, hence the alter- carbonyl chloride (phosgene; COCl2) native name fatty acids. Carboxylic acids A colorless toxic gas with a choking smell. with one COOH group are monobasic, It is used as a chlorinating agent and to those with two, dibasic, and those with make polyurethane plastics and insecti- three, tribasic. The methods of preparation cides; it was formerly employed as a war are: gas. 1. Oxidation of a primary alcohol or an aldehyde: → carbonyl group The group –C=O. It RCH2OH + 2[O] RCOOH + H2O 42 iranchembook.ir/edu

Carothers, Wallace Hume

2. Hydrolysis of a nitrile using dilute hy- ume, and temperature, and transfers en- drochloric acid: ergy to or from mechanical work. The effi- → RCN + HCl + 2H2O RCOOH + ciency of the Carnot cycle is the maximum NH4Cl attainable in a heat engine. See Carnot’s The acidic properties of carboxylic acids principle. are due to the carbonyl group, which attracts electrons from the C–O and O–H Carnot’s principle (Carnot theorem) bonds. The CARBOXYLATE ION formed, The principle that efficiency of any heat en- R–COO–, is also stabilized by delocaliza- gine cannot be greater than that of a re- tion of electrons over the O–C–O group- versible heat engine operating over the ing. same temperature range. It follows directly Other reactions of carboxylic acids in- from the second law of thermodynamics, clude the formation of ESTERS and the reac- and means that all reversible heat engines tion with phosphorus(V) chloride to form have the same efficiency, independent of ACYL HALIDES. the working substance. If heat is absorbed at temperature T1 and given out at T2, then carburize See carbonize. the Carnot efficiency is (T1–T2)/T1.

carbylamine reaction See isocyanide Carnot theorem See Carnot’s principle. test. carotene A carotenoid pigment, exam- carcinogen Any substance that causes ples being lycopene and α- and β-carotene. living tissues to become cancerous. Chemi- The latter compounds are important in an- cal carcinogens include many organic com- imal diets as a precursor of vitamin A. See pounds, e.g. hydrocarbons in tobacco carotenoids; photosynthetic pigments. smoke, as well as inorganic ones, e.g. as- bestos. Carcinogenic physical agents include carotenoid Any of a group of yellow, ultraviolet light, x-rays, and radioactive orange, or red pigments comprising the materials. Some viruses (e.g. hepatitis B) CAROTENES and XANTHOPHYLLS. They are are also carcinogens. Many carcinogens found in all photosynthetic organisms, are mutagenic, i.e. they cause changes in where they function mainly as accessory the DNA; dimethylnitrosamine, for exam- pigments in photosynthesis, and in some ple, methylates the bases in DNA. A poten- animal structures, e.g. feathers. They con- tial carcinogen may therefore be identified tribute, with anthocyanins, to the autumn by determining whether it causes muta- colors of leaves since the green pigment tions. chlorophyll, which normally masks the carotenoids, breaks down first. They are Carius method A method in quantita- also found in some flowers and fruits, e.g. tive analysis for determining the amounts tomato. Carotenoids have three absorption of halogens, phosphorus, and sulfur in or- peaks in the blue-violet region of the spec- ganic compounds. The compound is trum. heated with concentrated nitric acid and Carotenes are hydrocarbons. The most silver nitrate in a sealed tube. The silver widespread is β-carotene. This is the or- compounds produced are separated and ange pigment of carrots whose molecule is weighed. split into two identical portions to yield vitamin A during digestion in vertebrates. Carnot cycle The idealized reversible Xanthophylls resemble carotenes but con- cycle of four operations occurring in a per- tain oxygen. See also photosynthetic pig- fect heat engine. These are the successive ments. adiabatic compression, isothermal expansion, adiabatic expansion, and isothermal Carothers, Wallace Hume (1896–1937) compression of the working substance. American organic chemist. Carothers is The cycle returns to its initial pressure, vol- best known for having discovered nylon.

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carrier gas

He also produced neoprene, a synthetic ple, finely divided nickel (a solid) will cat- rubber, in 1931. He did so by treating viny- alyze the hydrogenation of oil (liquid). lacetylene with hydrochloric acid. This In increasing a reaction rate a catalyst produced the monomer chlorobutadiene provides a new pathway for which the rate- which readily polymerizes to give the poly- determining step has a lower activation en- mer neoprene. In the search to find artifical ergy than in the uncatalyzed reaction. A versions of silk and cellulose he used many catalyst does not change the products in an types of condensation polymers. In 1935 equilibrium reaction and their concentra- he discovered the polyamide usually tion is identical to that in the uncatalyzed known as nylon by the condensation of reaction; i.e. the position of the equilib- adipic acid and hexamethylenediamine. rium remains unchanged. The catalyst sim- Carothers suffered from depression and ply increases the rate at which equilibrium committed suicide in 1937. He therefore is attained. did not live to see the commercial produc- In autocatalysis, one of the products of tion of nylon in 1940. the reaction itself acts as a catalyst. In this type of reaction the reaction rate increases carrier gas The gas used to carry the with time to a maximum and finally slows sample in GAS CHROMATOGRAPHY. down. For example, in the hydrolysis of ethyl ethanoate, the ethanoic acid pro- casein A phosphorus-containing protein duced catalyzes the reaction. that occurs in milk and cheese. It is easily digested by young mammals and is their catalytic converter A device fitted to major source of protein and phosphorus. the exhaust system of gasoline-fuelled vehi- The protein has been used for making cer- cles to remove pollutant gases from the ex- tain items, such as billiard balls and but- haust. It consists of a honeycomb structure tons, and has also been used to make glue. (to provide maximum area) coated with These uses have declined because of com- platinum, palladium, and rhodium cata- petition from synthetic polymers. lysts. Such devices can convert carbon catabolism All the metabolic reactions monoxide to carbon dioxide, oxides of ni- that break down complex molecules to trogen to nitrogen, and unburned fuel to simpler compounds. The function of cata- carbon dioxide and water. bolic reactions is to provide energy. See also metabolism. catalytic cracking The conversion, using a catalyst, of long-chain hydrocar- catalyst A substance that alters the rate bons from the refining of petroleum into of a chemical reaction without itself being more useful shorter-chain compounds such changed chemically in the reaction. The as those occurring in kerosene and gaso- catalyst can, however, undergo physical line. change; for example, large lumps of catalyst can, without loss in mass, be converted catalytic reaction A chemical reaction into a powder. Small amounts of catalyst that occurs at a measurable rate only in the are often sufficient to increase the rate of presence of a catalyst. reaction considerably. A positive catalyst increases the rate of a reaction and a nega- catechol (1,2-dihydroxybenzene) A tive catalyst reduces it. Homogeneous cat- colourless crystalline PHENOL. It is used in alysts are those that act in the same phase photographic developing. as the reactants (i.e. in gaseous and liquid systems). For example, nitrogen(II) oxide catecholamine Any of a group of im- gas will catalyze the reaction between sul- portant amines that contain a catechol ring fur(IV) oxide and oxygen in the gaseous in their molecules. They are neurotransmit- phase. Heterogeneous catalysts act in a dif- ters and hormones. Examples are epineph- ferent phase from the reactants. For exam- rine and norepinephrine.

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centrifugal pump

catenation The formation of chains of cellulose acetate (cellulose ethanoate) A atoms in molecules. polymeric substance made by acetylating cellulose using a mixture of ethanoic acid, cathode In electrolysis, the electrode ethanoic anhydride, and sulfuric acid. It is that is at a negative potential with respect used in plastics, in acetate film, and in to the anode. In any electrical system, such acetate rayon. as a discharge tube or electronic device, the cathode is the terminal at which electrons cellulose ethanoate See cellulose ac- enter the system. etate.

cation A positively charged ion, formed cellulose trinitrate (guncotton; nitrocel- by removal of electrons from atoms or lulose) A highly flammable substance molecules. In electrolysis, cations are at- made by treating cellulose with a nitric– tracted to the negatively charged electrode sulfuric acid mixture. Cellulose trinitrate is (the cathode). Compare anion. used in explosives and in lacquers. It is an ester of nitric acid (i.e. not a true nitro cationic detergent See detergent. compound).

cationic resin An ION-EXCHANGE ma- Celsius scale A temperature scale in terial that can exchange cations, such as H+ + which the temperature of melting pure ice and Na , for ions in the surrounding is taken as 0° and the temperature of boil- medium. Such resins are used for a wide ing water 100° (both at standard pressure). range of purification and analytical pur- The degree Celsius (°C) is equal to the poses. kelvin. This was known as the centigrade They are often produced by adding a scale until 1948, when the present name sulfonic acid group (–SO –H+) or a car- 3 became official. It is named for the Swedish boxylate group (–COO–H+) to a stable astronomer Anders Celsius (1701–44). polyphenylethene resin. A typical exchange Celsius’ original scale (1742) was inverted reaction is: – + – + (i.e. had 0° as the steam temperature and resin–SO3 H + NaCl = resin–SO3 Na ° + HCl 100 as the ice temperature). See also tem- They have been used to great effect to sep- perature scale. arate mixtures of cations of similar size –2 having the same charge. Such mixtures can centi- Symbol: c A prefix denoting 10 . –2 be attached to cationic resins and progres- For example, 1 centimeter (cm) = 10 sive elution will recover them in order of meter (m). decreasing ionic radius. centigrade scale See Celsius scale. cell A system having two plates (electrodes) in a conducting liquid (electrolyte). centrifugal pump A device commonly An electrolytic cell is used for producing a used for transporting fluids around a chemical reaction by passing a current chemical plant. Centrifugal pumps usually through the electrolyte (i.e. by electrolysis). have a set of blades rotating inside a fixed A voltaic (or galvanic) cell produces an circular casing. As the blades rotate, the e.m.f. by chemical reactions at each elec- fluid is impelled out of the pump along a trode. Electrons are transferred to or from pipe. Centrifugal pumps do not produce the electrodes, giving each a net charge. high pressures but they have the advantage of being relatively cheap because they are

cellulose A POLYSACCHARIDE (C6H10O5)n simple in design, have no valves, and work of glucose, which is the main constituent of at high speeds. In addition they are not the cell walls of plants. It is obtained from damaged if a blockage develops. Compare wood pulp. displacement pump.

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centrifuge

centrifuge An apparatus for rotating a charcoal An amorphous form of carbon container at high speeds, used to increase made by heating wood or other organic the rate of sedimentation of suspensions or material in the absence of air. Activated the separation of two immiscible liquids. charcoal is charcoal heated to drive off ab- See also ultracentrifuge. sorbed gas. It is used for absorbing gases and for removing impurities from liquids. CFC Chlorofluorocarbon. See halocarbon. Chardonnet, Louis-Marie-Hilaire Bernigaud, Comte de (1839–1924) c.g.s. system A system of units that uses French organic chemist. Chardonnet is best the centimeter, the gram, and the second as known for inventing rayon. This was the the base mechanical units. Much early sci- first type of artificial silk to be produced. In 1884 he produced nitrocellulose (guncot- entific work used this system, but it has ton) by treating a pulp made from mul- now almost been abandoned in favor of SI berry leaves with a mixture of nitric acid UNITS. and sulphuric acid. The resulting cellulose compound was dissolved in a mixture of chain When two or more atoms form alcohol and ether, with this solution then bonds with each other in a molecule, a being forced into cold water through capil- chain of atoms results. This chain may be a lary tubes. Nitrocellulose was very inflam- straight chain, in which each atom is added mable, so Chardonnet sought a non- to the end of the chain, or it may be a inflammable fibre. This culminated in the branched chain, in which the main chain of development of rayon in 1889. atoms has one or more smaller side chains branching off it. Charles’ law For a given mass of gas at constant pressure, the volume increases by chain reaction A self-sustaining chemi- a constant fraction of the volume at 0°C cal reaction consisting of a series of steps, for each Celsius degree rise in temperature. each of which is initiated by the one before The constant fraction (α) has almost the it. An example is the reaction between hy- same value for all gases – about 1/273 – drogen and chlorine: and Charles’ law can be written in the form → α θ Cl2 2Cl• V = V0(1 + v ) → where V is the volume at temperature θ°C H2 + Cl• HCl + H• → and V the volume at 0°C. The constant α H• + Cl2 HCl + Cl• 0 v → is the thermal expansivity of the gas. For an 2H• H2 → ideal gas its value is 1/273.15. 2Cl• Cl2 The first stage, chain initiation, is the dis- A similar relationship exists for the pressure of a gas heated at constant vol- sociation of chlorine molecules into atoms; ume: this is followed by two chain propagation α θ p = p0(1 + p ) reactions. Two molecules of hydrogen α Here, p is the pressure coefficient. For an chloride are produced and the ejected chlo- ideal gas rine atom is ready to react with more hy- α α p = v drogen. The final steps, chain termination, although they differ slightly for real gases. stop the reaction. Chain reactions are im- It follows from Charles’ law that for a gas portant in certain types of free-radical heated at constant pressure, POLYMERIZATION reactions. V/T = K where T is the thermodynamic temperature chair conformation See cyclohexane. and K is a constant. Similarly, at constant volume, p/T is a constant. chalcogens The elements of group 16 of Charles’ volume law is sometimes the periodic table: oxygen, sulfur, sele- called Gay-Lussac’s law after its indepen- nium, tellurium, and polonium. dent discoverer.

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chemiluminescence

chelate A metal coordination complex depends on the fact that, in living organ- in which one ligand coordinates at two or isms, amino acids are optically active. more points to the same metal ion. The re- After death a slow racemization reaction sulting complex contains rings of atoms occurs and a mixture of L- and D-isomers that include the metal atom. An example of forms. The age of bones can be accurately a chelating agent is 1,2-diaminoethane determined by measuring the relative (H2NCH2CH2NH2), which can coordinate amounts of L- and D-amino acids present. both its amine groups to the same atom. It is an example of a bidentate ligand (having chemical engineering The branch of two ‘teeth’). EDTA, which can form up to engineering concerned with the design and six bonds, is another example of a chelat- maintenance of a chemical plant and its ing agent. The word chelate comes from ability to withstand extremes of tempera- the Greek word meaning ‘claw’. ture and pressure, corrosion, and wear. It enables laboratory processes producing chemical bond A link between atoms grams of material to be converted into a that leads to an aggregate of sufficient sta- large-scale plant producing tonnes of ma- bility to be regarded as an independent mo- terial. Chemical engineers plan large-scale lecular species. Chemical bonds include chemical processes by linking together the covalent bonds, electrovalent (ionic) appropriate unit processes and by studying bonds, coordinate bonds, and metallic such parameters as heat and mass transfer, bonds. Hydrogen bonds and van der Waals separations, and distillations. forces are not usually regarded as true chemical bonds. chemical equation A method of representing a chemical reaction using chemical chemical combination, laws of A formulae. The formulae of the reactants group of chemical laws developed during are given on the left-hand side of the equa- the late 18th and early 19th centuries, tion, with the formulae of the products on which arose from the recognition of the im- the right. The two halves are separated by portance of quantitative (as opposed to qualitative) study of chemical reactions. a directional arrow or arrows (or an equals The laws are: sign). A number preceding a formula 1. the law of conservation of mass (matter); (called a stoichiometric coefficient) indi- 2. the law of constant (definite) propor- cates the number of molecules of that sub- tions; stance involved. The equation must 3. the law of multiple proportions; balance – that is, the number of atoms of 4. the law of equivalent (or reciprocal) pro- any one element must be the same on both portions, sides of the equation. These laws played a significant part in Dal- ton’s development of his atomic theory chemical equilbrium See equilibrium. (1808). chemical formula See formula. chemical conversions See unit processes. chemical reaction A process in which chemical dating A method of using one or more elements or chemical com- chemical analysis to find the age of an ar- pounds (the reactants) react to produce a chaeological specimen in which composi- different substance or substances (the tional changes have taken place over time. products). For example, the determination of the amount of fluorine in bone that has been chemical shift See nuclear magnetic res- buried gives an indication of its age be- onance. cause phosphate in the bone has gradually been replaced by fluoride ions from chemiluminescence The emission of groundwater. Another dating technique light during a chemical reaction.

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chemisorption

chemisorption See adsorption. chloral See trichloroethanal.

chiral Having the property of CHIRALITY. chloral hydrate See trichloroethanal. For example, lactic acid is a chiral compound because it has two possible struc- chloramine (NH2Cl) A colorless liquid tures that cannot be superposed. See made by reacting ammonia with sodium optical activity. chlorate(I) (NaOCl). It is formed as an intermediate in the production of hydrazine. chirality The property of existing in left- Chloramine is unstable and changes explo- and right-handed forms; i.e. forms that are sively into ammonium chloride and nitro- not superposable. In chemistry the term is gen trichloride. applied to the existence of optical isomers. See optical activity. chloride See halide.

chirality element A part of a molecule chlorination 1. Treatment with chlo- that causes it to display chirality. The most rine; for instance, the use of chlorine to dis- common type of element is a chirality cen- infect water. ter, which is an atom attached to four dif- 2. See halogenation. ferent atoms or groups. This is also referred to as an asymmetric atom. Less chlorine A green reactive gaseous el- commonly a molecule may have a chirality ement belonging to the halogens; i.e. group 17 (formerly VIIA) of the periodic table. It axis, as in the case of certain substituted al- occurs in sea-water, salt lakes, and under- lenes of the type R R C=C=CR R . In this 1 2 3 4 ground deposits of halite, NaCl. It ac- form of compound the R and R groups 1 2 counts for about 0.055% of the Earth’s do not lie in the same plane as the R and 3 crust. Chlorine is strongly oxidizing and R groups because of the nature of the dou- 4 can be liberated from its salts only by ble bonds. The chirality axis lies along the strong oxidizing agents, such as man- C=C=C chain. It is also possible to have ganese(IV) oxide, potassium perman- molecules that contain a chirality plane. ganate(VII), or potassium dichromate; See optical activity. note that sulfuric acid is not sufficiently oxidizing to release chlorine from chlorides. chitin A nitrogen-containing heteropoly- Industrially, chlorine is prepared by the saccharide found in some animals and the electrolysis of brine and in some processes cell walls of most fungi. It is a polymer of chlorine is recovered by the high-tempera- N-acetylglucosamine. It consists of many ture oxidation of waste hydrochloric acid. glucose units, in each of which one of the Chlorine is used in large quantities, both as hydroxyl groups has been replaced by an the element, to produce chlorinated or- acetylamine group (CH3CONH). The ganic solvents, and for the production of outer covering of arthropods, the cuticle, is polyvinyl chloride (PVC), the major ther- impregnated in its outer layers with chitin, moplastic in use today, and in the form of which makes the exoskeleton more rigid. It hypochlorites for bleaching. is associated with protein to give a The solubility of inorganic metal chlo- uniquely tough yet flexible and light skel- rides is such that they are not an environ- eton, which also has the advantage of being mental problem unless the metal ion itself waterproof. The chitinous plates are thin- is toxic but many organochlorine com- ner for bending and flexibility or thicker pounds are sufficiently stable for the accu- for stiffness as required. The plates cannot mulated residues of chlorine-containing grow once laid down and are broken down pesticides to present a severe problem in at each molt. Chitin is also found in the some areas. This arises because they can hard parts of several other groups of ani- accumulate in food chains and concentrate mals. in the tissues of higher animals (see DDT).

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chromatography

Symbol: Cl; m.p. –100.38°C; b.p. chloroform See trichloromethane. –33.97°C; d. 3.214 kg m–3 (0°C); p.n. 17; r.a.m. 35.4527. chloromethane (methyl chloride; CH3Cl) A colorless flammable haloalkane gas chloroacetic acid See chloroethanoic made by chlorination of methane. It is used acid. as a refrigerant and a local anesthetic.

chlorobenzene (monochlorobenzene; chlorophyll A pigment present in plants C6H5Cl) A colorless liquid made by the that acts as a catalyst in the photosynthesis catalytic reaction of chlorine with benzene of carbohydrates from carbon dioxide and or by the RASHIG PROCESS. It can be con- water. There are four types, known as verted to phenol by reaction with sodium chlorophylls a, b, c, and d. The chloro- hydroxide under extreme conditions phylls are PORPHYRINS containing magne- (300°C and 200 atmospheres pressure). It sium. is also used in the manufacture of other organic compounds. chloroprene (CH2:CClCH:CH2) A colorless conjugated diene used in the manufacture of synthetic chlorinated rubbers chloroethane (ethyl chloride; C2H5Cl) A gaseous compound made by the addition (such as neoprene). The systematic name is of hydrogen chloride to ethene. It is used as 2–chlorobuta-1,3–diene. a refrigerant and a local anesthetic. cholecalciferol See vitamin D. chloroethanoic acid (chloroacetic acid; CH ClCOOH) A colorless crystalline choline An amino alcohol often classi- 2 fied as a member of the vitamin B complex. solid made by substituting one of the hy- It can be synthesized in humans from drogen atoms of the methyl group of lecithin by putrefaction in the bowel, but is ethanoic acid with chlorine, using red required as an essential nutrient for some phosphorus. It is a stronger acid than animals and microorganisms. It acts to dis- ethanoic acid because of the electron-with- perse fat from the liver or prevent its excess drawing effect of the chlorine atom. accumulation. Its ester acetylcholine func- Dichloroethanoic acid (dichloroacetic tions in the transmission of nerve impulses. acid, CHCl2COOH) and trichloroethanoic acid (trichloroacetic acid, CCl3COOH) are chromatography A technique used to made in the same way. The acid strength separate or analyze complex mixtures. A increases with the number of chlorine number of related techniques exist; all de- atoms present. pend on two phases: a mobile phase, which may be a liquid or a gas, and a stationary chloroethene (vinyl chloride; H2C:CHCl) phase, which is either a solid or a liquid A gaseous organic compound used in held by a solid. The sample to be separated the manufacture of PVC (polyvinyl chlo- or analyzed is carried by the mobile phase ride). Chloroethene is manufactured by the through the stationary phase. Different reaction between ethyne and hydrogen components of the mixture are absorbed or chloride using a mercury(II) chloride cata- dissolved to different extents by the sta- lyst: tionary phase, and consequently move → C2H2 + HCl H2C:CHCl along at different rates. In this way the An alternative source, making use of the components are separated. There are many ready supply of ethene, is via dichloro- different forms of chromatography de- ethane: pending on the phases used and the nature → → H2C:CH2+ Cl2 CH2Cl.CH2Cl of the partition process between mobile H2C:CHCl and stationary phases. The main classification is into column chromatography and chlorofluorocarbon See halocarbon. planar chromatography.

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chromophore

A simple example of column chro- number taking precedence. So in matography is in the separation of liquid C(NH2)(NO2)(CH3)(C2H6 ) the order is > > > mixtures. A vertical column is packed with NO2 NH2 C2H6 CH3. The system is an absorbent material, such as alumina named after the British chemists Robert (aluminum oxide) or silica gel. The sample Cahn (1899–1981) and Sir Christopher In- is introduced into the top of the column gold (1893–1970) and the Bosnian–Swiss and washed down it using a solvent. This chemist Vladimir Prelog (1906– ). process is known as elution; the solvent used is the eluent and the sample being sep- cis- Designating an isomer with groups arated is the eluate. If the components are that are adjacent. See isomerism. colored, visible bands appear down the column as the sample separates out. The cis-trans isomerism See isomerism. components are separated as they emerge from the bottom of the column. In this par- citric acid A white crystalline dibasic ticular example of chromatography the carboxylic acid important in plant and an- partition process is adsorption on the par- imal cells. It is present in many fruits, espe- ticles of alumina or silica gel. Column cially citrus fruits. The systematic name is chromatography can also be applied to 2-hydroxypropane-1,2,3-tricarboxylic mixtures of gases. See gas chromatogra- acid. The formula is: phy. In the other main type of chromatog- HOOCCH2C(OH)(COOH)CH2COOH raphy, planar chromatography, the stationary phase is a flat sheet of absorbent Claisen condensation A reaction in material. See paper chromatography; thin- which two molecules of ester combine to layer chromatography. give a keto-ester – a compound containing Components of the mixture are held a ketone group and an ester group. The re- back by the stationary phase either by ad- action is base-catalyzed by sodium ethox- sorption (e.g. on the surface of alumina) or ide; the reaction of ethyl ethanoate because they dissolve in it (e.g. in the mois- refluxed with sodium ethoxide gives: → ture within chromatography paper). 2CH3.CO.OC2H5 CH3.CO.CH2.CO.OC2H5 + C2H5OH chromophore A group of atoms in a The mechanism is similar to that of the molecule that is responsible for the color of ALDOL REACTION, the first step being for- the compound. Usually a chromophore is a mation of a carbanion from the ester: – → group of atoms having delocalized elec- CH3COC2H5 + OC2H5 – trons. CH2COC2H5 + C2H5OH This attacks the carbon atom of the car- cinnamic acid See 3-phenylpropenoic bonyl group on the other ester molecule, acid. forming an intermediate anion that decomposes to the keto-ester and the ethanoate CIP system (Cahn–Ingold–Prelog system) ion. It is named for the German organic A method of producing a sequence rule chemist Ludwig Claisen (1851–1930) who used in the absolute description of described it in 1890. stereoisomers in the R–S convention (see optical activity) or the E-Z CONVENTION. clathrate (enclosure compound) A sub- The rule is to consider the atoms that are stance in which small (guest) molecules are bound directly to a chiral center (or to a trapped within the lattice of a crystalline double bond). The group in which this (host) compound. Clathrates are formed atom has the highest proton number has when suitable host compounds are crystal- the highest priority. So, for example, in lized in the presence of molecules of the ap- HCClBr(NH2), the order of priority is propriate size. Although the term ‘clathrate > > > Br Cl NH2 H. If two atoms are the compound’ is often used, they are not true same, the substituents are considered, compounds; no chemical bonds are with the substituents of highest proton formed, and the guest molecules interact

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collagen

by weak van der Waals forces. The phine and is used as an analgesic. It is a clathrate is maintained by the cagelike lat- controlled substance in the USA. tice of the host. The host lattice must be broken down, for example, by heating or coenzyme Any of a group of molecules dissolution in order to release the guest. (which are small compared to the size of an This should be compared with zeolites, in enzyme) that enable enzymes to carry out which the holes in the host lattice are large their catalytic activity. Examples include enough to permit entrance or emergence of nicotinamide adenine dinucleotide (NAD) the guest without breaking bonds in the and ubiquinone (coenzyme Q). Some co- host lattice. Quinol forms many clathrates, enzymes are capable of catalyzing reactions in the absence of an enzyme but the e.g. with SO2; water (ice) forms a clathrate with xenon. rate of reaction is never as high as when a catalyst is present. A coenzyme is not a true CoA See coenzyme A. catalyst because it undergoes chemical change during the reaction. coagulation The association of particles (e.g. in colloids) into clusters. See floccula- coenzyme A (CoA) A complex nu- tion. cleotide containing an active –SH group that is readily acetylated to CoAS–COCH3 coal A black mineral that consists (acetyl CoA). Acetyl CoA is the source of mainly of carbon, used as a fuel and as a the two-carbon units that feed into the source of organic chemicals. It is the fos- KREBS CYCLE. It is produced from glycolysis and the breakdown of fatty acids and some silized remains of plants that grew in the amino acids. It is also a key intermediate in Carboniferous and Permian periods and the biosynthesis of lipids and other ana- were buried and subjected to high pres- bolic reactions. sures underground. There are various types of coal, classified according to their in- coenzyme Q See ubiquinone. creasing carbon content. cofactor A nonprotein substance that A fuel gas made by heating coal coal gas helps an enzyme to carry out its activity. in a limited supply of air. It consists mainly Cofactors may be cations or organic mol- of hydrogen and methane, with some car- ecules, known as coenzymes. Unlike en- bon monoxide (which makes the gas highly zymes they are, in general, stable to heat. poisonous). Coal tar and coke are formed When a catalytically active enzyme forms a as by-products. Coal gas was a major fuel complex with a cofactor a holoenzyme is in some countries in the 19th and early produced. An enzyme without its cofactor 20th century. See coal tar. is termed an apoenzyme.

coal tar Tar produced by heating coal in coherent units A system or subset of the absence of oxygen. It is a mixture of units (e.g. SI units) in which the derived many organic compounds (e.g. benzene, units are obtained by multiplying or divid- toluene, and naphthalene) and also con- ing together base units, with no numerical tains free carbon. factor involved.

cocaine An alkaloid obtained from the collagen The protein of fibrous connec- dried leaves of a South American shrub tive tissues, present in bone, skin, and car- Erythroxylon coca. It is a stimulant and tilage. It is the most abundant of all the narcotic. Its use is restricted in many coun- proteins in the higher vertebrates. Collagen tries. contains about 35% glycine, 11% alanine, 12% proline and small percentages of codeine A derivative of morphine, other amino acids. The amino acid se- methylmorphine. It is less potent than mor- quence is remarkably regular with almost

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colligative properties

every third residue being glycine. Collagen Milk, rubber, and emulsion paints are typ- is chemically inert (and insoluble) which ical examples of colloids. See also sol. suggests that its reactive side groups are immobilized by ionic bonding. Collagen colorimetric analysis Quantitative fibrils are highly complex and have a vari- analysis in which the concentration of a ety of orientations depending on the bio- colored solute is measured by the intensity logical function of the particular type of of the color. The test solution can be com- connective tissue. The secondary structure pared against standard solutions. of collagen is that of a triple helix of peptide chains. Its tertiary structure is one of column chromatography See chro- three alpha helices in a ‘super helix’, which matography; gas chromatography. is responsible for its high tensile strength and therefore its role in support tissues. combustion A reaction with oxygen with the production of heat and light. The colligative properties A group of prop- combustion of solids and liquids occurs erties of solutions that depends on the when they release flammable vapor, which number of particles present, rather than reacts with oxygen in the gas phase. Com- the nature of the particles. Colligative bustion reactions usually involve a com- properties include: plex sequence of free-radical chain 1. The lowering of vapor pressure. reactions. The light is produced by excited 2. The elevation of boiling point. atoms, molecules, or ions. In highly lumi- 3. The lowering of freezing point. nous flames it comes from small incandes- cent particles of carbon. 4. Osmotic pressure. Sometimes the term is also applied to The explanation of these closely related slow reactions with oxygen, and also to re- phenomena depends on intermolecular actions with other gases (for example, cer- forces and the kinetic behavior of the par- tain metals ‘burn’ in chlorine). ticles, which is qualitatively similar to those used in deriving the kinetic theory of complex (coordination compound) A gases. type of compound in which molecules or ions form coordinate bonds with a metal collimator An arrangement for produc- atom or ion. The coordinating species ing a parallel beam of radiation for use in a (called ligands) have lone pairs of elec- spectrometer or other instrument. A sys- trons, which they can donate to the metal tem of lenses and slits is utilized. atom or ion. They are molecules such as ammonia or water, or negative ions such as colloid A heterogeneous system in Cl– or CN–. The resulting complex may be which the interfaces between phases, neutral or it may be a complex ion. For ex- though not visibly apparent, are important ample: factors in determining the system proper- 2+ → 2+ Cu + 4NH3 [Cu(NH3)4] ties. The three important attributes of col- 3+ – → 3– Fe + 6CN [Fe(CN)6] loids are: 2+ – → 4– Fe + 6CN [Fe(CN)6] 1. They contain particles, commonly made The formation of such coordination up of large numbers of molecules, form- complexes is typical of transition metals. ing the distinctive unit or disperse phase. Often the complexes contain unpaired 2. The particles are distributed in a contin- electrons and are paramagnetic and col- uous medium (the continuous phase). ored. See also chelate; sandwich com- 3. There is a stabilizing agent, which has an pound. affinity for both the particle and the medium; in many cases the stabilizer is a component One of the separate chemi- polar group. cal substances in a mixture in which no Particles in the disperse phase typically chemical reactions are taking place. For ex- have diameters in the range 10–6–10–4 mm. ample, a mixture of ice and water has one

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concentrated

Nucleophile Product Name

NH3

ammonia 1 hydroxy amine R C NH2

H2NNH2 R2 hydrazine NC NH2 hydrazone R1

C6H5NH NH2 R2 H phenylhydrazine NNC phenylhydrazone R1 C6H5

HONH 2 R2 OH hydroxlamine oxime C N R1 1 2 In the above the reactant is R COR

Condensation reaction: some reactions of aldehydes and ketones

component; a mixture of nitrogen and oxy- substance. The constituent atoms cannot gen has two components. When chemical be separated by physical means; a chemical reactions occur between the substances in a reaction is required for the compounds to mixture, the number of components is de- be formed or to be changed. The existence fined as the number of chemical substances of a compound does not necessarily imply present minus the number of equilibrium that it is stable. Many compounds have reactions taking place. Thus, the system: ˆ lifetimes of less than a second. See also N2 + 3H2 2NH3 is a two-component system. mixture.

compound A chemical combination of concentrated Denoting a solution in atoms of different elements to form a sub- which the amount of solute in the solvent is stance in which the ratio of combining relatively high. The term is always relative; atoms remains fixed and is specific to that for example, whereas concentrated sulfuric

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concentration

acid may contain 96% H2SO4, concen- nation by indicators. The operation is car- trated potassium chlorate may contain as ried out in a conductance cell, which is part little as 10% KClO3. Compare dilute. of a resistance bridge circuit. The method depends on the fact that ions have different concentration The amount of sub- ionic mobilities, H+ and OH– having stance per unit volume or mass in a solu- particularly high values. The method is es- tion. Molar concentration is amount of pecially useful for weak acid–strong base substance (in moles) per cubic decimeter and strong acid–weak base titrations for (liter). Mass concentration is mass of solute which color-change titrations are unreli- per unit volume. Molal concentration is able. amount of substance (in moles) per kilogram of solute. configuration 1. The arrangement of electrons about the nucleus of an atom. concerted reaction A reaction that Configurations are represented by sym- takes place in a single stage rather than as bols, which contain: a series of simple steps. In a concerted re- 1. An integer, which is the value of the action there is a transition state in which principal quantum number (shell num- bonds are forming and breaking at the ber). same time. An example is the SN2 mecha- 2. A lower-case letter representing the nism in NUCLEOPHILIC SUBSTITUTION. See value of the azimuthal quantum number also pericyclic reaction. (l), i.e. s means l = 0, p means l = 1, condensation The conversion of a gas d means l = 2, f means l = 3. or vapor into a liquid or solid by cooling. 3. A numerical superscript giving the number of electrons in that particular set; for condensation polymerization See example, 1s2, 2p3, 3d5. polymerization. The ground state electronic configuration (i.e. the most stable or lowest energy state) condensation reaction A reaction in may then be represented as follows, for ex- which addition of two molecules occurs ample, He, 1s2; N, 1s22s22p5. However, el- followed by elimination of a smaller mol- ements are commonly abbreviated by using ecule, usually water. Condensation reac- an inert gas to represent the ‘core’, e.g. Zr tions (addition–elimination reactions) are has the configuration [Kr]4d25s2. characteristic of ALDEHYDES and KETONES 2. The arrangement of atoms or groups in reacting with a range of nucleophiles. a molecule. There is typically nucleophilic addition at the C atom of the carbonyl group followed conformation A particular shape of by elimination of water. molecule that arises through the normal rotation of its atoms or groups about single conducting polymer A type of organic bonds. Any of the possible conformations polymer that conducts electricity like a that may be produced is called a conformer metal. Conducting polymers are crystalline (or rotamer), and there will be an infinite substances containing conjugated unsatu- number of these possibilities, differing in rated carbon–carbon bonds. In principle, the angle between certain atoms or groups they provide lighter and cheaper alterna- on adjacent carbon atoms. Sometimes, the tives to metallic conductors. term ‘conformer’ is applied more strictly to possible conformations that have mini- conductiometric titration A titration mum energies – as in the case of the boat in which measurement of the electrical and chair conformations of CYCLOHEXANE. conductance is made continuously In considering conformations about a sin- throughout the addition of the titrant and gle bond, it is convenient to consider the di- well beyond the equivalence point. This is hedral angle between a bond from one in place of traditional end-point determi- carbon atom and a bond from the other

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conjugate acid

carbon. This is the angle between the is O°. The minimum is the staggered con- bonds as viewed along the C–C bond. It is formation, with a dihedral angle of 60°. also called the torsion angle. Conforma- If different atoms are attached to the tions are also visualized using Newman carbon atoms, the conformational analysis projection diagrams. In these the molecule is more complicated. For example, the is viewed along a particular bond. The compound XH2C–CH2X, where X is an- nearer atoms is represented by a point with other group (e.g. Me), has conformers bonds drawn to this point. The further known as syn-periplanar, synclinal (or atom is represented by a large circle, with gauche), anticlinal, and anti-periplanar bonds drawn to the edge of this circle. A (see illustration). conformation in which bonds on the back Conformational analysis is also impor- atom would be hidden by bonds in front is tant in the structures of nonplanar rings. drawn slightly displaced. In the case of See cyclohexane. ethane, the maximum energy is the eclipsed conformation, in which the dihedral angle conjugate acid See acid.

H H H H H H CC H H H H H H staggered

H H H H CC H H H H H H H H

eclipsed

Conformation: the conformations of ethane resulting from rotation about the C–C bond. The diagrams on the right are Newman projections, with the molecule viewed along the C–C bond.

EE Y

Z W energy X C D

SS A B U 0 60 120 180 240 dihedral angle

Dihedral angle: the angle θ between the two Conformation: the way in which energy planes (UVWX and UVYZ) is the dihedral angle changes with dihedral angle as a result of between lines AC and BD. If AC, BD, and AB rotation about the double bond in ethane. are bonds, this angle is also known as the E indicates an eclipsed conformation and torsion angle. S indicates a staggered conformation.

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X X X H X

H H H H H H H

syn-periplanar synclinal

X X H HH

H H H X H H X

anticlinal anti-periplanar

X X H X H

X H H H H H H

anticlinal synclinal

Conformation: the conformations of a disubstituted ethane CH2XCH2X for rotation about the C–C bond.

syn-periplanar syn-periplanar

anticlinal anticlinal energy

synclinal synclinal

anti-periplanar

0 60 120 180 240 300 360 dihedral angle

Conformation: the way in which energy changes with dihedral angle in CH2XCH2X for rotation about the C–C bond.

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coordinate bond

conjugate base See acid. constant composition, law of See constant proportions; law of. conjugated Describing compounds that have alternating double and single bonds constant proportions, law of Formu- in their structure. For example, but-1,3-ene lated by Proust in 1779 after the analysis of (H2C:CHCH:CH2) is a typical conjugated a large number of compounds, the princi- compound. In such compounds there is de- ple that the proportion of each element in localization of the electrons in the double a compound is fixed or constant. It follows bonds over part of the molecule. that the composition of a pure chemical compound is independent of the method of conjugated protein A protein that on preparation. It is also called the law of def- hydrolysis yields not only amino acids but inite proportions and the law of constant also other organic and inorganic sub- composition. stances. They are simple proteins combined with nonprotein groups (prosthetic continuous phase See colloid. groups). See also glycoprotein; lipoprotein; phosphoprotein. continuous process A manufacturing process in which the raw materials are con- conservation of energy, law of Enun- stantly fed into the plant. These react as ciated by Helmholtz in 1847, this law they flow through the equipment to give a states that in all processes occurring in an continuing flow of product. At any point, isolated system the energy of the system re- only a small amount of material is at a par- mains constant. The law does of course ticular stage in the process but material at all stages of the reaction is present. The permit energy to be converted from one fractional distillation of crude oil is an ex- form to another (including mass, since en- ample of a continuous process. Such ergy and mass are equivalent). processes are relatively easy to automate and can therefore be used to manufacture a conservation of mass, law of Formu- product cheaply. The disadvantages of lated by Lavoisier in 1774, this law states continuous processing are that it usually that matter cannot be created or destroyed. caters for a large demand and the plant is Thus in a chemical reaction the total mass expensive to install and cannot normally of the products equals the total mass of the be used to make other things. Compare reactants (the term ‘mass’ must include any batch process. solids, liquids, and gases – including air – that participate). continuous spectrum A spectrum composed of a continuous range of emitted or constant-boiling mixture A general absorbed radiation. Continuous spectra observation for most liquids is that the are produced in the infrared and visible re- vapor phase above a liquid is richer in the gions by hot solids. See also spectrum. more volatile component (a deviation from Raoult’s law). Consequently most liquid coordinate bond (dative bond) A co- mixtures show a regular increase in the valent bond in which the bonding pair is boiling point as the liquid is progressively visualized as arising from the donation of a distilled. In distillation, a point is reached lone pair from one species to another at which a constant boiling mixture or species, which behaves as an electron AZEOTROPE distills over. Further attempts acceptor. The definition includes such to fractionate the distillate do not lead to a examples as the ‘donation’ of the lone pair change in composition. An example of an of the ammonia molecule to H+ (an accep- + 2+ azeotropic mixture of minimum boiling tor) to form NH4 or to Cu to form ° 2+ point is water (b.p. 100 C) and ethanol [Cu(NH3)4] . (b.p. 78.3°C), the azeotrope being 4.4% The donor groups are known as Lewis water and boiling at 78.1°C. bases and the acceptors are either hydro-

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coordination compound

gen ions or Lewis acids. Simple combina- Scottish organic chemist. Couper was the → tions, such as H3N BF3, are known as first person to recognize that carbon has a adducts. See also complex. valence of four and can combine with itself. He put forward these views in a paper coordination compound See complex. entitled On a New Chemical Theory which he wrote in 1858. He asked Charles coordination number The number of Adolphe Wurtz to present his paper to the coordinate bonds formed to a metal atom French Academy. Wurtz delayed doing so. or ion in a complex. Couper’s work on structure in organic molecules was eventually published but by copolymer See polymerization. that time August KEKULÉ had published similar ideas and claimed priority. Couper Cornforth, Sir John Warcup (1917– ) quarrelled violently with Wurtz, returned Australian organic chemist. Cornforth is to Scotland and was mentally ill for most best known for his work on the problem of of the rest of his life. how the steroid cholesterol is synthesized in a cell. To investigate this problem he coupling A chemical reaction in which used the three isotopes of hydrogen – nor- two groups or molecules join together. An mal hydrogen (H-one), deuterium (H-two) example is the formation of AZO COM- and tritium (H-three) – and observed the POUNDS. different speeds of reactions found with these isotopes to infer how cholesterol was covalent bond A bond formed by the formed. He shared the 1975 Nobel Prize sharing of an electron pair between two for chemistry with Vladimir PRELOG for atoms. The covalent bond is convention- this work. Cornforth has synthesized a ally represented as a line, thus H–Cl indi- number of other compounds including cates that between the hydrogen atom and alkenes and oxazoles. the chlorine atom there is an electron pair formed by electrons of opposite spin im- corn rule See optical activity. plying that the binding forces are strongly localized between the two atoms. Mol- coumarin (1,2–benzopyrone; C9H6O2) A ecules are combinations of atoms bound colorless crystalline compound with a together by covalent bonds; covalent bond- pleasant odor, used in making perfumes. ing energies are of the order 103 kJ mol–1. On hydrolysis with sodium hydroxide it Modern bonding theory treats the elec- forms coumarinic acid. tron pairing in terms of the interaction of electron (atomic) ORBITALS and describes coumarinic acid See coumarin. the covalent bond in terms of both ‘bonding’ and ‘anti-bonding’ molecular orbitals. coumarone See benzfuran. covalent crystal A crystal in which the Couper, Archibald Scott (1831–92) atoms present are covalently bonded. They

CH C H

CO CO2H O OH

coumarin coumarinic acid

Coumarin

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crown ether

are sometimes referred to as giant lattices critical point The conditions of temper- or macromolecules. The best-known com- ature and pressure under which a liquid pletely covalent crystal is diamond. being heated in a closed vessel becomes in- distinguishable from the gas or vapor covalent radius The radius an atom is phase. At temperatures below the critical assumed to have when involved in a cova- temperature (Tc) the substance can be lent bond. For homonuclear diatomic mol- liquefied by applying pressure; at temperatures above T this is not possible. For each ecules (e.g. Cl2) this is simply half the c measured internuclear distance. For het- substance there is one critical point; for ex- ° eroatomic molecules substitutional meth- ample, for carbon dioxide it is at 31.1 C ods are used. For example, the internuclear and 73.0 atmospheres. distance of bromine fluoride (BrF) is about critical pressure The lowest pressure 180 pm, therefore using 71 pm for the co- needed to bring about liquefaction of a gas valent radius of fluorine (from F ) we get 2 at its critical temperature. 109 pm for bromine. The accepted value is 114 pm. critical temperature The temperature below which a gas can be liquefied by ap- creosote A colorless oily liquid contain- plying pressure and above which no ing phenols and distilled from wood tar, amount of pressure is sufficient to bring used as a disinfectant. The name is also about liquefaction. Some gases have criti- given to creosote oil, a dark brown liquid cal temperatures above room temperature distilled from coal tar and used for pre- (e.g. carbon dioxide 31.1°C and chlorine serving timber. It also consists of phenols, 144°C) and have been known in the liquid mixed with some methylphenols. state for many years. Liquefaction proved much more difficult for those gases (e.g. cresol See methylphenol. oxygen –118°C and nitrogen –146°C) that have very low critical temperatures. Crick, Francis Harry Compton (1916– ) British molecular biologist. critical volume The volume of one Crick is best known for determining the mole of a substance at its critical point. structure of DNA with James WATSON in 1953. Based on a combination of model cross linkage An atom or short chain building, previous knowledge of the physi- joining two longer chains in a polymer. cal and chemical features of DNA and the x-ray diffraction photographs of Rosalind crown ether A compound that has a large ring composed of –CH –CH –O– FRANKLIN they found the famous double 2 2 helix structure. Together with Sydney units. For example, 18-crown-6 has the formula C H O (six CH CH O units). Brenner, he worked on the problem of the 12 24 6 2 2 The rings of these compounds are not pla- genetic code. Crick put forward the Cen- nar – the name comes from the shape of the tral Dogma of molecular genetics. This as- molecule. The oxygen atoms of these cyclic serts that genetic information passes from ethers can coordinate to central metal ions DNA to RNA protein. It was subsequently + or to other positive ions (e.g. NH4 ). The shown that sometimes information can crown ethers have a number of uses in flow from RNA to DNA. In his later years analysis, separation of mixtures, and as Crick worked on how the mind works. He catalysts. Cryptands are similar com- gave an account of this work in The As- pounds in which the ether chains are linked tonishing Hypothesis (1994). In 1988 his by nitrogen atoms to give a three-dimen- autobiography What Mad Pursuit was sional cage structure. They are similar in published. Crick, Watson, and Maurice action to crown ethers but generally form WILKINS won the 1962 Nobel Prize for more strongly bound complexes. See also medicine for their work on DNA. host–guest chemistry.

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crude oil

crude oil See petroleum. crystalloid A substance that is not a colloid and which will therefore not pass cryoscopic constant See depression of through a semipermeable membrane. See freezing point. colloid; semipermeable membrane.

cryptand See crown ether. crystal structure The particular repeating arrangement of atoms, molecules, or crystal A solid substance that has a def- ions in a crystal. ‘Structure’ refers to the in- inite geometric shape. A crystal has fixed ternal arrangement of particles, not the ex- angles between its faces, which have dis- ternal appearance. tinct edges. The crystal will sparkle if the faces are able to reflect light. The constant crystal system A classification of crys- angles are caused by the regular arrange- tals based on the shapes of their unit cell. If ments of particles (atoms, ions, or mol- the unit cell is a parallelopiped with lengths ecules) in the crystal. If broken, a large a, b, and c and the angles between these crystal will form smaller crystals. edges are α (between b and c), β (between In crystals, the atoms, ions, or mol- a and c), and γ (between a and b), then the ecules of the substance form a distinct reg- classification is: ular array in the solid state. The faces and cubic: a = b = c; α = β = γ = 90° their angles bear a definite relationship to tetragonal: a = b ≠ c; α = β = γ = 90° the arrangement of these particles. orthorhombic: a ≠ b ≠ c; α = β = γ = 90° hexagonal: a = b ≠ c; α = β = 90°; γ = 120° crystal habit The shape of a crystal. The trigonal: a = b ≠ c; α = β = γ≠90° habit depends on the way in which the monoclinic: a ≠ b ≠ c; α = γ = 90°≠β crystal has grown; i.e. the relative rates of triclinic: a ≠ b ≠ c; α≠β≠γ development of different faces. The orthorhombic system is also called the rhombic system. crystalline Denoting a substance that forms crystals. Crystalline substances have CS gas ((2-chlorobenzylidine-)-malanoni- a regular internal arrangement of atoms, trile; C6H4ClCH:C(CN)2) A white or- even though they may not exist as geomet- ganic compound that is a nasal irritant rically regular crystals. For instance, lead used in powder form as a tear gas for riot (and other metals) are crystalline. Such control. substances are composed of accumulations of tiny crystals. cumene process An industrial process for the manufacture of phenol from iso- crystallite A small crystal that has the propylbenzene (cumene), which is itself potential to grow larger. It is often used in made by passing benzene vapor and mineralogy to describe specimens that con- propene over a phosphoric acid catalyst tain accumulations of many minute crys- (250°C and 30 atmospheres): → tals of unknown chemical composition and C6H6 + CH2:CH(CH3) crystal structure. C6H5CH(CH3)2 The isopropylbenzene is oxidized by air to crystallization The process of forming a ‘hydroperoxide’: crystals. When a substance cools from the C6H5C(CH3)2–O–O–H gaseous or liquid state to the solid state, This is hydrolyzed by dilute acid to phenol crystallization occurs. Crystals will also (C6H5OH) and propanone (CH3COCH3), form from a solution saturated with a which is a valuable by-product. solute. curie Symbol: Ci A unit of radioactivity, crystallography The study of the for- equivalent to the amount of a given radio- mation, structure, and properties of crys- active substance that produces 3.7 × 1010 tals. See also x-ray crystallography. disintegrations per second, the number of

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cyclohexane

disintegrations produced by one gram of monophosphate (see AMP) formed from radium. ATP in a reaction catalyzed by the enzyme adenylate cyclase. It has many functions, cyanide See nitrile. acting as an enzyme activator, genetic reg- ulator, chemical attractant, secondary mes- cyanocobalamin (vitamin B12) One of senger, and as a mediator in the activity of the water-soluble B-group of vitamins. It many hormones, including epinephrine, has a complex organic ring structure at the norepinephrine, vasopressin, ACTH, and center of which is a single cobalt atom. the prostaglandins. Foods of animal origin are the only important dietary source. A deficiency in humans cyclic compound A compound con- leads to the development of pernicious an- taining a ring of atoms. If the atoms form- emia since the vitamin is required for the ing the ring are all the same the compound development of red blood cells. See also is homocyclic; if different atoms are in- vitamin B complex. volved it is heterocyclic.

ROH cyclization Any reaction in which a straight-chain compound is converted into C a cyclic compound. H CN from aldehyde cycloaddition See pericyclic reaction. 2 R OH cycloalkane A saturated cyclic hydro- C carbon comprising a ring of carbon atoms, 1 R CN each carrying two hydrogen atoms, general from ketone formula CnH2n. Cyclopropane (C3H6), and cyclobutane (C4H8) both have strained Cyanohydrin rings and are highly reactive. Other cy- cloalkanes have similar properties to the cyanohydrin An addition compound alkanes, although they are generally less reformed between an aldehyde or ketone and active than their corresponding alkane. hydrogen cyanide. The general formula is RCH(OH)(CN) (from an aldehyde) or cyclohexadiene-1,4-dione See quinone. RR′C(OH)(CN) (from a ketone). Cyano- hydrins are easily hydrolyzed to hydroxy- cyclohexane (C6H12) A colorless liquid carboxylic acids. For instance, the alkane that is commonly used as a solvent compound 2-hydroxypropanonitrile (CH3- and in the production of hexanedioic acid CH(OH)(CN)) is hydrolyzed to 2-hydroxy- (adipic acid) for the manufacture of nylon. propanoic acid (CH3CH(OH)(COOH)). Cyclohexane is manufactured by the refor- mation of longer chain hydrocarbons pre- cyclic AMP (cAMP; adenosine-3′,5′- sent in crude-oil fractions. It is also monophosphate) A form of adenosine interesting from a structural point of view,

equatorial axial (lower energy) (higher energy)

Cyclohexane: axial and equatorial positions

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cyclonite

chair

half chair

twist boat

boat

twist boat

half chair

chair

Cyclohexane: ring conformations

existing as a ‘puckered’ six-membered ring, cyclo-octatetraene See annulene. having all bonds between carbon atoms at 109.9° (the tetrahedral angle). The mol- cyclopentadiene A cyclic hydrocarbon ecule undergoes rapid interconversion be- made by cracking petroleum. The mol- tween two chair conformations, which are ecules have a five-membered ring contain- energetically equivalent, passing through a ing two carbon-carbon double bonds and boat conformation of higher energy. In one CH2 group. It forms the negative cy- – passing from a chair to a boat, the cyclo- clopentadienyl ion C5H5 , present in SAND- hexane ring passes through a half-chair WICH COMPOUNDS and is a nonbenzenoid conformation, which is the CONFORMATION aromatic. See aromatic compound. of highest energy. This converts to a twist- boat conformation, which has a higher en- cyclopentadienyl ion See cyclopentadi- ergy than the chair but lower than the true ene. boat. If cyclohexane has a substituent, there cysteine See amino acid. are also two different chair conformations, corresponding to whether the substituent is cystine A compound formed by the join- axial or equatorial (see illustration). ing of two cysteine amino acids through a –S–S– linkage (a cystine link). Bonds of this cyclonite A high explosive made from type are important in forming and main- hexamine. taining the tertiary structure of proteins.

62 iranchembook.ir/edu cytosine

cytokinin One of a class of plant hor- NH 2 mones concerned with the stimulation of cell division, nucleic acid metabolism, and N root-shoot interactions. Cytokinins are often purine derivatives: e.g. kinetin (6-fur- N O HOCH2 O furyl aminopurine), an artificial cytokinin commonly used in experiments; and zeatin, found in maize cobs.

OH OH cytosine A nitrogenous base found in DNA and RNA. Cytosine has the pyrimidine ring structure. Cytidine

cytidine (cytosine nucleoside) A nucleoside formed when cytosine is linked to D-ribose via a β-glycosidic bond. NH2

N cytochrome Any of a group of conju- 3 gated proteins containing heme, that act as 1 N O intermediates in the electron-transport H chain. There are four main classes, designated a, b, c, and d. Cytosine

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2,4-D (2,4-dichlorophenoxyacetic acid) deactivation A reduction in the reactiv- A synthetic auxin used as a potent selective ity of a substance or in the activity of a cat- weedkiller. Monocotyledenous species with alyst. narrow erect leaves (e.g. cereals and grasses) are generally resistant to 2,4-D de Broglie wave A wave associated while dicotyledenous plants are often very with a particle, such as an electron or pro- susceptible. The compound is thus used for ton. In 1924, Louis de Broglie suggested controlling weeds in cereal crops and that, since electromagnetic waves can be lawns. See auxin. described as particles (photons), particles of matter could also have wave properties. Dalton’s law (of partial pressures) The The wavelength (λ) has the same relation- principle that the pressure of a mixture of ship to momentum (p) as in electromag- gases is the sum of the partial pressures of netic radiation: each individual constituent. The partial λ = h/p pressure of a certain amount of a gas in a where h is the Planck constant. See also mixture is the pressure that it would exert quantum theory. if it alone were present in the container. Dalton’s law is strictly true only for ideal debye Symbol: D A unit of electric di- gases. In real gases there are effects caused pole moment equal to 3.335 64 × 10–30 by intermolecular forces. It is named for coulomb meter. It is used in expressing the the British chemist John Dalton (1766– dipole moments of molecules. The unit is 1844), who proposed it in 1803. named for the Dutch-born physical chemist Peter Debye (1884–1966). dative bond See coordinate bond. Debye–Hückel theory A theory of the d-block elements The transition el- behavior (e.g. conductivity) of ions in di- ements of the first, second, and third long lute solutions of electrolytes. It assumes periods of the periodic table, i.e. Sc to Zn, that electrolytes in dilute solution are com- Y to Cd, and La to Hg. They are so called pletely dissociated into ions but takes into because in general they have inner d-levels account interionic attraction and repul- with configurations of the type (n – 1)dxns2 sion. Agreement between the theory and where x = 1–10. experiment occurs only with very dilute solutions (less than 10–3M). DDT (dichlorodiphenyltrichloroethane; (ClC6H4)2CH(CCl3)) A colorless crys- deca- Symbol: da A prefix denoting 10. talline organic compound that was once For example, 1 decameter (dam) = 10 me- widely used as an insecticide. It is very sta- ters (m). ble and tends to accumulate in the soil, and passes up the food chain to accumulate in decahydronaphthalene See decalin. the fatty tissues of carnivorous animals. Its systematic name is 1,1-bis(4-chlorophenyl)- decalin (decahydronaphthalene; C10H18) 2,2,2-trichloroethane. A liquid hydrocarbon made by the hydro-

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degrees of freedom

H H H H H H H H H H H H H H H H H H H H

trans-decalin cis-decalin

Decalin

genation of naphthalene at high tem- magnetic field is applied or if the arrange- perature and pressure. There are two iso- ment of ligands around the atom is not mers. symmetrical. The degeneracy is then said to be ‘lifted’. decant To pour off the liquid above a sediment. De Gennes, Pierre Gilles (1932– ) French physicist. De Gennes is a versatile decay 1. The spontaneous breakdown of theoretical physicist who has made impor- a radioactive isotope. See half life; radioac- tant contributions to the theory of liquid tivity. crystals and polymers. In particular, he has 2. The transition of excited atoms, ions, shown that although these forms of matter molecules, etc., to a state of lower energy. do not have order in the same sense as solid crystals they do have order that character- deci- Symbol: d A prefix denoting 10–1. izes them. This enabled him to analyze For example, 1 decimeter (dm) = 10–1 them by using concepts such as order para- meter (m). meters and scaling taken from the theory of phase transitions. De Gennes gave an ac- decomposition The process in which a count of his work on liquid crystals in the compound is broken down into com- book The Physics of Liquid Crystals pounds with simpler molecules. (1974) and on polymers in Scaling Con- cepts of Polymer Physics (1979). In 1991 decrepitation The process in which a he won the Nobel Prize for physics for his crystalline solid emits a crackling noise on contributions to liquid crystals and poly- heating, usually because of loss of water of mers. crystallization. degradation A type of chemical reac- definite proportions, law of See con- tion involving the decomposition of a mol- stant proportions; law of. ecule into simpler molecules, usually in stages. The HOFMANN DEGRADATION of degassing The removal of dissolved or amides is an example. absorbed gases from liquids or solids, either on heating or in a vacuum. degrees of freedom The independent ways in which particles can take up energy. degenerate Describing different quan- In a monatomic gas, such as helium or tum states that have the same energy. For argon, the atoms have three translational instance, the five d orbitals in a transition- degrees of freedom (corresponding to mo- metal atom all have the same energy but tion in three mutually perpendicular direc- different values of the magnetic quantum tions). The mean energy per atom for each number m. Differences in energy occur if a degree of freedom is kT/2, where k is the

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dehydration

Boltzmann constant and T the thermody- delocalized bond A type of bonding in namic temperature; the mean energy per molecules that occurs in addition to sigma atom is thus 3kT/2. bonding. The electrons forming the delo- A diatomic gas has in addition two ro- calized bond are not localized between two tational degrees of freedom (about two atoms; i.e. the electron density of the delo- axes perpendicular to the bond) and one vi- calized electrons is spread over several brational degree (along the bond). The ro- atoms and may spread over the whole mol- tations also each contribute kT/2 to the ecule. average energy. The vibration contributes The electron density of the delocalized kT (kT/2 for kinetic energy and kT/2 for bond is spread by means of a delocalized potential energy). Thus, the average energy molecular orbital and may be regarded as a per molecule for a diatomic molecule is series of pi bonds extending over several 3kT/2 (translation) + kT (rotation) + kT atoms, for example the pi bonds in butadi- (vibration) = 7kT/2. ene and the C–O pi bonds in the CARBOXY- Linear triatomic molecules also have LATE ION. two significant rotational degrees of freedom; nonlinear molecules have three. For denaturation The changes in structure nonlinear polyatomic molecules, the num- that occur when a PROTEIN is heated. These ber of vibrational degrees of freedom is 3N changes are irreversible and affect the – 6, where N is the number of atoms in the properties of the protein. molecule. denatured alcohol Alcohol (ethanol) The molar energy of a gas is the average that has been contaminated by the addition energy per molecule multiplied by the Avo- of small amounts of substances to make it gadro constant. For a monatomic gas it is unfit for drinking. Ethanol treated in this 3RT/2, etc. way may still be useful for many purposes (e.g. as a solvent) but not have restrictions dehydration 1. Removal of water from or taxes on its sale. a substance. 2. Removal of the elements of water (i.e. dendritic growth The growth of crys- hydrogen and oxygen in a 2:1 ratio) from a tals with a branching habit. compound to form a new compound. An example is the dehydration of propanol to dendritic polymer See supramolecular propene over hot pumice: chemistry. → C3H7OH CH3CH:CH2 + H2O density Symbol: ρ A property of sub- deionization The removal of ions from stances equal to the mass of substance per a solution. The usual method is to use an unit volume. The units are g dm–3, etc. See ion-exchange resin. The term is commonly also relative density. applied to the purification of tap water; deionized water is cheaper to produce than deoxyribonucleic acid See DNA. distilled water and is adequate for many applications. deoxyribose See ribose.

deliquescent Describing a solid com- depression of freezing point A colliga- pound that absorbs water from the atmos- tive property of solutions in which the phere, eventually forming a solution. See freezing point of a given solvent is lowered also hygroscopic. by the presence of a solute. The amount of the reduction is proportional to the molal delocalization A spreading out of concentration of the solute. The depression bonding electrons in a molecule over the depends only on the concentration and is molecule. See delocalized bond. independent of solute composition. The

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detergent

proportionality constant, Kf, is called the vapor pressure above the frozen solvent. freezing point constant or sometimes the The addition of solute depresses the former ∆ ∆ cryoscopic constant. t = KfCM, where t is curve but as the solid phase that separates the lowering of the temperature and CM is is always pure solvent (above the eutectic the molal concentration; the unit of Kf is point), there is no attendant depression of kelvin kilogram mole–1 (K kg mol–1). Al- the latter curve. Consequently the point of though closely related to the property of intersection is depressed, resulting in a boiling-point elevation, the cryogenic lowering of the freezing point. See also method can be applied to measurement of lowering of vapor pressure. relative molecular mass with considerable precision. A known weight of pure solvent derivative A compound that could be is slowly frozen, with stirring, in a suitable produced from another compound by cold bath and the freezing temperature chemical reaction. Usually, the term is ap- measured using a Beckmann thermometer. plied to a compound that has a structural A known weight of solute of known mo- similarity to the parent compound; for ex- lecular mass is introduced, the solvent ample, chlorobenzene (C6H5Cl) is a deriv- thawed out, and the cooling process and ative of benzene (C6H6). measurement repeated. The addition is repeated several times and an average value derived unit A unit defined in terms of ∆ of Kf for the solvent obtained by plotting t base units, and not directly from a stan- against CM. The whole process is then re- dard value of the quantity it measures. For peated using the unknown solute and its example, the newton is a unit of force de- relative molecular mass determined using fined as a kilogram meter second–2 (kg m –2 the value of Kf previously obtained. s ). See also SI units. The effect is applied to more precise measurement of relative molecular mass by desiccation Removal of moisture from using a pair of Dewar flasks (pure solvent a substance. and solution) and measuring ∆t by means of thermocouples. The theoretical explana- desiccator A piece of laboratory appa- tion is similar to that for lowering of vapor ratus for drying solids or for keeping solids pressure. The freezing point of the solvent free of moisture. Typically, a dessicator is is that point at which the curve represent- an air-tight container in which is kept a hy- ing the vapor pressure above the liquid groscopic material (e.g. calcium chloride or phase intersects the curve representing the silica gel) to absorb moisture from the atmosphere.

destructive distillation The process of Beckmann thermometer heating an organic substance in the absence of air, so that it wholly or partially decomposes to produce volatile products, tube for which are subsequently condensed. The de- introduction structive distillation of coal was the of solute process for manufacturing coal gas and coal tar. At one time, methanol was made by the destructive distillation of wood. container for freezing detergent Any of a group of substances mixture that improve the cleansing action of solvents, particularly water. The majority of detergents, including SOAP, have the same basic structure. Their molecules have a hydrocarbon chain (tail) that does not attract Depression of freezing point water molecules. The tail is said to be hy-

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deuterated compound

drophobic (water hating). Attached to this forming analogous compounds, although tail is a small group (head) that readily ion- reactions of deuterium compounds are izes and attracts water molecules. It is said often slower than those of the correspond- to be hydrophilic (water loving). Deter- ing 1H compounds. This is made use of in gents reduce the surface tension of water kinetic studies where the rate of a reaction and thus improve its wetting power. Be- may depend on transfer of a hydrogen cause the detergent ions have their hy- atom. drophilic heads anchored in the water and

their hydrophobic tails protruding above deuterium oxide (D2O) See heavy it, the water surface is broken up, enabling water. the water to spread over the material to be cleaned and penetrate between the material deuteron The nucleus of a deuterium and the dirt. With the assistance of agita- atom. tion, the dirt can be floated off. The hydrophobic tails of the detergent molecules Dewar flask (vacuum flask) A double- ‘dissolve’ in grease and oils. The protrud- walled container of thin glass with the ing hydrophilic heads repel each other space between the walls evacuated and causing the oil to roll up and form a drop, sealed to stop conduction and convection which floats off into the water as an emul- of energy through it. The glass is often sil- sion. More recently synthetic detergents, vered to reduce radiation. It is named for often derived from petrochemicals, have the British chemist and physicist Sir James been developed. Unlike soaps these deter- Dewar (1842–1923). gents do not form insoluble scums with hard water. Dewar structure A representation of Synthetic detergents are of three types. the structure of benzene in which there is a Anionic detergents form ions consisting of single bond between two opposite corners a hydrocarbon chain to which is attached of the hexagonal ring and two double either a sulfonate group, –SO –O–, or a 2 bonds at the sides of the ring. The Dewar sulfate group, –O–SO –O–. The corre- 2 structures contribute to the resonance hy- sponding metal salts are soluble in water. Cationic detergents have organic positive brid of benzene. It is named for the + British–American chemist Michael Dewar ions of the type RNH3 , in which R has a long hydrocarbon chain. Non-ionic deter- (1918–nn). The nonplanar compound with gents are complex chemical compounds this structure, having two fused four- called ethoxylates. They owe their deter- membered rings, was synthesized in 1963. gent properties to the presence of a number See benzene. of oxygen atoms in one part of the molecule, which are capable of forming hydro- dextrin A polysaccharide SUGAR pro- gen bonds with the surface water duced from starch by the action of amylase molecules, thus reducing the surface ten- enzymes or by chemical hydrolysis. Dex- sion of the water. See also soap. trins are used as adhesives.

deuterated compound A compound in dextro-form See optical activity. which one or more 1H atoms have been replaced by deuterium (2H) atoms. dextronic acid See gluconic acid.

deuterium Symbol: D, 2H A naturally dextrorotatory See optical activity. occurring stable isotope of hydrogen in which the nucleus contains one proton and dextrose (grape-sugar) The dextrorota- one neutron. The atomic mass is thus ap- tory naturally occurring form of GLUCOSE, proximately twice that of 1H; deuterium is D-(+)-glucose. Because other stereochemi- known as ‘heavy hydrogen’. Chemically it cal forms of glucose have no significance in behaves almost identically to hydrogen, biological systems the term ‘glucose’ is

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Diels, Otto Paul Hermann

often used interchangeably with ’dextrose’ method of nucleophilic substitution onto in biology. the benzene ring.

D-form See optical activity. dibasic acid An acid that has two acidic hydrogen atoms, such as sulfuric acid. 1,6-diaminohexane (hexamethylene di- Dibasic acids can give rise to two series of amine; H2N(CH2)6NH2) An organic salts. For example, sulfuric acid (H2SO4) 2– compound used as a starting material in forms sulfates (SO4 ) and hydrogensul- – the production of nylon. It is manufactured fates (HSO 4). from cyclohexane. See nylon. dibenzo-4-pyrone See xanthone. diastereoisomer See isomerism. 1,2-dibromoethane (ethylene dibromide; diatomic molecule A molecule that BrCH2CH2Br) A colorless volatile organic liquid, made by reacting bromine consists of two atoms. Hydrogen (H2), with ethene. It is used as a fuel additive to oxygen (O2), nitrogen (N2), and the halogens are examples of diatomic elements. remove lead (as lead bromide, which is also volatile). diazine See pyrazine. dicarboxylic acid An organic acid that diazole See pyrazole. has two carboxyl groups (–COOH). An example is hexanedioic acid, diazonium compound A compound of HOOC(CH2)4COOH (adipic acid). + – the type ArN2 X , where Ar is an aromatic group and X– a negative ion. Diazonium dichloroacetic acid See chloroethanoic salts are made by diazotization. They can acid. be isolated but are very unstable, and are dichlorodiphenyltrichloroethane See usually prepared in solution. The –N + 2 DDT. group renders the benzene ring susceptible to nucleophilic substitution (rather than dichloroethanoic acid See chloroethanoic electrophilic substitution). Typical reac- acid. tions are: 1. Reaction with water on warming the so- dichromate(VI) A salt containing the lution: ion Cr O –. Dichromates are strong oxidiz- + → + 2 7 ArN2 + H2O ArOH + N2 + H ing agents. See potassium dichromate. 2. Reaction with halogen ions (CuCl catalyst for chloride ions): Diels, Otto Paul Hermann (1876– + – → ArN2 + I ArI + N2 1954) German organic chemist. The first Diazonium ions can also act as elec- major discovery which Diels made was car- trophiles and undergo substitution reacting bon suboxide (C3O2). He discovered this with other benzene rings (diazo coupling). compound in 1906 by dehydrating mal- See also azo compound. onic acid with phosphorus pentoxide. His second major discovery was the process of diazotization The reaction of an aro- removing hydrogen from steroids by heat- matic amine (e.g. aniline) with nitrous acid ing them with selenium. He used this at low temperatures (below 5°C). process on cholesterol. He was able to use → + – C6H5NH2 + HNO2 C6H5N N + OH the process he found to determine the + H2O structures of steroids. In 1928 Diels and his The acid is prepared in situ by reaction be- colleague Kurt ALDER discovered what tween nitric acid and sodium nitrite. The came to be known as the Diels–Alder reac- resulting diazonium ion is susceptible to at- tion for producing a ring compound from tack by nucleophiles and provides a a diene. Diels and Alder shared the 1950

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Diels–Alder reaction

H H

H C H H COOH H C COOH C C C C

C C C C H C H H COOH H C COOH

H H

butadiene maleic acid phthalic acid

Diels–Alder reaction

Nobel Prize for chemistry for this discov- sion in solids occurs very slowly at normal ery. temperatures. See also Graham’s law.

Diels–Alder reaction A type of reac- dihedral angle See conformation. tion in which a conjugated DIENE adds to a compound containing a double C=C bond dihydrate A crystalline compound with (called the dienophile) to give a ring com- two molecules of water of crystallization pound. To be effective, the dienophile has per molecule of compound. to have electron-withdrawing groups on the double bond. The diene has to have a dihydric alcohol See diol. cis-conformation or to be able to adopt a cis-conformation. The reactants are mixed dihydroxypurine See xanthine. together and heated. The mechanism involves a single step in which electrons diluent A solvent that is added to reduce move to form different bonds. The reaction the strength of a solution. is an example of a cycloaddition reaction (see pericyclic reaction). It was named for dilute Denoting a solution in which the the German chemists Otto Diels and Kurt amount of solute is low relative to that of Alder (1902–58), who described it in 1928. the solvent. The term is always relative.

diene An organic compound containing dimensionless units The radian and two carbon–carbon double bonds. In a steradian in SI units. See SI units. conjugated diene the two double bonds are separated by a single C–C bond. dimer A compound (or molecule) formed by combination or association of dienophile See Diels–Alder reaction. two molecules of a monomer. Cyclopenta- diene, for example, exists as a dimer at diesel fuel A petroleum fraction consist- room temperature. On heating it dissoci- ing of various alkanes in the boiling range ates. 200–350°C, used as a fuel for diesel (compression-ignition) engines. dimethylbenzene (xylene; C6H4(CH3)2) An organic hydrocarbon present in the diethylether See ethoxyethane. light-oil fraction of crude oil. It is used extensively as a solvent. There are three iso- diffusion Movement of a gas, liquid, or meric compounds with this name and solid as a result of the random thermal mo- formula, distinguished as 1,2-, 1,3-, and tion of its particles (atoms or molecules). A 1,4-dimethylbenzene according to the posi- drop of ink in water, for example, will tions of the methyl groups on the benzene slowly spread throughout the liquid. Diffu- ring.

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disperse dyes

2,4-dinitrophenylhydrazine (Brady’s re- manent dipoles. Induced dipoles can also agent) An orange solid commonly used occur. See also van der Waals force. in solution with methanol and sulfuric acid to produce crystalline derivatives by con- dipole moment Symbol: µ A quantita- densation with aldehydes and ketones. The tive measure of polarity in either a bond derivatives, known as 2,4-dinitrophenyl- (bond moment) or a molecule as a whole hydrazones, can easily be purified by re- (molecular dipole moment). The unit is the crystallization and have characteristic debye (equivalent to 3.34 × 10–30 coulomb melting points, used to identify the original meter). Molecules such as HF, H2O, NH3, aldehyde or ketone. and C6H5NH2 possess dipole moments; CCl4, N2, C6H6, and PF5 do not. dinucleotide A compound of two nu- The molecular dipole moment can be cleotides linked by their phosphate groups. estimated by vector addition of individual Important examples are the coenzymes bond moments if the bond angles are NAD and FAD. known. The possession of a dipole moment permits direct interaction with electric diol (dihydric alcohol; glycol) An alco- fields or interaction with the electric com- hol that has two hydroxyl groups (–OH) ponent of radiation. per molecule of compound. dipyridyl (bipyridyl) A compound 1,4-dioxan ((CH2)2O2) A colorless liq- formed by linking two pyridine rings. uid cyclic ether. It is an inert compound There are various isomers, some of which miscible with water used as a solvent. are used in herbicides.

dioxin Any of a related group of highly direct dyes A group of dyes that are toxic chlorinated compounds. Particularly mostly azo-compounds derived from ben- important is the compound 2,3,7,8-tetra- zidene or benzidene derivatives. They are chlorodibenzo-p-dioxin (TCDD), which is used to dye cotton, viscose rayon, and produced as a by-product in the manufac- other cellulose fibers directly, using a neu- ture of 2,4,5-T, and may consequently tral bath containing sodium chloride or occur as an impurity in certain types of sodium sulfate as a mordant. weedkiller. The defoliant known as AGENT ORANGE used in Vietnam contained signifi- disaccharide A SUGAR with molecules cant amounts of TCDD. Dioxins cause a composed of two monosaccharide units. skin disease (chloracne) and birth defects. These are linked by a –O– linkage (glyco- Dioxins have been released into the atmos- sidic link). Sucrose and maltose are exam- phere as a result of explosions at herbicide ples. manufacturing plants, most notably at Seveso, Italy, in 1976. disconnection See retrosynthetic analysis. dipeptide See peptide. disperse dyes Water-insoluble dyes, diphosphane (diphosphine, P2H4)A which, when held in fine suspension, can yellow liquid that can be condensed out be applied to acetate rayon fabrics. The from phosphine in a freezing mixture. It ig- dye, together with a dispersing agent, is nites spontaneously in air. warmed to a temperature of 45–50°C and the fabric added. By modifying the method diphosphine See diphosphane. of application it is possible to dye poly- acrylic and polyester fibers. The yellow/ dipole A system in which two equal and orange shades are nitroarylamine deriva- opposite electric charges are separated by a tives and the green to bluish shades are finite distance. Polar molecules have per- derivatives of 1-amino anthraquinone.

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disperse phase

Certain azo compounds are disperse dyes Often the degree of dissociation is used and these give a range of colors. – the fraction (α) of the original compound that has dissociated at equilibrium. For an disperse phase See colloid. original amount of AB of n moles in a volume V, the dissociation constant is given dispersing agent A compound used to by: assist emulsification or dispersion. K = α2n/(1 – α)V Note that this expression is for dissociation dispersion force A weak type of inter- into two molecules. molecular force. See van der Waals force. Acid dissociation constants (or acidity constants, symbol: Ka) are dissociation displacement pump A commonly used constants for the dissociation into ions in device for transporting liquids and gases solution: ˆ + – around chemical plants. It works on the HA + H2O H3O + A principle of the bicycle pump: a piston The concentration of water can be taken as raises the pressure of the fluid and, when it unity, and the acidity constant is given by: + – is high enough, a valve opens and the fluid Ka = [H3O ][A ]/[HA] is discharged through an outlet pipe. As the The acidity constant is a measure of the piston moves back the pressure falls and strength of the acid. Base dissociation con- the cycle continues. Displacement pumps stants (Kb) are similarly defined. The ex- can be used to generate very high pressures pression: α2 α but because of the system of valves, they K = n/(1 – )V are more expensive than other types of applied to an acid is known as Ostwald’s dilution law (for the German chemist pump. Compare centrifugal pump. Friedrich Wilhelm Ostwald (1853–1932), who formulated it in 1888). In particular if displacement reaction A chemical re- α is small (a weak acid) then K = α2n/V, or action in which an atom or group displaces α = C√V, where C is a constant. The degree another atom or group from a molecule. of dissociation is then proportional to the square root of the dilution. disproportionation A chemical reaction in which there is simultaneous oxida- distillation The process of boiling a liq- tion and reduction of the same compound. uid and condensing the vapor. Distillation The CANNIZZARRO REACTION is an example is used to purify liquids or to separate com- in organic chemistry. ponents of a liquid mixture. See also destructive distillation; fractional distillation; dissociation Breakdown of a molecule steam distillation; vacuum distillation. into two molecules, atoms, radicals, or ions. Often the reaction is reversible, as in distilled water Water that has been pu- the ionic dissociation of weak acids in rified by distillation, perhaps several times. water: ˆ – CH3COOH + H2O CH3COO + diterpene See terpene. + H3O divalent (bivalent) Having a valence of dissociation constant The equilibrium two. constant of a dissociation reaction. For example, the dissociation constant of a reac- Djerassi, Carl (1923– ) Austrian- tion: born American chemist. The first notable AB ˆ A + B work which he and his colleagues at Syn- is given by: tex, in Mexico City, performed was to ex- K = [A][B]/[AB] tract cortisone from a vegetable source. where the brackets denote concentration Djerassi and his colleagues then investi- (activity). gated the steroid hormone progesterone

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double bond

which acts as a natural contraceptive. They deoxyribose sugar molecules along both produced progesterone artificially in the chains and each sugar molecule is also early 1950s, thus reducing its price. joined to one of four nitrogenous bases – Djerassi improved the power of proges- adenine (A), guanine (G), cytosine (C), or terone by removing a particular methyl thymine (T). The two chains are joined to group. He used a similar trick with testos- each other by bonding between bases. The terone. This led to the development of the two purine bases (adenine and guanine) al- contraceptive pill. Djerassi published his ways bond with the pyrimidine bases autobiography The Pill, Pigmy Chimps, (thymine and cytosine), and the pairing is and Degas Horse in 1992. quite specific: adenine with thymine and guanine with cytosine. The two chains are D-L convention See optical activity. therefore complementary. The sequence of bases along the chain makes up a code – DNA (deoxyribonucleic acid) A nucleic the genetic code – that determines the pre- acid, mainly found in the chromosomes, cise sequence of amino acids in proteins. that contains the hereditary information of DNA is the hereditary material of all organisms. The molecule is made up of two organisms with the exception of RNA antiparallel helical polynucleotide chains viruses. Together with histones (and RNA coiled around each other to give a double in some instances) it makes up the chromo- helix. It is also known as the Watson-Crick somes of eukaryotic cells. See also RNA. model after James Watson and Francis See illustration overleaf. Crick who first proposed this model in 1953. Phosphate molecules alternate with dodecanoic acid (lauric acid; CH3- (CH2)10COOH) A white crystalline carboxylic acid, used as a plasticizer and for making detergents and soaps. Its glycerides AT occur naturally in coconut and palm oils.

donor 1. The atom, ion, or molecule hydrogen that provides the pair of electrons in form- bond ing a covalent bond. TA 2. The impurity atoms used in doping semi- conductors. GC

CG dopamine A catecholamine precursor of epinephrine and norepinephrine. In sugar– phosphate mammals it is found in highest concentra- backbone tion in the corpus striatum of the brain, where it functions as an inhibitory neuro- TA transmitter. High levels of dopamine are associated with Parkinson’s disease in hu- GC mans. AT 3.4 nm dormin A former name for abscisic acid. base double bond A covalent bond between two atoms that includes two pairs of elec- AT trons, one pair being the single bond equivalent (the sigma pair) and the other CG forming an additional bond, the pi bond (π bond). It is conventionally represented by two lines, for example H2C=O. See multi- DNA: the double helix ple bond; orbital.

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sugar sugar

phosphate phosphate

sugar sugar

phosphate phosphate

sugar sugar

phosphate phosphate

sugar sugar

KEY adenine Part of the structure of DNA showing hydrogen bonding (dotted lines) between complementary bases cytosine

thymine

guanine

DNA: the structure and bonding

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dynamite

double helix See DNA. surplus vapor has been expelled, the bulb is sealed off, cooled, dried, and weighed. The double salt When equivalent quantities tip of the tube is then broken under water of certain salts are mixed in aqueous solu- so that the water completely fills the tube tion and the solution evaporated, a salt and the whole weighed again. This enables may form, e.g. FeSO4.(NH4)2SO4.6H2O. the volume of the bulb to be calculated In aqueous solution the salt behaves as a from the known density of water, and mixture of the two individuals. These salts knowing the density of air one can com- are called double salts to distinguish them pute the mass of vapor in a known volume from complex salts, which yield complex of the sample. ions in solution. 2. A method of finding the amount of nitrogen in an organic compound by heating dryers Devices used in chemical the compound with copper oxide to con- processes to remove a liquid from a solid vert the nitrogen into nitrogen oxides. by evaporation. Drying equipment is clas- These are reduced by passing them over sified by the method of transferring heat to hot copper and the volume of nitrogen col- a wet solid. This can be by direct contact lected is measured. between hot gases and the solid (direct dry- Both methods are named for the French ers), heat transfer by conduction through a chemist Jean Baptiste André Dumas retaining metallic wall (indirect dryers), or (1800–84). infrared rays (infrared dryers).

dry ice Solid carbon dioxide, used as a dye A coloring material for fabric, refrigerant. leather, etc. Most dyes are now synthetic organic compounds (the first such was the drying oil A natural oil, such as linseed dye mauve synthesized from aniline in oil, that hardens in air. Such oils contain 1856 by William Perkin). Dyes are often unsaturated fatty acids, which polymerize unsaturated organic compounds contain- on oxidation. ing conjugated double bonds – the bond system responsible for the color is called Dumas’ method 1. A method for de- the chromophore. See also azo compound. termining the relative molecular mass of a volatile liquid. The method utilizes a glass dynamite A high explosive made by ab- bulb with a narrow entrance tube. The sorbing nitroglycerine into an earthy ma- bulb is weighed ‘empty’ (i.e. full of air) terial such as diatomite (kieselguhr). Solid then the sample is introduced and the bulb sticks of dynamite are much safer to handle immersed in a heating bath so that the sam- than the highly sensitive liquid nitroglycer- ple boils and expels all the air. When the ine.

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ebonite See vulcanite. als for reactions that involve ions in solution. In decreasing order of activity, the se- ebulioscopic constant See elevation of ries is boiling point. K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Pt, Au ebullition The boiling or bubbling of a Any member of the series will displace ions liquid. of a lower member from solution. For example, zinc metal will displace Cu2+ ions: eclipsed conformation See conforma- Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) tion. Zinc has a greater tendency than copper to form positive ions in solution. Similarly, edta (ethylenediamine tetraacetic acid) A compound with the formula metals above hydrogen displace hydrogen (HOOCCH ) N(CH ) N(CH COOH) from acids: 2 2 2 2 2 2 → It is used in forming chelates of transition Zn + 2HCl ZnCl2 + H2 metals. See chelate. The series is based on electrode potentials, which measure the tendency to form posi- effervescence The evolution of gas in tive ions. The series is one of increasing the form of bubbles in a liquid. electrode potential for half cells of the type Mn+|M. Thus, copper (EŠ for Cu2+|Cu = + efflorescence The process in which a 0.34 V) is lower than zinc (EŠ for Zn2+|Zn crystalline hydrated solid spontaneously = –0.76V). The hydrogen half cell has a loses water of crystallization to the air. A value EŠ = 0. powdery deposit is gradually formed. electrochemistry The study of the for- elastin A structural protein found in mation and behavior of ions in solutions. It mammalian connective tissues, especially includes electrolysis and the generation of in elastic fibers. Glycine is the main com- electricity by chemical reactions in cells. ponent; proline, alanine, and valine are the other main residues. electrochromatography See electrophoresis. elastomer An elastic substance, e.g. a natural or synthetic rubber. electrocyclic reaction See pericyclic re- electrochemical equivalent Symbol: z action. The mass of an element released from a solution of its ion when a current of one electrode Any part of an electrical de- ampere flows for one second during ELEC- vice or system that emits or collects elec- TROLYSIS. trons or other charge carriers. An electrode may also be used to deflect charged parti- electrochemical series (electromotive se- cles by the action of the electrostatic field ries) A series giving the activities of met- that it produces.

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electromagnetic radiation

electrode potential Symbol: E A meas- The current is conducted by migration of ure of the tendency of an element to form ions – positive ones (cations) to the cathode ions in solution. For example, a metal in a (negative electrode), and negative ones (an- solution containing M+ ions may dissolve ions) to the anode (positive electrode). Re- in the solution as M+ ions; the metal then actions take place at the electrodes by has an excess of electrons and the solution transfer of electrons to or from them. an excess of positive ions – thus, the metal In the electrolysis of water (containing a becomes negative with respect to the solu- small amount of acid to make it conduct tion. Alternatively, the positive ions may adequately) hydrogen gas is given off at the gain electrons from the metal and be de- cathode and oxygen is evolved at the posited as metal atoms. In this case, the anode. At the cathode the reaction is: metal becomes positively charged with re- H+ + e– → H → spect to the solution. In either case, a po- 2H H2 tential difference is developed between At the anode: solid and solution, and an equilibrium OH– → e– + OH → state will be reached at which further reac- 2OH H2O + O → tion is prevented. The equilibrium value of 2O O2 this potential difference would give an in- In certain cases the electrode material dication of the tendency to form aqueous may dissolve. For instance, in the electrol- ions. ysis of copper(II) sulfate solution with cop- It is not, however, possible to measure per electrodes, copper atoms of the anode this for an isolated half cell – any measure- dissolve as copper ions ment requires a circuit, which sets up an- Cu → 2e– + Cu2+ other half cell in the solution. Therefore, electrode potentials (or reduction poten- electrolyte A liquid containing positive tials) are defined by comparison with a hy- and negative ions that conducts electricity drogen half cell, which is connected to the by the flow of those charges. Electrolytes half cell under investigation by a salt can be solutions of acids or metal salts bridge. The e.m.f. of the full cell can then (‘ionic compounds’), usually in water. Al- be measured. ternatively they may be molten ionic com- In referring to a given half cell the more pounds – again the ions can move freely reduced form is written on the right for a through the substance. Liquid metals (in half-cell reaction. For the half cell Cu2+|Cu, which conduction is by free electrons the half-cell reaction is a reduction: rather than ions) are not classified as elec- Cu2+(aq) + 2e → Cu trolytes. See also electrolysis. The cell formed in comparison with a hydrogen electrode is: electrolytic Relating to the behavior or + 2+ Pt(s)H2(g)|H (aq)|Cu (aq)|Cu reactions of ions in solution. The e.m.f. of this cell is +0.34 volt measured under standard conditions. Thus, the electrolytic cell See cell; electrolysis. standard electrode potential (symbol: EŠ) is +0.34 V for the half cell Cu2+|Cu. The electromagnetic radiation Energy prop- standard conditions are 1.0 molar solu- agated by vibrating electric and magnetic tions of all ionic species, standard pressure, fields. Electromagnetic radiation forms a and a temperature of 298 K. whole electromagnetic spectrum, depend- Half cells can also be formed by a solu- ing on frequency and ranging from high- tion of two different ions (e.g. Fe2+ and frequency radio waves to low-frequency Fe3+). In such cases, a platinum electrode is gamma rays. used under standard conditions. Electromagnetic radiation can be thought of as waves (electromagnetic electrolysis The production of chemical waves) or as streams of photons. The fre- change by passing electric charge through quency and wavelength are related by: certain conducting liquids (electrolytes). λv = c

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electromagnetic spectrum

ELECTROMAGNETIC SPECTRUM (note: the figures are only approximate) Radiation Wavelength (m) Frequency (Hz) gamma radiation –10–10 1019– x-rays 10–12 –10–9 1017 –1020 ultraviolet radiation 10–9 –10–7 1015 –1017 visible radiation 10–7 –10–6 1014 –1015 infrared radiation 10–6 –10–4 1012 –1014 microwaves 10–4 –1 109 –1013 radio waves 1 – –109

where c is the speed of light. The energy for the bonding orbital. See also multicen- carried depends on the frequency. ter bond.

electromagnetic spectrum See electro- electron diffraction A technique used magnetic radiation. to determine the structure of substances, principally the shapes of molecules in the electromotive series See electrochemi- gaseous phase. A beam of electrons di- cal series. rected through a gas at low pressure produces a series of concentric rings on a electron An elementary particle of nega- photographic plate. The dimensions of tive charge (–1.602 192 × 10–19C) and rest these rings are related to the interatomic mass 9.109 558 × 10–31 kg. Electrons are distances in the molecules. See also x-ray present in all atoms in shells around the nu- diffraction. cleus. electron donor See reduction. electron affinity Symbol: A The energy released when an atom (or molecule or electronegative Describing an atom or group) gains an electron in the gas phase to molecule that attracts electrons, forming form a negative ion. It is thus the energy of: negative ions. Examples of electronegative A + e– → A– elements include the halogens (chlorine A positive value of A (often in electron- etc.), which readily form negative ions (F–, volts) indicates that heat is given out. Often Cl–, etc.). See also electronegativity. the molar enthalpy is given for this process of electron attachment (∆H). Here the electronegativity A measure of the ten- units are joules per mole (J mol–1), and, by dency of an atom in a molecule to attract the usual convention, a negative value indi- electrons to itself. Elements to the right- cates that energy is released. hand side of the periodic table are strongly electronegative (values from 2.5 to 4); electron-deficient compounds Com- those on the left-hand side have low elec- pounds in which the number of electrons tronegativities (0.8–1.5) and are sometimes available for bonding is insufficient for the called electropositive elements. Different bonds to consist of conventional two-elec- electronegativities of atoms in the same tron covalent bonds. Diborane, B2H6, is an molecule give rise to polar bonds and example in which each boron atom has sometimes to polar molecules. two terminal hydrogen atoms bound by As the concept of electronegativity is conventional electron-pair bonds and in not precisely defined it cannot be precisely addition the molecule has two hydrogen measured and several electronegativity atoms bridging the boron atoms (B–H–B). scales exist. Although the actual values dif- In each bridge there are only two electrons fer the scales are in good relative agree-

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electrophilic addition

ment. See also electron affinity; ionization of NADH. If FADH2 is the electron donor, potential. only four H+ ions are pumped across, as it donates electrons directly to ubiquinone. electronic energy levels See atom; en- The function of electron transport in ergy level. the mitochondrion is to provide the energy required to phosphorylate ADP to ATP. electronic transition The demotion or According to the chemiosmotic theory, the promotion of an electron between elec- H+ ions in the intermembrane space diffuse tronic energy levels in an atom or molecule. back to the matrix through the inner mitochondrial membrane down a concentra- electron pair Two electrons in one or- tion gradient. As they do so they pass bital with opposing spins (spin paired), through the protein channel (the F0 unit) of such as the electrons in a COVALENT BOND the enzyme ATP synthase. The energy re- or LONE PAIR. leased allows the catalytic F1 unit of ATP synthase to synthesize ATP from ADP and + electron spin See atom. inorganic phosphate. Each pair of H ions catalyzes the formation of one molecule of electron spin resonance (ESR) A simi- ATP, so for each NADH molecule, three lar technique to nuclear magnetic reso- molecules of ATP may be synthesized (two nance, but applied to unpaired electrons in molecules of ATP per molecule of FADH2). a molecule (rather than to the nuclei). It is A similar mechanism is involved in ATP a powerful method of studying free radi- formation by components of the light reac- cals. ESR is also used in inorganic chem- tion in photosynthesis. See photosynthesis; istry to study transition-metal complexes. oxidative phosphorylation.

electron-transport chain (respiratory electronvolt Symbol: eV A unit of energy equal to 1.602 191 7 × 10–19 joule. It chain) A chain of chemical reactions in- is defined as the energy required to move volving proteins and enzymes, resulting in an electron charge across a potential differ- the formation of ATP and the transfer of ence of one volt. It has been used to meas- hydrogen atoms to oxygen to form water. ure the kinetic energies of elementary The enzymes and other proteins are, in eu- particles or ions, or the ionization poten- karyotic cells, located in the inner mem- tials of molecules. brane of the mitochondria and are grouped into discrete complexes. The reduced coen- electrophile An electron-deficient ion zyme NADH gives up two electrons to the or molecule that takes part in an organic first component in the chain, NADH dehy- reaction. The electrophile can be either a + drogenase, and two hydrogen ions (H ) are + + positive ion (H , NO2 ), a molecule that discharged from the matrix of the mito- can accept an electron pair (SO3, O3), or an chondrion into the intermembrane space. electron-deficient group (e.g. a CARBENE). The electrons are transferred along the The electrophile attacks negatively charged chain to a carrier molecule (ubiquinone). areas of molecules, which usually arise Ubiquinone passes them to the next com- from the presence in the molecule of a plex, which contains cytochromes b and c1. polar single bond or group or of pi-bonds. Another carrier (cytochrome c) transfers Compare nucleophile. the electrons to the final complex in the chain. There they act with the enzyme cy- electrophilic addition A reaction in- tochrome oxidase to reduce an oxygen volving the addition of a small molecule to atom, which combines with two H+ ions to an unsaturated organic compound, across form water. During this electron transfer, a the atoms joined by a double or triple bond further two pairs of H+ ions are pumped with an ELECTROPHILE as the initial attack- into the intermembrane space by the coming species. The reaction is initiated by the plexes, making a total of six per molecule attack of the electrophile on the electron-

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electrophilic substitution

rich area of the molecule. The mechanism tive elements include the alkali metals of electrophilic addition is thought to be (lithium, sodium, etc.), which readily form ionic, as in the addition of HBr to ethene: positive ions (Li+, Na+, etc.). + – → H2C:CH2 + H Br + – → H3CCH2 + Br electrovalent bond (ionic bond) A H3CCH2Br binding force between the ions in com- In the case of higher alkenes (more than pounds in which the ions are formed by two carbon atoms) several isomeric prod- complete transfer of electrons from one el- ucts are possible. The particular isomer ement to another element or radical. For produced depends on the stability of the al- example, Na + Cl becomes Na+ + Cl–. The ternative intermediates and this is summa- electrovalent bond arises from the excess rized empirically by MARKOVNIKOFF’S RULE. of the net attractive force between the ions See also addition reaction. of opposite charge over the net repulsive force between ions of like charge. The mag- electrophilic substitution A reaction nitude of electrovalent interactions is of the involving substitution of an atom or group 2 3 –1 of atoms in an organic compound with an order 10 –10 kJ mol and electrovalent compounds are generally solids with rigid ELECTROPHILE as the attacking substituent. Electrophilic substitution is very common lattices of closely packed ions. in aromatic compounds, in which electrophiles are substituted onto the ring. An element A substance that cannot be example is the nitration of benzene: chemically decomposed into more simple + → + substances. The atoms of an element all C6H6 + NO2 C6H5NO2 + H + The nitronium ion (NO2 ) is formed by have the same proton number (and thus the mixing concentrated nitric and sulfuric same number of electrons, which deter- acids: mines the chemical activity). → + – HNO3 + H2SO4 H2NO3 + HSO4 At present there are 114 reported chem- + → + H2NO3 NO2 + H2O ical elements, although research is continu- The accepted mechanism for a simple ing all the time to synthesize new ones. The electrophilic substitution on benzene in- elements from hydrogen (p.n. 1) to ura- volves an intermediate of the form nium (92) all occur naturally, with the ex- + C6H5HNO2 . See also substitution reac- ception of technetium (43), which is tion. produced artificially by particle bombardment. Technetium and elements with pro- electrophoresis The use of an electric ton numbers higher than 84 (polonium) field (produced between two electrodes) to are radioactive. Radioactive isotopes also cause charged particles of a colloid to move exist for other elements, either naturally in through a solution. The technique is used small amounts or synthesized by particle to separate and identify colloidal sub- bombardment. The elements with proton stances such as carbohydrates, proteins, number higher than 92 are the transuranic and nucleic acids. Various experimental elements. Neptunium (93) and plutonium arrangements are used. One simple technique uses a strip of adsorbent paper (94) both occur naturally in small quanti- soaked in a buffer solution with electrodes ties in uranium ores, but the transuranics placed at two points on the paper. This are all synthesized. Thus, neptunium and technique is sometimes called electrochro- plutonium are made by neutron bombard- matography. In gel electrophoresis, used to ment of uranium nuclei. Other transuran- separate DNA fragments, the medium is a ics are made by high-energy collision layer of gel. processes between nuclei. The higher proton number elements have been detected electropositive Describing an atom or only in very small quantities – in some molecule that tends to lose electrons, form- cases, only a few atoms have been pro- ing positive ions. Examples of electroposi- duced.

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emission spectrum

elevation of boiling point A colliga- There are several disadvantages with tive property of solutions in which the boil- this method and it is therefore used largely ing point of a solution is raised relative to for demonstration purposes. The main that of the pure solvent. The elevation is di- problem is that the exact amount of solvent rectly proportional to the number of solute remaining in the liquid phase is unknown molecules introduced rather than to any and varies with the rate of boiling. The the- specific aspect of the solute composition. oretical explanation of the effect is identi- The proportionality constant, kB, is called cal to that for the lowering of vapor the boiling-point elevation constant or pressure; the boiling points are those tem- sometimes the ebulioscopic constant. The peratures at which the vapor pressure relationship is equals the atmospheric pressure. ∆ t = kBCM ∆ where t is the rise in boiling point and CM elimination reaction A reaction in- is the molal concentration; the units of kB volving the removal of a small molecule, are kelvins kilograms moles–1 (K kg mol–1). e.g. water or hydrogen chloride, from an The property permits the measurement of organic molecule to give an unsaturated relative molecular mass of involatile compound. An example is the elimination solutes. An accurately weighed amount of of a water molecule from an alcohol to pure solvent is boiled until the temperature produce an alkene. is steady, a known weight of solute of An elimination reaction is often in com- known molecular mass is quickly intro- petition with a substitution reaction and duced, the boiling continued, and the ele- the predominant product will depend on vation measured using a Beckmann the reaction conditions. The reaction of thermometer. The process is repeated sev- bromoethane with sodium hydroxide eral times and the average value of kB ob- could yield either ethene (by elimination of ∆ tained by plotting t against CM. The HBr) or ethanol (by substitution of the Br whole process is then repeated with the un- with OH). The former product predomi- known material and its relative molecular nates if the reaction is carried out in an al- mass obtained using the value of kB previ- coholic solution and the latter if the ously obtained. solution is aqueous.

eluate See elution.

eluent See elution. Beckmann thermometer elution The removal of an adsorbed substance in a CHROMATOGRAPHY column condenser or ion-exchange column using a solvent (eluent), giving a solution called the eluate. The chromatography column can selec- tube for tively adsorb one or more components introducing solute from the mixture. To ensure efficient re- covery of these components graded elution is used. The eluent is changed in a regular manner starting with a nonpolar solvent and gradually replacing it by a more polar one. This will wash the strongly polar components from the column.

Embden–Meyerhoff pathway See gly- heater colysis.

Elevation of boiling point emission spectrum See spectrum.

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empirical formula

empirical formula See formula. cussion. For example, chemical energy is the kinetic and potential energies of elec- emulsion A colloid in which a liquid trons in a chemical compound. phase (small droplets with a diameter range 10–5–10–7 cm) is dispersed or sus- energy level One of the discrete energies pended in a liquid medium. Emulsions are that an atom, molecule, ion, etc., can have classed as lyophobic (solvent-repelling and according to quantum theory. Thus in an generally unstable) or lyophilic (solvent- atom there are certain definite orbits that attracting and generally stable). the electrons can be in, corresponding to definite electronic energy levels of the enantiomer (enantiomorph) A com- atom. Similarly, a vibrating or rotating pound whose structure is not superimpos- molecule can have discrete vibrational and able on its mirror image; one of any pair of rotational energy levels. optical isomers. See also isomerism; optical activity. energy profile A diagram that traces the changes in the energy of a system during enantiomorph See enantiomer. the course of a reaction. Energy profiles are obtained by plotting the potential energy encephalin See endorphin. of the reacting particles against the reaction coordinate. To obtain the reaction co- enclosure compound See clathrate. ordinate the energy of the total interacting system is plotted against position for the endorphin (encephalin; enkephalin) One molecules. The reaction coordinate is the of a group of peptides produced in the pathway for which the energy is a mini- brain and other tissues that are released mum. after injury and have pain-relieving effects similar to those of opiate alkaloids, such as enkephalin See endorphin. morphine. They include the enkephalins, which consist of just five amino acids. enol An organic compound containing Other larger endorphins occur in the pitui- the C:CH(OH) group; i.e. one in which a tary, while some are polypeptides, found hydroxyl group is attached to one of the mainly in pancreas, adrenal gland, and carbon atoms of a double bond between other tissues. two carbon atoms. See keto–enol tautomerism. endothermic Describing a process in which heat is absorbed (i.e. heat flows enthalpy Symbol: H The sum of the in- from outside the system, or the tempera- ternal energy (U) and the product of pres- ture falls). The dissolving of a salt in water, sure (p) and volume (V) of a system: for instance, is often an endothermic H = U + pV process. Compare exothermic. In a chemical reaction carried out at constant pressure, the change in enthalpy end point See equivalence point; volu- measured is the internal energy change plus metric analysis. the work done by the volume change: ∆H = ∆U + p∆V energy Symbol: W A property of a system; a measure of its capacity to do work. entropy Symbol: S In any system that Energy and work have the same unit: the undergoes a reversible change, the change joule (J). It is convenient to divide energy of entropy is defined as the heat absorbed into kinetic energy (energy of motion) and divided by the thermodynamic tempera- potential energy (‘stored’ energy). Names ture: are given to many different forms of energy δS = δQ/T (chemical, electrical, nuclear, etc.); the only A given system is said to have a certain en- real difference lies in the system under dis- tropy, although absolute entropies are sel-

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equatorial conformation

dom used: it is change in entropy that is cheese and junkets. Enzymes are also used important. The entropy of a system meas- to determine the concentration of reactants ures the availability of energy to do work. or products in specific reactions catalyzed In any real (irreversible) change in a by them. closed system the entropy always increases. Although the total energy of the system has epimerism A form of isomerism exhib- not changed (first law of thermodynamics) ited by carbohydrates in which the isomers the available energy is less – a consequence (epimers) differ in the positions of –OH of the second law of thermodynamics. groups. The α- and β- forms of glucose are The concept of entropy has been epimers. See sugar. widened to take in the general idea of disorder – the higher the entropy, the more epinephrine (adrenaline) A hormone disordered the system. For instance, a produced by the adrenal glands. The mid- chemical reaction involving polymeriza- dle part of these glands, the adrenal tion may well have a decrease in entropy medulla, secretes the hormone, which is because there is a change to a more ordered chemically almost identical to the transmit- system. The ‘thermal’ definition of entropy ter substance norepinephrine produced at is a special case of this idea of disorder – the ends of sympathetic nerves. Epineph- here the entropy measures how the energy rine secretion into the bloodstream in transferred is distributed among the parti- stress causes acceleration of the heart, con- cles of matter. striction of arterioles, and dilation of the pupils. In addition, epinephrine produces a enzyme A macromolecule that catalyzes marked increase in metabolic rate thus biochemical reactions. Enzymes act with a preparing the body for emergency. given compound (the substrate) to produce a complex, which then forms the products epoxide A type of organic compound of the reaction. The enzyme itself is un- containing a three-membered ring contain- changed in the reaction; its presence allows ing two carbon atoms and one oxygen the reaction to take place. The names of atom. most enzymes end in -ase, added to the substrate (e.g. lactase) or the reaction (e.g. epoxyethane (ethylene oxide; C2H4O) A hydrogenase). colorless gaseous cyclic ether. Epoxyethane Enzymes are extremely efficient cata- is the simplest EPOXIDE. It is made by oxi- lysts for chemical reactions, and very spe- dation of ethene over a silver catalyst. The cific to particular reactions. Most enzymes ring is strained and the compound is con- are proteins. They may have a nonprotein sequently highly reactive. It polymerizes to part (cofactor), which may be an inorganic produce epoxy polymers (resins). The com- ion or an organic constituent (coenzyme). pound hydrolyzes to give 1,2-ethanediol The mechanism of action of most enzymes (CH2(OH)CH2(OH)). appears to be by active sites on the enzyme molecule. The substrate acting with the en- epoxy resin See epoxyethane. zyme changes shape to fit the active site, and the reaction proceeds. Enzymes are equation See chemical equation. very sensitive to their environment – e.g. temperature, pH, and the presence of other equation of state An equation that in- substances. Catalytic activity has also been terrelates the pressure, temperature, and found in some RNA molecules. volume of a system, such as a gas. The ideal gas equation (see gas laws) and the van der enzyme technology (enzyme engi- Waals equation are examples of equations neering) A branch of biotechnology that of state. utilizes enzymes for industrial purposes. For example rennet (impure rennin) is equatorial conformation See cyclo- manufactured on a large scale to make hexane.

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equilibrium

equilibrium In a reversible chemical reform compounds with a third element, a action: compound of the first two elements con- A + B ˆ C + D tains them in the relative proportions that The reactants are forming the products: they have in compounds with the third el- A + B → C + D ement. For example, the mass ratio of car- which also react to give the original reac- bon to hydrogen in methane (CH4) is 12:4; tants: the ratio of oxygen to hydrogen in water → C + D A + B (H2O) is 16:2. In carbon monoxide (CO), The concentrations of A, B, C, and D the ratio of carbon to oxygen is 12:16. change with time until a state is reached at which both reactions are taking place at equivalent weight A measure of ‘com- the same rate. The concentrations (or pres- bining power’ formerly used in calcula- sures) of the components are then constant tions for chemical reactions. The – the system is said to be in a state of chem- equivalent weight of an element is the num- ical equilibrium. Note that the equilibrium ber of grams that could combine with or is a dynamic one; the reactions still take displace one gram of hydrogen (or 8 grams place but at equal rates. The relative proof oxygen or 35.5 grams of chlorine). It is portions of the components determine the the relative atomic mass (atomic weight) ‘position’ of the equilibrium, which may be divided by the valence. For a compound displaced by changing the conditions (e.g. the equivalent weight depends on the reac- temperature or pressure). tion considered. An acid, for instance, in acid–base reactions has an equivalent equilibrium constant In a chemical weight equal to its relative molecular mass equilibrium of the type (molecular weight) divided by the number xA + yB ˆ zC + wD of acidic hydrogen atoms. The expression: [A]x[B]y/[C]z[D]w ergosterol A sterol present in plants. It where the square brackets indicate concen- is converted, in animals, to vitamin D by trations, is a constant (K ) when the system 2 c ultraviolet radiation, and is the most im- is at equilibrium. Kc is the equilibrium constant of the given reaction; its units depend portant of vitamin D’s provitamins. on the stoichiometry of the reaction. For gas reactions, pressures are often used in- Erlenmeyer flask A glass laboratory stead of concentration. The equilibrium flask with conical shape and a narrow n neck. It is named for the German chemist constant is then Kp, where Kp = Kc . Here n is the number of moles of product minus Richard Erlenmeyer (1825–1909). the number of moles of reactant; for instance, in essential amino acid See amino acid. ˆ 3H2 + N2 2NH3 n is 2 – (1 + 3) = –2. essential fatty acid A polyunsaturated fatty acid (see carboxylic acid) required for equivalence point The point in a TITRA- growth and health that cannot be synthe- TION at which the reactants have been sized by the body and therefore must be in- added in equivalent proportions, so that cluded in the diet. Linoleic acid and there is no excess of either. It differs (9,12,15)-linolenic acid are the only essen- slightly from the end point, which is the tial fatty acids in humans, being required observed point of complete reaction, be- for cell membrane synthesis and fat me- cause of the effect of the indicator, errors, tabolism. Arachidonic acid is essential in etc. some animals, such as the cat, but in humans it is synthesized from linoleic acid. equivalent proportions, law of (law of Essential fatty acids occur mainly in reciprocal proportions) The principle vegetable-seed oils, e.g. safflower-seed and that when two chemical elements both linseed oils.

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ethanal

essential oil Any pleasant-smelling ethanal (acetaldehyde; CH3CHO) A volatile oil obtained from various plants, water-soluble liquid aldehyde used as a widely used in making flavorings and per- starting material in the manufacture of sev- fumes. Most consist of terpenes and they eral other compounds. Ethanal can be pre- are obtained by steam distillation or sol- pared by the oxidation of ethanol. It is vent extraction. manufactured by the catalytic oxidation of ethyne with oxygen using copper(II) chlo- ester A type of organic compound ride and palladium(II) chloride as catalysts. formed, or regarded as formed, by reaction The mixture of gases is bubbled through an between an alcohol and an acid. If the acid aqueous solution of the catalysts; the reac- is a carboxylic acid, esters have the general tion involves formation of an intermediate formula R1COOR2, where R1 and R2 are organometallic complex with Pd2+ ions. alkyl or aryl groups. For example, ethanol With dilute acids ethanal polymerizes to ethanal trimer (C O H (CH ) , formerly (C2H5OH) reacts with ethanoic acid 3 3 3 3 3 called paraldehyde), which is a sleep-in- (acetic acid; CH3COOH) to give the ester ducing drug. Below 0°C ethanal tetramer is ethyl ethanoate (C2H5OCOCH3) along with water: formed (C4O4H4(CH3)4, formerly called ˆ metaldehyde), which is used as a slug poi- C2H5OH + CH3COOH son and a fuel in small portable stoves. C2H5OCOCH3 + H2O. Methanol reacts with propanoic acid to give methyl propanoate: CH ˆ 3 CH3OH + C2H5COOH CH3OCOC2H5 + H2O This type of reaction, called esterification, CH is reversible and, in preparing esters, the equilibrium can be displaced toward the O O ester by using a large excess of alcohol or acid. It can also be displaced by distilling off the water or by removing it with a de- HC CH hydrating agent (e.g. sulfuric acid). Esters CH O CH can also be made from alcohols with ACYL 3 3 HALIDES or ACID ANHYDRIDES. Ethanal trimer The reverse reaction of esterification is

hydrolysis. Both esterification and ester hy- CH3 drolysis are acid-catalyzed. The mechanism involves protonation of the oxygen of the carbonyl group, allowing nucleophilic CH attack by water or alcohol at the carbon atom of the carbonyl group. Ester forma- O O tion cannot be base-catalyzed but the hydrolysis can be catalyzed by OH– ions, H3C CH HC CH 3 which attack the carbon atom of the carbonyl group. This type of hydrolysis is known as saponification (because it is the O O reaction used to make SOAP from fats and CH oils). Simple esters are volatile compounds, often with pleasant odors. They are used as CH3 flavorings. Esters of triols occur as fats and oils. See glyceride. Ethanal tetramer

esterification See ester. Ethanal: polymer forms of ethanal

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ethanimide

ethanimide (acetamide; CH3CONH2) A ume) the reaction stops because the yeast is colourless solid crystallizing in the form of killed. Higher concentrations of alcohol long white crystals with a characteristic are produced by distillation. smell of mice. It is made by the dehydration Apart from its use in drinks, alcohol of ammonium ethanoate or by the action is used as a solvent and to form ethanal. of ammonia on ethanoyl chloride, ethanoic Formerly, the main source was by fermen- anhydride, or ethyl ethanoate. tation of molasses, but now catalytic hydration of ethene is used to manufacture ethane (C2H6) A gaseous alkane ob- industrial ethanol. tained either from the gaseous fraction of crude oil or by the ‘cracking’ of heavier ethanoyl chloride (acetyl chloride; fractions. Ethane is the second member of CH3COCl) A liquid acyl chloride used as the homologous series of alkanes. an acetylating agent.

ethanedioic acid (oxalic acid; (COOH)2) ethanoyl group (acetyl group) The A white crystalline organic acid that occurs group RCO–. naturally in rhubarb, sorrel, and other plants of the genus Oxalis. It is slightly sol- ethene (ethylene; C2H4) A gaseous uble in water, highly toxic, and used in alkene. Ethene is not normally present in dyeing and as a chemical reagent. the gaseous fraction of crude oil but can be obtained from heavier fractions by cat- ethane-1,2-diol (ethylene glycol; glycol; alytic cracking. This is the principal indus- CH2(OH)CH2(OH)) A syrupy organic trial source. The compound is important as liquid commonly used as antifreeze and as a starting material in the organic-chemicals a starting material in the manufacture of industry (e.g. in the manufacture of Dacron. The compound is manufactured ethanol) and as the starting material for the from ethene by oxidation over suitable cat- production of polyethene. Ethene is the alysts to form epoxyethane, with subse- first member of the homologous series of quent hydrolysis to the diol. alkenes.

ethanoate (acetate) A salt or ester of ether A type of organic compound con- ethanoic acid (acetic acid). See ethanoic taining the group –O–. Simple ethers have acid. the formula R1–O–R2, where R1 and R2 are alkyl or aryl groups, which may or may not ethanoic acid (acetic acid; CH3COOH) be the same. They are either gases or very A colorless viscous liquid organic acid with volatile liquids and are very flammable. a pungent odor (it is the acid in vinegar). The commonest example is ethoxyethane ° Below 16.7 C it solidifies to a glassy solid (diethylether; C2H5OC2H5) used formerly (glacial ethanoic acid). It is made by the ox- as an anesthetic. Ethers now find applica- idation of ethanol or butane, or by the con- tion as solvents. They are prepared in the tinued fermentation of beer or wine. It is laboratory by the dehydration of alcohols made into ethenyl ethanoate (vinyl acetate) with concentrated sulfuric acid. An excess for making polymers. Cellulose ethanoate of alcohol is used to ensure that only one (acetate) is made from ethanoic anhydride. molecule of water is removed from each See cellulose acetate. pair of alcohol molecules. They are generally unreactive, but the C–O bond can be ethanol (ethyl alcohol; alcohol; C2H5OH) cleaved by reaction with HI or PCl5.

A colorless volatile liquid alcohol. Ethanol

. occurs in intoxicating drinks, in which it is .. . produced by fermentation of a sugar: O → 1 2 C6H12O6 2C2H5OH + 2CO2 R R Yeast is used to cause the reaction. At about 15% alcohol concentration (by vol- Ether

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excited state

ethoxyethane (ether; diethylether; C2H5- in oxy-acetylene welding torches, since its OC2H5) A colorless volatile liquid. Ether combustion with oxygen produces a flame is well known for its characteristic smell of very high temperature. It is also impor- and anesthetic properties, also for its ex- tant in the organic chemicals industry for treme flammability. It still finds some ap- the production of chloroethene (vinyl chlo- plication as an anesthetic when more ride), which is the starting material for the modern materials are unsuitable; it is also production of polyvinyl chloride (PVC), an excellent solvent. Its manufacture is an and for the production of other vinyl com- extension of the laboratory synthesis: pounds. Formerly, ethyne was manufac- ethanol vapor is passed into a mixture of tured by the synthesis and subsequent excess ethanol and concentrated sulfuric hydrolysis of calcium dicarbide. Modern acid at 140°C: methods increasingly employ the cracking C H OH + H SO → 2 5 2 4 of alkanes. C2H5.O.SO2.OH + H2O C H O.SO .OH + C H OH → 2 5 2 2 5 ethynide See carbide. C2H5OC2H5 + H2SO4

ethyl acetate See ethyl ethanoate. eudiometer An apparatus for the volumetric analysis of gases. ethyl alcohol See ethanol. evaporation 1. A change of state from liquid to gas (or vapor). Evaporation can ethylamine (aminoethane; C2H5NH2) A colorless liquid amine. It can be prepared take place at any temperature, the rate in- from chloroethane heated with concen- creasing with temperature. Some molecules trated aqueous ammonia: in the liquid have enough energy to escape → C2H5Cl + NH3 C2H5NH2 + HCl into the gas phase (if they are near the sur- It is used in manufacturing certain dyes. face and moving in the right direction). Be- cause these are the molecules with higher ethyl bromide See bromoethane. kinetic energies, evaporation results in a cooling of the liquid. ethyl carbamate See urethane. 2. A change from solid to vapor, especially occurring at high temperatures close to the ethyl chloride See chloroethane. melting point of the solid. Thin films of metal can be evaporated onto a surface in ethylene See ethene. this way. ethylenediamine tetraacetic acid See exa- Symbol: E A prefix denoting 1018. edta. excitation The process of producing an ethylene glycol See ethane-1,2-diol. excited state of an atom, molecule, etc. ethylene oxide See epoxyethane. excitation energy The energy required to change an atom, molecule, etc. from one ethyl ethanoate (ethyl acetate; C2H5- quantum state to a state with a higher en- OOCCH3) An ester formed from ethanol and ethanoic acid. It is a fragrant liquid ergy. The excitation energy (sometimes used as a solvent for plastics and in flavor- called excitation potential) is the difference ing and perfumery. between two energy levels of the system.

ethyl iodide See iodoethane. excited state A state of an atom, molecule, or other system, with an energy

ethyne (acetylene; C2H2) A gaseous greater than that of the ground state. Com- alkyne. Traditionally ethyne has found use pare ground state.

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exclusion principle

exclusion principle The principle, 2. Initiators which are very sensitive and enunciated by the Austrian–Swiss physicist are used in small amounts to detonate Wolfgang Pauli (1900–58) in 1925, that no less sensitive explosives. two electrons in an atom can have an iden- 3. High explosives which need an initiator, tical set of quantum numbers. but are very powerful.

exothermic Denoting a chemical reac- E–Z convention A convention for the tion in which heat is evolved (i.e. heat description of a molecule showing cis-trans flows from the system or the temperature ISOMERISM. In a molecule ABC=CDE, rises). Combustion is an example of an where A, B, D, and E are different groups, exothermic process. Compare endother- the sequence rule (see CIP system) is ap- mic. plied to the pair A and B to find which has explosive A substance or mixture that priority and it is similarly applied to the can rapidly decompose upon detonation pair C and D. If the two groups of highest producing large amounts of heat and gases. priority are on the same side of the bond The three most important classes of explo- then the isomer is designated Z (from Ger- sives are: man zusammen, together). If they are on 1. Propellants which burn steadily, and are opposite sides the isomer is designated E used as rocket fuels. (German entgegen, opposite).

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FAD (flavin adenine dinucleotide) A de- hydroxide. It is named for the German rivative of riboflavin that is a coenzyme chemist H. C. von Fehling (1812–85). in electron-transfer reactions. Its reduced form is written as FADH2. See also flavo- femto- Symbol: f A prefix denoting protein. 10–15. For example, 1 femtometer (fm) = 10–15 meter (m). Fahrenheit scale A temperature scale in which the ice temperature is taken as 32° fermentation A chemical reaction pro- and the steam temperature is taken as 212° duced by microorganisms (molds, bacteria, (both at standard pressure). The scale is or yeasts). A common example is the for- not used for scientific purposes. To convert mation of ethanol from sugars: → between degrees Fahrenheit (F) and de- C6H12O6 2C2H5OH + 2CO2 grees Celsius (C) the formula C/5 = (F – 32)/9 is used. It is named for the German ferredoxins A group of red-brown pro- physicist Gabriel Daniel Fahrenheit (1686– teins found in green plants, many bacteria 1736) who proposed a scale of this type in and certain animal tissues. They contain nonheme iron in association with sulfur at 1714. the active site. They are strong reducing agents (very negative redox potentials) and faraday Symbol: F A unit of electric function as electron carriers, for example charge equal to the charge required to dis- in photosynthesis and nitrogen fixation. charge one mole of a singly-charged ion. They have also been isolated from mito- One faraday is 9.648 670 × 104 coulombs. chondria. The unit is named for the British chemist and physicist Michael Faraday (1791– ferrocene (Fe(C5H5)2) An orange crys- 1867). talline solid. It is an example of a sandwich compound, in which an iron(II) ion is co- fat See glyceride. ordinated to two cyclopentadienyl ions. The bonding involves overlap of d orbitals fatty acid See carboxylic acid. on the iron with the pi electrons in the cyclopentadienyl ring. The compound can Fehling’s solution A solution used to undergo substitution reactions on the test for the aldehyde group (–CHO). It is a rings, which have aromatic character. The freshly made mixture of copper(II) sulfate systematic name is di-π-cyclopentadienyl solution with alkaline potassium sodium iron(II). 2,3-dihydroxybutanedioate (tartrate). The aldehyde, when heated with the mixture, is filler A solid material used to modify the oxidized to a carboxylic acid, and a red physical properties or reduce the cost of precipitate of copper(I) oxide and copper synthetic compounds, such as rubbers, metal is produced. The tartrate is present plastics, paints, and resins. Slate powder, to complex with the original copper(II) glass fiber, mica, and cotton are all used as ions to prevent precipitation of copper(II) fillers.

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filter

filter See filtration. rate = k[radioactive material] For a first-order reaction, the time for a filter pump A type of vacuum pump in definite fraction of the reactant to be con- which a jet of water forced through a noz- sumed is independent of the original con- zle carries air molecules out of the system. centration. The units of k, the RATE Filter pumps cannot produce pressures CONSTANT, are s–1. below the vapor pressure of water. They are used in the laboratory for vacuum fil- Fischer, Emil Hermann (1852–1919) tration, distillation, and similar techniques German organic chemist. Fischer studied requiring a low-grade vacuum. many compounds of biological interest. He is sometimes referred to as the father of filtrate See filtration. biochemistry. In 1874 he discovered phenylhydrazine. He studied peptides, filtration The process of removing sus- purines and sugars very thoroughly. His pended particles from a fluid by passing or work on purines (a name he coined) led to forcing the fluid through a porous material the synthesis of many compounds such as (the filter). The fluid that passes through caffeine and purine. In his early work he the filter is the filtrate. In laboratory filtra- put forward incorrect structures but by tion, filter paper or sintered glass is com- 1897 he and his colleagues had established monly used. the correct structures. Fischer was awarded the 1902 Nobel Prize for chemistry for his fine organic chemicals Carbon com- work on purines and sugars. pounds, such as dyes and drugs, that are produced only in small quantities. Their Fischer, Hans (1881–1945) German main requirement is that they must have a organic chemist. Fischer devoted his career high degree of purity, often higher than to the study of the molecular structures of 95%. They are manufactured for special the biologically significant molecules purposes, e.g. for use in spectroscopy, hemoglobin, chlorophyll and the bile pig- pharmacology, and electronics. ment bilirubin. Fischer started investigating hemoglobin in 1921. He showed that fine structure Closely spaced lines seen at high resolution in a spectral line or band. the iron-containing nonprotein part con- Fine structure may be caused by vibration sists of four pyrrole rings surrounding an of the molecules or by electron spin. Hy- iron atom. He synthesized this part by perfine structure, seen at very high resolu- 1929 and thoroughly investigated the portion, is caused by the atomic nucleus phyrins. He won the 1930 Nobel prize for affecting the possible energy levels of the chemistry for this work. He then investi- atom. gated the chlorophylls and demonstrated that they are substituted porphins sur- firedamp Methane present in coal rounding a magnesium atom. He also mines. demonstrated that bile acids are degraded porphins. In 1944 he synthesized bilirubin first-order reaction A reaction in completely. which the rate of reaction is proportional to the concentration of one of the reacting Fischer projection A way of represent- substances. The concentration of the reacting the three-dimensional structure of a ing substance is raised to the power one; molecule in two dimensions. The molecule i.e. rate = k[A]. For example, the decompo- is drawn using vertical and horizontal sition of hydrogen peroxide is a first-order lines. Horizontal lines represent bonds that reaction: come out of the paper. Vertical lines repre- rate = k[H2O2] sent bonds that go into the paper (or are in Similarly the rate of decay of radioactive the plane of the paper). Named for Emil material is a first-order reaction: Fischer, the convention was formerly used

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flavonol

for representing the absolute configuration flavin adenine dinucleotide See FAD. of SUGARS. flavin mononucleotide See FMN. Fischer–Tropsch process A method of making a mixture of hydrocarbons using flavone See flavonoid. hydrogen and carbon monoxide (2:1 ratio) passed over a nickel or cobalt catalyst at a O temperature of 200°C. The mixture, which also contains alcohols and carbonyl compounds, can be distilled to make fuels for diesel and gasoline engines. It was used for O this by Germany in World War II. Now the process is one way of making SNG. It is named for the German chemist Franz Fis- Flavonoid cher (1852–1932) and the Czech chemist Hans Tropsch (1839–1935), who invented flavonoid One of a common group of it in 1933. plant compounds having the C6–C3–C6 chemical skeleton in which C6 is a benzene Fittig reaction See Wurtz reaction. ring. They are an important source of non- photosynthetic pigments in plants. They flame-ionization detector See gas are classified according to the C3 portion chromatography. and include the yellow chalcones and au- rones; the pale yellow and ivory flavones and flavonols and their glycosides; the red, flare stack A chimney at the top of blue, and purple anthocyanins and antho- which unwanted gases are burnt in an oil cyanidins; and the colorless isoflavones, refinery or other chemical plant. catechins, and leukoanthocyanidins. They are water soluble and usually located in the flash photolysis A technique for inves- cell vacuole. See anthocyanin. tigating free radicals in gases. The gas is held at low pressure in a long glass or quartz tube, and an absorption spectrum O taken using a beam of light passing down OH the tube. The gas is subjected to a very brief intense flash of light from a lamp outside O the tube, producing free radicals, which are identified by their spectra. Measurements of the intensity of spectral lines can be Flavonol made with time using an oscilloscope, and the kinetics of very fast reactions can thus be investigated. flavonol A plant pigment that modifies the effects of certain growth substances. flash point The lowest temperature at See flavonoid. which sufficient vapor is given off by a flammable liquid to ignite in the presence of a spark. See also ignition temperature. O

flavanone A type of flavonoid. Fla- vanone glycosides are found in flowering plants. O

flavin A derivative of riboflavin occurring in the flavoproteins; i.e. FAD or FMN. Flavanone

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flavoprotein

flavoprotein A conjugated protein in lower energy states. Compare phosphores- which a flavin (FAD or FMN) is joined to cence. a protein component. Flavoproteins are enzymes in the electron-transport chain. fluoridation The introduction of small quantities of fluoride compounds into the flocculation (coagulation) The combin- water supply as a public-health measure to ing of the particles of a finely divided pre- reduce the incidence of tooth decay. cipitate, such as a colloid, into larger particles or clumps that sink and are easier fluoride See halide. to filter off. fluorination See halogenation. flocculent Describing a precipitate that has aggregated in wooly masses. fluorine A slightly greenish-yellow highly reactive gaseous element belonging Flory, Paul John (1910–85) American to the halogens (group 17 of the periodic polymer chemist. Flory’s early work con- table, formerly VIIA). It occurs notably as sisted of helping Wallace CAROTHERS to de- fluorite (CaF2) and cryolite (Na3AlF3) but velop nylon and neoprene. He began to traces are also widely distributed with investigate the properties of polymers in other minerals. It is slightly more abundant the 1930s. Flory solved the difficulty that a than chlorine, accounting for about polymer molecule does not have a fixed 0.065% of the Earth’s crust. The high re- size and structure by using statistical tech- activity of the element delayed its isolation. niques to calculate a distribution of poly- Fluorine is now prepared by electrolysis of mer chain lengths. Flory also worked on molten KF/HF electrolytes, using copper or polymers in which there are links between steel apparatus. Its preparation by conven- chains. This led to work on the elasticity of tional chemical methods is impossible. rubber. Flory summarized his work in the Fluorine is strongly electronegative and classic books Principles of Polymer Chem- exhibits a strong electron withdrawing ef- istry (1953) and Statistical Mechanics of fect on adjacent bonds, thus CF3COOH is Chain Molecules (1969). Flory won the a strong acid (whereas CH3COOH is not). 1974 Nobel Prize for chemistry for his Fluorine and hydrogen fluoride are ex- work on polymers. tremely dangerous and should only be used in purpose-built apparatus; gloves and face fluid A state of matter that is not a solid shields should be used when working with – that is, a liquid or a gas. All fluids can hydrofluoric acid and accidental exposure flow, and the resistance to flow is the vis- should be treated as a hospital emergency. cosity. Symbol: F; b.p. –188.14°C; m.p. –219.62°C; d. 1.696 kg m–3 (0°C); p.n. 9; fluidization The suspension of a finely- r.a.m. 18.99840.32. divided solid in an upward-flowing liquid or gas. This suspension mimics many prop- fluorocarbon A compound derived erties of liquids, such as allowing objects to from a hydrocarbon by replacing hydrogen ‘float’ in it. Fluidized beds so constructed atoms with fluorine atoms. Fluorocarbons are important in the chemical industry. are unreactive and most are stable up to high temperatures. They have a variety of fluorescein A fluorescent dye used as an uses – in aerosol propellants, oils and absorption indicator. It has a yellow solu- greases, and synthetic polymers such as tion with green fluorescence. PTFE. See also halocarbon.

fluorescence The absorption of energy fluxional molecule A molecule in by atoms, molecules, etc., followed by im- which the constituent atoms change their mediate emission of electromagnetic radia- relative positions so quickly at room tem- tion as the particles make transitions to perature that the normal concept of struc-

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formula

ture is inadequate; i.e. no specific structure molecular formula gives the numbers and exists for longer than about 10–2 second types of atom present. For example, and the relative positions become indistin- ethanoic acid (acetic acid) has the molecu- ° guishable. For example ClF3 at –60 C has lar formula C2H4O2. The empirical for- a distinct ‘T’ shape but at room tempera- mula gives the simplest ratios of atoms. ture the fluorine atoms are visualized as Thus, the empirical formula of ethanoic moving rapidly over the surface of the acid is CH2O. This is the formula that chlorine atom in a state of exchange and would be obtained by experimental deter- are effectively identical. mination of the amounts of each element present. The molecular formula can then FMN (flavin mononucleotide) A derivative of riboflavin that is a coenzyme in electron-transfer reactions. See also flavoprotein. butane

foam A dispersion of bubbles of gas in a liquid, usually stabilized by a SURFACTANT. Solid foams, such as expanded polystyrene OH or foam rubber, are made by allowing liquid foams to set. propanol

folic acid (pteroylglutamic acid) One of the water-soluble B-group of vitamins. The principal dietary sources of folic acid are leafy vegetables, liver, and kidney. Defi- ciency of the vitamin exhibits itself in ane- butadiene mia in a similar manner to vitamin B12 deficiency, while deficiency during preg- nancy increases the risk of birth defects in children. O Folic acid is important in metabolism in various coenzyme forms, all of which are specifically concerned with the transfer OH and utilization of the single carbon (C1) ethanoic acid group. Before functioning in this manner folic acid must be reduced to either dihy- drofolic acid (FH2) or tetrahydrofolic acid (FH4). It is important in the growth and re- production of cells, participating in the synthesis of purines and thymine. See also vitamin B complex.

cyclohexane formaldehyde See methanal.

formalin See methanal.

formate See methanoate.

formic acid See methanoic acid.

formula A representation of a molecule benzene using symbols for the atoms. Subscripts in- Formula: representations of some simple dicate the numbers of atoms present. The compounds

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formyl group

HOCH 2 Often a general formula is used to rep- O OH resent a class of compounds. For instance OH CnH2n for alkenes. It is also common to use HO the symbol R for an organic group. So RCOOH is any carboxylic acid. When two OH different groups are needed, R and R′ are used (or R1, R2, R3, etc.). Ar is sometimes Formula: representation of the β-D-anomer of glucose used for any aryl group.

be obtained if the relative molecular mass formyl group The group HCO–. is known. More information is given by the struc- fossil fuel A mineral fuel that forms un- tural formula, which shows how the atoms derground from the remains of living or- are joined together. The formula of ganisms. Fossil fuels include coal, natural ethanoic acid is usually written as CH3- gas, peat, and petroleum. COOH, showing that it is formed from a methyl group (CH3–) and a carboxylate fraction A mixture of liquids with simi- group (–COOH). Sometimes full stops are lar boiling points collected by fractional used in such formulae to divide up the distillation. groups. Often it is necessary to show a ring compound or to show the disposition of fractional crystallization Crystalliza- the atoms or groups in space (see isomer; tion of one component from a mixture in optical activity). In such cases a diagram of solution. When two or more substances are the structure has to be given. present in a liquid (or in solution), on cool- In certain cases symbols are used for ing to a lower temperature one substance groups of atoms. Common ones are Me for will preferentially form crystals, leaving methyl, Et for ethyl, Pr for propyl, Bu for the other substance in the liquid (or dis- butyl, and Ph for phenyl. These symbols solved) state. Fractional crystallization can are sometimes called organic elements. So, thus be used to purify or separate sub- for example, ethanol is EtOH, phenol is stances if the correct conditions are PhOH, and ethanoic acid is MeCOOH. known. In representing three-dimensional structures certain conventions are used. fractional distillation (fractionation) A Particular types of projection formulae are distillation carried out with partial reflux, used for certain types of compound. For using a long vertical column (fractionating example, the FISCHER PROJECTION has been column). It utilizes the fact that the vapor extensively used for representing the open- phase above a liquid mixture is generally chain form of sugars. The NEWMAN PROJEC- richer in the more volatile component. If TION is used for discussions of rotation the region in which refluxing occurs is suf- about C–C single bonds (see also confor- ficiently long, fractionation permits the mation). More generally, it is conventional complete separation of two or more to use a straight line for a single bond in the volatile liquids. Fractionation is the funda- plane of the paper. A bond coming out of mental process for producing petroleum the paper is represented as a solid narrow from crude oil. wedge, intended to give the impression of Unlike normal reflux, the fractionating perspective. A bond into the paper is repre- column may be insulated to reduce heat sented by a dotted or dashed line. Also, or- loss, and special designs are used to maxi- ganic chemists commonly represent mize the liquid-vapour interface. structures without the C or H atoms, except where these appear in functional fractionation See fractional distillation. groups. A hydrocarbon chain is drawn as a zig-zag line and the BENZENE ring is drawn Frankland, Sir Edward (1825–99) as one of the Kekulé structures. British chemist. Frankland is best known

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frontier orbital

for introducing the concept of what is now freezing mixtures Two or more sub- known as valence. In 1852 he noticed that stances mixed together to produce a low nitrogen and phosphorus frequently form temperature. A mixture of sodium chloride compounds in which there are either three and ice in water (–20°C) is a common ex- or five atoms of the other elements. This ample. suggested to Frankland that each element has a definite combining power which is freezing point The temperature at satisfied by a certain number of atoms. He which a liquid is in equilibrium with its elaborated what came to be known as the solid phase at standard pressure and below theory of valence in 1866. In 1864 Frank- which the liquid freezes or solidifies. This land pointed out that the carboxyl group temperature is always the same for a particular liquid and is numerically equal to (CO2H) is present in many organic acids. Frankland was also concerned with tech- the melting point of the solid. See also de- nological applications of chemistry, no- pression of freezing point. tably to coal gas and to water purification. Freon (Trademark) Any of a number of Franklin, Rosalind (1920–58) British chlorofluorocarbons (CFCs) and fluoro- x-ray crystallographer. Rosalind Franklin carbons used as refrigerants. See fluorocar- is best known for having played a key role bon; halocarbon. in the discovery of the structure of DNA and for having been portrayed in an un- Friedel–Crafts reaction A type of reaction in which an alkyl or acyl group is sub- flattering way by James WATSON in his stituted on a benzene ring. In Friedel– book The Double Helix (1968). Her early Crafts alkylation the reactant is a work was on coal. Her x-ray photographs haloalkane, and an alkylbenzene is proof DNA in 1952 led Francis CRICK and duced: Watson to postulate the double helix struc- CH Cl + C H → C H CH + HCl ture of DNA. Franklin subsequently con- 3 6 6 6 5 3 In Friedel–Crafts acylation the reactant is firmed the double helix picture using x-ray an acyl halide and the product is an aro- crystallography. In her later years she matic ketone: worked on tobacco mosaic virus. Her early CH COCl + C H → death prevented the possibility of her win- 3 6 6 C6H5COCH3 + HCl ning a Nobel Prize for her work on DNA. These reactions occur at about 100°C using aluminum chloride as a catalyst. This free energy A measure of the ability of a accepts a lone pair of electrons from the system to do useful work. See Gibbs func- halogen atom on the haloalkane or acyl tion; Helmholtz function. halide, which results in a positive charge on the adjoining carbon atom. The reaction is free radical An atom or group of atoms then ELECTROPHILIC SUBSTITUTION. It is also with a single unpaired electron. Free radi- possible to use alkenes (for alkylation) and cals are produced by breaking a covalent acid anhydrides (for acylation). It is named bond; for example: for the French chemist Charles Friedel → CH3Cl CH3• + Cl• (1832–99) and the US chemist James They are often formed in light-induced Crafts (1832–99). reactions. Most free radicals are extremely reactive and can be stabilized and isolated frontier orbital Either of two orbitals in only under special conditions. They can be a molecule: the highest occupied molecular studied by electron spin resonance. See also orbital (the HOMO) or the lowest unoccu- carbene. pied molecular orbital (the LUMO). The HOMO is the orbital with the highest en- freezing The process by which a liquid is ergy level occupied at absolute zero tem- converted into a solid by cooling; the re- perature. The LUMO is the lowest-energy verse of melting. unoccupied orbital at absolute zero. For a

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fructan

CH3

+ CH3CI

benzene chloromethane methylbenzene (toluene)

Friedel–Crafts methylation O

C CH3 + CH3COCI

benzene ethanoyl chloride phenyl methyl ketone

Friedel-Crafts acetylation

Friedel–Crafts reactions

particular molecule the nature of these two form, hydrogen gas and oxygen gas are fed orbitals is very important in determining to the surfaces of two porous nickel elec- the chemical properties. Frontier-orbital trodes immersed in potassium hydroxide theory, which considers the symmetry of solution. The oxygen reacts to form hy- these orbitals, has been very successful in droxyl (OH–) ions, which it releases into explaining such reactions as the DIELS– the solution, leaving a positive charge on ALDER REACTION. See also Woodward– the electrode. The hydrogen reacts with the Hoffmann rules. OH– ions in the solution to form water, giving up electrons to leave a negative fructan A polysaccharide made entirely charge on the other electrode. Large fuel of fructose residues. They are used as food cells can generate tens of amperes. Usually stores in many plants. the e.m.f. is about 0.9 volt and the efficiency around 60%. fructose (fruit sugar; C6H12O6)A SUGAR found in fruit juices, honey, and cane Fukui, Kenichi (1918–98) Japanese sugar. It is a ketohexose, existing in a pyra- physical and theoretical chemist. Fukui is nose form when free. In combination (e.g. best known for his work on frontier orbital in sucrose) it exists in the furanose form. theory, a theory which describes the fruit sugar See fructose. changes in molecular orbitals during a chemical reaction. He was particularly in- fucoxanthin A xanthophyll pigment of terested in applying frontier orbital theory diatoms, brown algae, and golden brown to the reactions of methyl radicals. He algae. The light absorbed is used with shared the 1981 Nobel Prize for chemistry high efficiency in photosynthesis, the with Roald HOFFMANN for his work on energy first being transferred to chloro- frontier orbital theory. He also studied the phyll a. It has three absorption peaks cov- reaction between nitrogen molecules and ering the blue and green parts of the transition metal complexes. spectrum. fullerene See buckminsterfullerene. fuel cell A type of cell in which fuel is converted directly into electricity. In one fullerite See buckminsterfullerene.

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fusion

fuller’s earth A natural clay used as an furan (furfuran; C4H4O) A heterocyclic absorbent and industrial catalyst. liquid organic compound. Its five-membered ring contains four carbon atoms and fumaric acid See butenedioic acid. one oxygen atom. The structure is characteristic of some monosaccharide sugars (fu- functional group A group of atoms in a ranoses). compound that is responsible for the characteristic reactions of the type of compound. Examples are: furanose A SUGAR that has a five- alcohol –OH membered ring (four carbon atoms and alkoxide –OR one oxygen atom). aldehyde –CHO amide –CO–NH2 furfuran See furan. amine –NH2 ketone =CO fused Describing a solid that has been carboxylic acid –CO.OH melted and solidified into a single mass. ester –CO–OR Fused silica, for example, is produced by acyl halide –CO.X (X = halogen) melting sand. nitro compound –NO2 sulfonic acid –SO2.OH fused ring See ring. nitrile –CN diazonium salt –N + 2 fusel oil A mixture of high-molecular- diazo compound –N=N– weight alcohols together with some esters fundamental units The units of length, and fatty acids, formed from alcoholic fer- mass, and time that form the basis of most mentation and obtained during distillation. systems of units. In SI, the fundamental It is used as a source of higher alcohols. units are the meter, the kilogram, and the second. fusion Melting.

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GA3 See gibberellic acid gent sample is passed through a detector, which registers the presence of the different GAG See glycosaminoglycan. components in the carrier gas. Two types of detector are in common galactose (C6H12O6)A SUGAR found in use: the katharometer, which measures lactose and many polysaccharides. It is an changes in thermal conductivity, and the aldohexose, isomeric with glucose. flame-ionization detector, which turns the volatile components into ions and registers gas The state of matter in which forces the change in electrical conductivity. Gas of attraction between the particles of a sub- chromatography is also used in other tech- stance are small. The particles have free- niques to identify the separated compo- dom of movement and gases and have no nents, as in gas chromatography–mass fixed shape or volume. The atoms or mol- spectroscopy (GCMS) and gas chromatog- ecules of a gas are in a continual state of raphy infrared (GCIR). motion and are continually colliding with each other and with the walls of the con- gas chromatography infrared See gas taining vessel. These collisions with the chromatography. walls create the pressure of a gas. gas chromatography–mass spectros- gas chromatography A type of CHRO- copy See gas chromatography. MATOGRAPHY widely used for the separation and analysis of mixtures. Gas gas constant (universal gas constant) chromatography employs a column packed Symbol: R The universal constant with either a solid stationary phase 8.314 34 J mol–1 K–1 appearing in the (gas–solid chromatography or GSC) or a equation of state for an ideal gas. See gas solid coated with a nonvolatile liquid laws. (gas–liquid chromatography or GLC). The whole column is placed in a thermostatic- gas equation See gas laws. ally controlled heating jacket. A volatile sample is introduced into the column using gas laws Laws relating the temperature, a syringe, and a carrier gas, such as hydro- pressure, and volume of a fixed mass of gen or nitrogen, is passed through it. The gas. The main gas laws are BOYLE’S LAW components of the sample will be carried and CHARLES’ LAW. The laws are not along in this mobile phase. Some of the obeyed exactly by any real gas, but many components will cling more readily to the common gases obey them under certain stationary phase than others, either be- conditions, particularly at high tempera- cause they become attached to the solid tures and low pressures. A gas that would surface or because they dissolve in the liq- obey the laws over all pressures and tem- uid. The time taken for different compo- peratures is a perfect or ideal gas nents to pass through the column is Boyle’s and Charles’ laws can be com- characteristic of a particular compound bined into an equation of state for ideal and can be used to identify it. The emer- gases:

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GCMS

pVm = RT cyanide rather than carbon monoxide. where Vm is the molar volume and R the Typically, a mixture of zinc cyanide and molar gas constant. For n moles of gas hydrochloric acid is used, to give zinc chlo- pV = nRT ride (which acts as a Lewis acid) and hy- All real gases deviate to some extent drogen cyanide. The electrophile in this from the gas laws, which are applicable case is protonated hydrogen cyanide: → + only to idealized systems of particles of HCN + HCl + ZnCl2 HCNH + – negligible volume with no intermolecular ZnCl3 forces. There are several modified equa- The phenol is first substituted to give an tions of state that give a better description IMINE: of the behavior of real gases, the best → C6H5OH HOC6H4CH=NH known being the VAN DER WAALS EQUATION. This then hydrolyzes to the aromatic aldehyde: gas–liquid chromatography See gas → HOC6H4CH=NH2 HOC6H4CHO chromatography. Similar reactions using alkyl cyanides (nitriles) rather than hydrogen cyanide give gasohol Alcohol (ethanol) obtained by aromatic ketones. This type of reaction, in the industrial fermentation of sugar for use which a cyanide is used to produce an alde- as a motor fuel. It has been produced on a hyde (or ketone) is often called the Gatter- large scale in Brazil. man reaction. The Gatterman–Koch reaction was reported by the German gas oil One of the main fractions ob- chemist Ludwig Gatterman (1860–1920) tained from petroleum by distillation, used in 1897 (with J. C. Koch). The use of hy- as a fuel for diesel engines. See diesel fuel; drogen cyanide was reported by Gatter- petroleum. man in 1907. gasoline See petroleum. Gatterman reaction 1. See Sandmeyer gas–solid chromatography See gas reaction. chromatography. 2. See Gatterman–Koch reaction.

Gatterman–Koch reaction A reaction gauche conformation See conforma- for substituting a formyl group (HCO–) tion. into a benzene ring of an aromatic hydrocarbon. It is used in the industrial produc- gauss Symbol: G The unit of magnetic tion of benzaldehyde from benzene: flux density in the c.g.s. system. It is equal → to 10–4 tesla. C6H6 C6H5CHO The aromatic hydrocarbon is mixed with a Lewis acid, such as aluminum chloride, Gay-Lussac’s law 1. The principle that and a mixture of carbon monoxide and the gases react in volumes that are in simple ra- hydrogen chloride is passed through. The tios to each other and to the products if first stage if the production of a H–C≡O+ they are gases (all volumes measured at the ion: same temperature and pressure). The law → + – HCl + CO + AlCl3 HCO + AlCl4 was first put forward in 1808 by the Copper(I) chloride (CuCl) is also added as French chemist and physicist Joseph-Louis a catalyst. The HCO+ ion acts as an elec- Gay Lussac (1778–1850). trophile in electrophilic substitution on the 2. See Charles’ law. benzene ring. The Gatterman–Koch reaction is used GCIR (gas chromatography infrared) See for introducing the HCO– group into hy- gas chromatography. drocarbons. A variation of the reaction in which the HCO– group is substituted into GCMS (gas chromatography–mass spec- the benzene ring of a phenol uses hydrogen troscopy) See gas chromatography.

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gel

gel A lyophilic colloid that is normally isolated, the first and one of the most com- stable but may be induced to coagulate mon being gibberellic acid, GA3. partially under certain conditions (e.g. lowering the temperature). This produces a Gibbs free energy See Gibbs function. pseudo-solid or easily deformable jellylike mass, called a gel, in which intertwining Gibbs function (Gibbs free energy) Sym- particles enclose the whole dispersing bol: G A thermodynamic function defined medium. Gels may be further subdivided by into elastic gels (e.g. gelatin) and rigid gels G = H – TS (e.g. silica gel). where H is the enthalpy, T the thermodynamic temperature, and S the entropy. It is gelatin (gelatine) A pale yellow protein useful for specifying the conditions of obtained from the bones, hides, and skins chemical equilibrium for reactions for of animals. It forms a colloidal jelly when constant temperature and pressure (G is a dissolved in hot water and is used in the minimum). It is named for the US math- food industry, to make capsules for various ematician and physicist Josiah Willard medicinal drugs, as an adhesive and sizing Gibbs (1839–1903), who first developed medium, and in photographic emulsions. the theory of chemical thermodynamics. See also free energy. gelatine See gelatin. giga- Symbol: G A prefix denoting 109. gel electrophoresis See electrophoresis. For example, 1 gigahertz (GHz) = 109 hertz (Hz). gel filtration A form of column chromatography in which a gel is used as the glacial acetic acid See glacial ethanoic stationary medium. Components move acid. through pores in the gel at a rate that depends on the size of their molecules. The glacial ethanoic acid (glacial acetic technique is used to separate proteins. acid) Pure water-free ethanoic acid.

gem positions Positions in a molecule GLC Gas–liquid chromatography. See on the same atom. For example, 1,1- gas chromatography. dichloroethane (CH3CHCl2), in which both chlorine atoms are on the same car- globulin One of a group of proteins that bon, is a gem dihalide. are insoluble in water but will dissolve in neutral solutions of certain salts. They gen- general formula See formula. erally contain glycine and coagulate when heated. Three types of globulin are found geometrical isomerism See isomerism. in blood: alpha (α), beta (β), and gamma (γ). α and β globulins are made in the liver gibberellic acid (GA3) A common GIB- and are used to transport nonprotein ma- BERELLIN and one of the first to be discov- terial. γ globulins are made in reticuloen- ered. Together with GA1 and GA2 it was dothelial tissues, lymphocytes, and plasma isolated from Gibberella fujikuroi, a fun- cells and most of them have antibody ac- gus that infects rice seedlings causing ab- tivity (see immunoglobulin). normally tall growth. glove box A sealed box with gloves fit- gibberellin A plant hormone involved ted to ports in one side and having a trans- chiefly in shoot extension. Gibberellins are parent top, used for safety reasons or to diterpenoids; their molecules have the gib- handle materials in an inert or sterile at- bane skeleton. More than thirty have been mosphere.

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glycine

glucan See glycan. glycan A polysaccharide made of more than 10 monosaccharide residues. A ho- gluconic acid (dextronic acid; CH2OH- moglycan is made up of a single type of > (CHOH)4COOH) A soluble crystalline sugar unit (i.e. 95%). As a class the gly- organic acid made by the oxidation of glucans serve both as structural units (e.g. cel- cose (using specific molds). It is used in lulose in plants and chitin in invertebrates) paint strippers. and energy stores (e.g. starch in plants and glycogen in animals). The most common glucosan A POLYSACCHARIDE that is homoglycans are made up of D-glucose formed of glucose units. Cellulose and units and called glucans. starch are examples.

glucose (dextrose; grape sugar; C6H12O6) A monosaccharide occurring CHO widely in nature as D-glucose. It occurs as HOHC glucose units in sucrose, starch, and cellulose. It is important to metabolism because CH2OH it participates in energy-storage and Glyceraldehyde: Fischer projection of energy-release systems. See also sugar. D-glyceraldehyde

glucoside See glycoside. glyceraldehyde A simple triose sugar glue An adhesive, of which there are var- used in determining absolute configura- ious types. Aqueous solutions of starch and tion. ethyl cellulose are used as pastes for stick- ing paper; traditional wood glue is made by glyceride An ester formed between glyc- boiling animal bones (see gelatin); quick- erol (propane-1,2,3-triol) and one or more drying adhesives are made by dissolving carboxylic acids. Glycerol has three alco- rubber or a synthetic polymer in a volatile hol groups, and if all three groups have solvent; and some polymers, such as epoxy formed esters, the compound is a triglyc- resins and polyvinyl acetate (PVA), are eride. Naturally occurring fats and oils are themselves used as glues. triglycerides of long-chain carboxylic acids (hence the name ‘fatty acid’). The main car- glutamic acid See amino acid. boxylic acids forming glycerides in fats and oils are: glutamine See amino acid. 1. octadecanoic acid (stearic acid), a saturated acid CH3(CH2)16COOH. glutathione A tripeptide of cysteine, 2. hexadecanoic acid (palmitic acid), a sat- glutamic acid, and glycine, widely distrib- urated acid CH3(CH2)14COOH. uted in living tissues. It takes part in many 3. cis-9-octadecenoic acid (oleic acid), an oxidation–reduction reactions, due to the unsaturated acid. reactive thiol group (–SH) being easily ox- CH3(CH2)7CH:CH(CH2)7C OOH idized to the disulfide (–S–S–), and acts as an antioxidant, as well as a coenzyme to glycerin See propane-1,2,3-triol. several enzymes. glycerine (glycerin) See propane-1,2,3- gluten A mixture of proteins found in triol. wheat flour. It is composed mainly of two proteins (gliaden and glutelin), the proteins glycerol See propane-1,2,3-triol. being present in almost equal quantities. Certain people are sensitive to gluten glyceryl trinitrate See nitroglycerine. (celiac disease) and must have a gluten-free diet. glycine See amino acid.

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glycogen

glycogen (animal starch) A polysaccha- cellular. Animal glycolipids are derived ride that is the main carbohydrate store of from sphingosine, an amino alcohol with a animals. It is composed of many glucose long unsaturated hydrocarbon chain. In units linked in a similar way to starch. glycolipids, the amino group of sphingo- Glycogen is readily hydrolyzed in a step- sine is joined to a fatty acid chain by an wise manner to glucose itself. It is stored amide bond and the primary hydroxyl largely in the liver and in muscle but is group is linked to a sugar residue. The sim- found widely distributed in the body. plest glycolipid (in animal cells) is cere- broside, which has one sugar residue glycol See diol. (either glucose or galactose). Glycolipids glycolipid A sugar-containing lipid with with branched chains of sugar residues one or more sugar residues attached to a are known as gangliosides. The fatty lipid by a glycosidic link (or ester link in acid chain and sphingosine chain are prokaryotes). Glycolipids play an impor- hydrophobic while the sugar residues are tant structural role in cell membranes, hydrophilic, making glycolipids amphipro- where the sugar residues are always extra- tic.

(starch, glycogen) (glucose) Production of fructose diphosphate (glucose 1-phosphate) ATP ADP

(glucose 6-phosphate) This stage requires energy and the conversion of ATP to ADP fructose 6-phosphate ATP ADP fructose 1,6-diphosphate

The 6C fructose diphosphate 2 x (glyceraldehyde breaks into two 3C molecules 3-phosphate) 2 NAD+ 2 NADH 2 x (1,3-diphosphoglycerate) This stage yields energy and 2 ADP 2 molecules of pyruvate 2 ATP ADP is converted to ATP. 2 x (3-phosphoglycerate) NADH is also formed, and may participate in subsequent reactions 2 x (2-phosphoglycerate)

2 x (phosphoenolpyruvate) 2 ADP 2 ATP 2 x (pyruvate)

Glycolysis

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gram-molecule

glycolysis (Embden–Meyerhof pathway; methoxy group, –OCH3. Such glycosides glycolytic pathway) The conversion of are described as alpha or beta according to glucose into pyruvate, with the release of whether the organic group is below or some energy in the form of ATP. Glycoly- above the ring at the anomeric carbon. If sis occurs in cell cytoplasm. It yields two the glycosyl group is attached through molecules of ATP and two of NADH2 per some other atom, it is often described as molecule of glucose. In anaerobic condi- ppa S-glycoside, C-glycoside, etc. Com- tions, breakdown proceeds no further and pounds in which the glycosyl group is at- pyruvate is converted into ethanol or lactic tached through nitrogen (i.e. N-glycosides) acid for storage or elimination. In aero- are also called glycosylamines. If the sugar bic conditions, glycolysis is followed by is glucose, the compound is a glucoside. the KREBS CYCLE. The rate of glycolysis is Glycosides occur in plants and include controlled by the enzyme phosphofructo- many useful substances. See also nucleo- kinase, which catalyzes an essentialy irre- side. versible reaction. There are two other irreversible reactions catalyzed by hexoki- glycosidic link See glycoside. nase and pyruvate kinase. glycosylamine See glycoside. glycolytic pathway See glycolysis. glycosyl group See glycoside. glycoprotein A conjugated protein formed by the combination of a protein glyoxylate cycle A modification of the with carbohydrate side chains. Certain Krebs cycle occurring in some microorgan- antigens, enzymes, and hormones are gly- isms, algae, and higher plants in regions coproteins. where fats are being rapidly metabolized, e.g. in germinating fat-rich seeds. Acetyl glycosaminoglycan (GAG) One of a groups formed from the fatty acids are group of compounds, sometimes called passed into the glyoxylate cycle, with the mucopolysaccharides, consisting of long unbranched chains of repeating disaccha- eventual formation of mainly carbohy- ride sugars, one of the two sugar residues drates. being an amino sugar – either N-acetylglucosamine or N-acetylgalactosamine. These graft copolymer See polymerization. compounds are present in connective tissue; they include heparin and hyaluronic Graham’s law (of diffusion) The princi- acid. Most glycosaminoglycans are linked ple that gases diffuse at a rate that is in- to protein to form proteoglycans (some- versely proportional to the square root of times called mucoproteins). See also glyco- their density. Light molecules diffuse faster protein. than heavy molecules. It is named for the Scottish chemist Thomas Graham (1805– glycoside A compound in which the 69), who reported it in 1829. ring form of a SUGAR is joined to some other organic group. The link in a glyco- gram (gramme; symbol: g) A unit of side occurs at the anomeric carbon atom. If mass defined as 10–3 kilogram. the hydroxyl group at this carbon is removed, the result is a glycosyl group. If this gram-atom See mole. group is joined to another organic group through an oxygen atom, then the resulting gram-equivalent The equivalent weight compound is an O-glycoside. The linkage of a substance in grams. ring–O–organic group is a glycosidic link. A simple example of a glycoside would be gramme An alternative spelling of gram. the compound formed by replacing the anomeric –OH group in a sugar by a gram-molecule See mole.

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granulation

granulation A process for enlarging piling his Treatise on Organic Chemistry particles to improve the flow properties of after World War I. The first volumes ap- solid reactants and products in industrial peared in 1935, with others helping to chemical processes. The larger a particle, complete this multi-volume treatise after and the freer from fine materials in a solid, his death. the more easily it will flow. Dry granulation produces pellets from dry materials, Grignard reagent A type of organo- which are crushed into the desired size. metallic compound with the general for- Wet granulation involves the addition of a mula RMgX, where R is an alkyl or aryl liquid to the material, and the resulting group and X is a halogen (e.g. CH3MgCl). paste is extruded and dried before cutting Grignard reagents are prepared by reacting to the required size. the haloalkane or haloaryl compound with magnesium in dry ether: grape sugar See glucose. → CH3Cl + Mg CH3MgCl Grignard reagents probably have the gravimetric analysis A method of quantitative analysis in which the final an- form R2Mg.MgCl2. They are used exten- alytical measurement is made by weighing. sively in organic chemistry. With methanal There are many variations in the method a primary alcohol is produced: → but in essence they all consist of: RMgX + HCHO RCH2OH + 1. taking an accurately weighed sample Mg(OH)X into solution; Other aldehydes give secondary alcohols: 2. precipitation as a known compound by RMgX + R′CHO → RR′CHOH + a quantitative reaction; Mg(OH)X 3. digestion and coagulation procedures; Alcohols and carboxylic acids give hydro- 4. filtration and washing; carbons: 5. drying and weighing as a pure com- RMgX + R′OH → RR′ + Mg(OH)X pound. Water also gives a hydrocarbon: → Filtration is a key element in the method RMgX + H2O RH + Mg(OH)X and a variety of special filter papers and Solid carbon dioxide in acid solution gives sinter-glass filters are available. a carboxylic acid: → RMgX + CO2 + H2O RCOOH + gray Symbol: Gy The SI unit of absorbed Mg(OH)X energy dose per unit mass resulting from They are named for Victor Grignard. the passage of ionizing radiation through living tissue. One gray is an energy absorp- ground state The lowest energy state of tion of one joule per kilogram of mass. The an atom, molecule, or other system. Com- unit is named for the British radiobiologist pare excited state. L. H. Gray (1905–65). group 1. In the periodic table, a series of Grignard, François Auguste Victor chemically similar elements that have simi- (1871–1935) Franch organic chemist. lar electronic configurations. A group is Grignard is best remembered for the organomagnesium compounds known as thus a column of the periodic table. For ex- Grignard reagents which he discovered in ample, the alkali metals, all of which have 1 1901. He found that these compounds, outer s configurations, belong to group 1. which have the general formular RMgX, See also periodic table. where R is an organic group and X is a 2. (functional group) In organic chemistry, halogen, can be used in the synthesis of an arrangement of atoms that bestows a many types of compounds, including alco- particular type of property on a molecule hols, hydrocarbons and carboxylic acids. and enables it to be placed in a particular Grignard shared the 1912 Nobel Prize for class, e.g. the aldehyde group –CHO. See chemistry for this work. He started com- also functional group.

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guncotton

GSC Gas–solid chromatography. See DNA and RNA. Guanine has a purine ring gas chromatography. structure.

GTP (guanosine triphosphate) A nucleo- O side triphosphate occurring in all cells as a coenzyme for various key processes. N HN Often it provides energy by undergoing hydrolysis to GDP (guanosine diphosphate) H N N N and a phosphate group, a reaction cat- 2 alyzed by an enzyme or other component HOCH2 O having GTPase activity. In protein synthesis, GTP is essential for the assembly of ri- bosomes and elongation of the polypeptide chain. It is also required for the assembly of OHOH microtubules, for protein transport within Guanosine cells, and for the relaying of messages to various cell components in signal transduction. guanosine (guanine nucleoside) A nucleoside present in DNA and RNA and guanidine (iminourea; HN:C(NH2)2)A consisting of guanine linked to D-ribose via strongly basic crystalline organic com- a β-glycosidic bond. pound which can be nitrated to make a powerful explosive. It is also used in mak- guanosine triphosphate See GTP. ing dyestuffs, medicines and polymer resins. gum One of a group of substances that guanine A nitrogenous base found in swell in water to form gels or sticky solutions. Similar compounds that produce O slimy solutions are called mucilages. Gums and mucilages are not distinguishable 6 N HN175 chemically. Most are heterosaccharides, 8 2 4 9 being large, complex, flexible, and often 3 N H2N N highly-branched molecules. H

Guanine guncotton See cellulose trinitrate.

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habit See crystal habit. 3. Refluxing with an alkoxide to give an ether: half-chair conformation See cyclo- RI + –OR′→ROR′ + I– hexane. 4. Reaction with alcoholic ammonia solution (100°C in a sealed tube) to give an half life For a certain radioactive nu- amine: → cleus, the time taken for half the original RI + NH3 RNH2 + HI nuclei in a sample to decay. 5. Boiling with alcoholic potassium hydroxide, to eliminate an acid and pro- halide A compound containing a halo- duce an alkene: → gen. The HALOALKANES are examples. RCH2CH2I + KOH KI + H2O + RCH:CH2 haloalkane (alkyl halide) A type of or- See also Grignard reagent; Wurtz reaction. ganic compound in which one or more hydrogen atoms of an alkane have been halocarbon A chemical compound that replaced by halogen atoms. Haloalkanes contains carbon atoms bound to halogen can be made by direct reaction of the atoms and (sometimes) hydrogen atoms. alkane with a halogen. Other methods are: The halocarbons include haloalkanes such 1. Reaction of an alcohol with the halogen as tetrachloromethane (CCl4) and the halo- acid (e.g. from NaBr + H SO ) or with forms (CHCl3, CHBr3, etc.). There are var- 2 4 ious types of halocarbon that are useful but phosphorus halides (red phosphorus are also significant pollutants. For exam- and iodine can be used): ple, the chlorofluorocarbons (CFCs) con- ROH + HBr → RBr + H O 2 tain carbon, fluorine, and chlorine. They ROH + PCl → RCl + POCl + HCl 5 3 are useful as refrigerants, aerosol propel- 2. Addition of an acid to an alkene: → lants, and in making rigid plastic foams. RCH:CH2 + HBr RCH2CH2Br However, they are also thought to damage The haloalkanes are much more reactive the ozone layer and an international agree- than the alkanes, and are useful starting ment exists to phase out their use. Similar compounds for preparing a wide range of compounds are the hydrochlorofluorocar- organic chemicals. In particular, they un- bons (HCFCs), which contain hydrogen as dergo nucleophilic substitutions in which well as chlorine and fluorine, and the hy- the halogen atom is replaced by some drofluorocarbons (HFCs), which contain other group (iodine compounds are the hydrogen and fluorine. most reactive). Some reactions of haloalka- The halons are a class of halocarbons nes are: that contain bromine as well as hydrogen 1. Refluxing with aqueous potassium hy- and other halogens. Their main use is in droxide to give an alcohol: fire extinguishers. They are, however, sig- RI + OH– → ROH + I– nificantly more active than CFCs in their 2. Refluxing with potassium cyanide in al- effect on the ozone layer. The halocarbons coholic solution to give a nitrile: are also thought to contribute to global RI + CN– → RCN + I– warming.

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hardening

haloform Any of the four compounds halogens A group of elements (group CHX3, where X is a halogen atom (F, fluo- 17, formerly VIIA, of the periodic table) roform; Cl, chloroform; Br, bromoform; I, consisting of fluorine, chlorine, bromine, iodoform). The systematic names are tri- iodine, and the short-lived radioactive el- fluoromethane, trichloromethane, tribro- ement astatine. The halogens all have outer momethane, and triiodomethane. valence shells that are one electron short of a rare-gas configuration. Because of this, haloform reaction A reaction of a the halogens are characterized by high elec- methyl ketone with NaOX, where X is Cl, tron affinities and high electronegativities, Br, or I, to give a haloform. With sodium fluorine being the most electronegative el- chlorate(I), for example: ement known. → A wide range of organic halides is RCOCH3 + 3NaOCl RCOCCl3 + 3NaOH formed in which the C–F bond is charac- → teristically resistant to chemical attack; the RCOCl3 + NaOH NaOCOR + C–Cl bond is also fairly stable, particularly CHCl3 The reaction can be used to make car- in aryl compounds, but the alkyl halogen boxylic acids (from the NaOCOR), and is compounds become increasingly suscepti- especially useful when R is an aromatic ble to nucleophilic attack and generally group because the starting ketone, more reactive. RCOCH3, can be produced by Friedel–Crafts acetylation. See also tri- halon See halocarbon. iodomethane. halothane (CHBrClCF3) A colorless nonflammable liquid halocarbon used as a halogenating agent A compound used general anesthetic. The systematic name is to introduce halogen atoms into a mol- 1-chloro-1-bromo-2,2,2-trifluoroethane. ecule. Examples are phosphorus trichloride (PCl ) and aluminum trichloride (AlCl ). 3 3 hammer mill A device used in the chemical industry for crushing and grinding halogenation A reaction in which a solid materials at high speeds to a specified halogen atom is introduced into a mol- size. The impact between the particles, ecule. Halogenations are specified as chlo- grinding plates, and grinding hammers pul- rinations, brominations, fluorinations, verizes the particles. Hammer mills can be etc., according to the element involved. used for a greater variety of soft material There are several methods. than other types of grinding equipment. 1. Direct reaction with the element using Compare ball mill. high temperature or ultraviolet radiation: hardening (of oils) The conversion of → CH4 + Cl2 CH3Cl + HCl liquid plant oils into a more solid form for 2. Addition to a double bond: use in margarine by hydrogenation using a → H2C:CH2 + HCl C2H5Cl nickel catalyst. In vegetable oils the fatty 3. Reaction of a hydroxyl group with a acids present (as glycerides) contain double halogenating agent, such as PCl3: bonds (i.e. they are unsaturated). The hy- → – C2H5OH C2H5Cl + OH drogenation process increases the amount 4. In aromatic compounds direct substitu- of unsaturated material, increasing the tion can occur using aluminum chloride melting point, but still leaves unsaturated as a catalyst: fatty acids. For this reason it is claimed that → 2C6H6 + Cl2 2C6H5Cl margarines are healthier than animal fats 5. Alternatively in aromatic compounds, (e.g. butter), which contains saturated fats, the chlorine can be introduced by react- because the unsaturated fats are less likely ing the diazonium ion with copper(I) to lead to cholesterol build-up in the body, chloride: and consequent risk of coronary heart dis- + – → C6H5N2 + Cl C6H5Cl + N2 ease. However, the hydrogenation process 107 iranchembook.ir/edu

Haworth, Sir Walter Norman

may also affect the nature of the double ditions, which are defined as 298 K (25°C) bonds. Natural unsaturated fatty acids and 101 325 Pa (1 atmosphere). Thus, the mostly have a cis configuration about the standard molar enthalpy of reaction is the double bonds. In the hydrogenation enthalpy change for reaction of substances process, a proportion of these are con- under these conditions producing reactants verted into fatty acids with a trans config- under the same conditions. The substances uration. Glycerides of these are known as involved must be in their normal equilib- trans-fats. It has been claimed that there is rium physical states under these conditions also a link between trans-fats and coronary (e.g. carbon as graphite, water as the liq- heart disease. See Sabatier–Senderens uid, etc.). Note that the measured enthalpy process. change will not usually be the standard change. In addition, it is common to spec- Haworth, Sir Walter Norman (1883– ify the entity involved. For instance 1950) British organic chemist. Haworth ∆ Š Hf (H2O) is the standard molar en- was a pioneer of the study of carbohy- thalpy of formation for one mole of H2O drates, particularly sugars. He showed that species. sugar molecules are ring molecules, with a puranose ring consisting of five carbon heat exchanger A device that enables atoms and an oxygen atom and a furanose the heat from a hot fluid to be transferred ring consisting of four carbon atoms and to a cool fluid without allowing the fluids an oxygen atom. He and his colleagues to come into contact. The normal arrange- subsequently investigated the chain struc- ment is for one of the fluids to flow in a tures of various polysaccharides. This es- coiled tube through a jacket containing the tablished the structures of cellulose, starch second fluid. Both the cooling and heating and glycogen. In 1929 he published the effect may be of benefit in conserving the book The Constitution of the Sugars which energy used in a chemical plant and in con- soon became the standard work on this trolling the process. topic. In 1933, together with his colleague Edmund Hirst, he synthesized vitamin C (ascorbic acid), having previously estab- heat of atomization The energy re- lished its structure. Haworth shared the quired in dissociating one mole of a sub- 1937 Nobel Prize for chemistry with Paul stance into atoms. See heat. KARRER. heat of combustion The energy liber- HCFC Hydrochlorofluorocarbon. See ated when one mole of a substance burns in halocarbon. excess oxygen. See heat.

heat Energy transferred as a result of a heat of crystallization The energy lib- temperature difference. The term is often erated when one mole of a substance crys- loosely used to mean internal energy (i.e. tallizes from a saturated solution of this the total kinetic and potential energy of the substance. particles). It is common in chemistry to define such quantities as heat of combustion, heat of dissociation The energy re- heat of neutralization, etc. These are in fact quired to dissociate one mole of a sub- molar enthalpies for the change, given the stance into its constituent elements. ∆ Š symbol HM . The superscript symbol de- notes standard conditions, while the sub- heat of formation The energy change script M indicates that the enthalpy change when one mole of a substance is formed is for one mole. The unit is usually the kilo- from its elements. See heat. joule per mole (kJ mol–1). By convention, ∆H is negative for an exothermic reaction. heat of neutralization The energy lib- Molar enthalpy changes stated for chemi- erated when one mole of an acid or base is cal reactions are changes for standard con- neutralized.

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hemoglobin

heat of reaction The energy change trix of plant cell walls together with pectic when molar amounts of given substances substances (and occasionally, in mature react completely. See heat. cells, with lignin, gums, and mucilages). They are heteropolysaccharides, i.e. poly- heat of solution The energy change saccharides built from more than one type when one mole of a substance is dissolved of sugar, mainly the hexoses (mannose and in a given solvent to infinite dilution (in galactose) and the pentoses (xylose and practice, to form a dilute solution). arabinose). They vary greatly in composition between species. In some seeds (e.g. heavy hydrogen See deuterium. the endosperm of dates) hemicelluloses are a food reserve. heavy water Deuterium oxide, D2O. hemiketal See acetal. hecto- Symbol: h A prefix denoting 102. For example, 1 hectometer (hm) = 102 hemin The hydrochloride form of heme. meters (m). Hemin is the crystalline form in which heme can be isolated and studied in the lab- helicate See supramolecular chemistry. oratory. The iron present is the trivalent state (iron(III)). Hemin can be made to Hell–Volard–Zelinsky reaction A crystallize by heating hemoglobin gently method for the preparation of halogenated with acetic acid and sodium chloride. A va- carboxylic acids using free halogen in the riety of crystal forms are known. presence of a phosphorus halide. The halogenation occurs at the carbon atom adja- hemocyanin A blue copper-containing cent to the –COOH group. With Br2 and blood pigment found in many mollusks PBr3: and arthropods. Hemocyanin is the second → → RCH2COOH RCHBrCOOH most abundant blood pigment after hemo- RCBr2COOH globin and functions similarly in acting as an oxygen-carrier in the blood. Helmholtz free energy See Helmholtz function. hemoerythrin A pigment occurring in the blood of certain invertebrates, similar Helmholtz function (Helmholtz free en- in structure to HEMOGLOBIN. ergy) Symbol: F A thermodynamic function defined by hemoglobin The pigment of the red F = U – TS blood cells in humans and other verte- where U is the internal energy, T the thermodynamic temperature, and S the entropy. It is a measure of the ability of a CH2 system to do useful work in an isothermal CH3 process. The function is named for the Ger- man physiologist and physicist Hermann H C Ludwig Ferdinand von Helmholtz (1821– 3 N N CH 94). See also free energy. 2 Fe heme (haeme) An iron-containing por- N N phyrin that is the prosthetic group in HEMO- H3C CH3 GLOBIN, myoglobin, and some cytochromes.

hemiacetal See acetal. COOH COOH hemicellulose One of a group of substances that make up the amorphous ma- Heme

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henry

brates that is responsible for the transport or wave motion. It is named for the Ger- of oxygen from the lungs to the tissues. It man physicist Heinrich Hertz (1857–94). consists of a basic protein, globin, linked with four heme groups. Heme is a complex Hess’s law A derivative of the first law compound containing an iron atom. The of thermodynamics. It states that the total most important property of hemoglobin is heat change for a given chemical reaction its ability to combine reversibly with one involving alternative series of steps is inde- molecule of oxygen per iron atom to form pendent of the route taken. It is named for oxyhemoglobin, which has a bright red the Russian chemist Germain Henri Hess color. The iron is present in the divalent (1802–50), who proposed it in 1840. state (iron(II)) and this remains unchanged with the binding of oxygen. There are vari- hetero atom See heterocyclic com- ations in the polypeptide chains, giving rise pound. to different types of hemoglobins in different species. The binding of oxygen depends heterocyclic compound A compound on the oxygen partial pressure; high pres- that has a ring containing more than one sure favors formation of oxyhemoglobin type of atom. Commonly, heterocyclic and low pressure favors release of oxygen. compounds are organic compounds with at least one atom in the ring that is not a henry Symbol: H The SI unit of induc- carbon atom. Pyridine and glucose are ex- tance, equal to the inductance of a closed amples. The noncarbon atom is called a circuit that has a magnetic flux of one hetero atom. Compare homocyclic com- weber per ampere of current in the circuit. pound. 1 H = 1 Wb A–1. It is named for the US physicist Joseph Henry (1797–1828). heterogeneous Relating to more than one phase. A heterogeneous mixture, for Henry’s law The concentration (C) of a instance, contains two or more distinct gas in solution is proportional to the par- phases. Heterogeneous catalysis involves a tial pressure (p) of that gas in equilibrium catalyst that is a different phase than that with the solution, i.e. p = kC, where k is a of the reactants (usually gaseous reactants proportionality constant. The relationship passed over a solid catalyst). is similar in form to that for RAOULT’S LAW, which deals with ideal solutions. A conse- heterolysis See heterolytic fission. quence of Henry’s law is that the ‘volume solubility’ of a gas is independent of pres- heterolytic fission (heterolysis) The sure. The law is named for the British breaking of a covalent bond so that both physician and chemist William Henry electrons of the bond remain with one frag- (1755–1836), who formulated it in 1801. ment. A positive ion and a negative ion are produced: heparin A polysaccharide that inhibits RX → R+ + X– the formation of thrombin from prothrom- Compare homolytic fission. bin and thereby prevents the clotting of blood. It is used in medicine as an antico- heteropolymer See polymerization. agulant. See polysaccharide. heteropolysaccharide See hemicellu- heptane (C7H16) A colorless liquid lose. alkane obtained from petroleum refining. It is used in gasoline and as a solvent. hexadecanoate (palmitate) A salt or ester of hexadecanoic acid. hertz Symbol: Hz The SI unit of frequency, defined as one cycle per second hexadecanoic acid (palmitic acid) A (s–1). Note that the hertz is used for regu- crystalline carboxylic acid: larly repeated processes, such as vibration CH3(CH2)14COOH 110 iranchembook.ir/edu

Hofmann, August Wilhelm von

It is present as glycerides in fats and HFC Hydrofluorocarbon. See halocar- oils. See glyceride. bon.

high-performance liquid chromatography See HPLC. N H C CH histamine An amine formed from the 2 CH2 2 amino acid histidine by decarboxylation N and produced mainly in connective tissue CH2 CH2 as a response to injury or allergic reaction. N N It causes contraction of smooth muscle, stimulates gastric secretion of hydrochloric CH2 acid and pepsin, and dilates blood vessels, Hexamethylenetetramine which lowers blood pressure and produces inflammation, itching, or allergic symptoms. hexamethylenetetramine (hexamine; C6H12N4) A white crystalline organic histidine See amino acid. compound made by condensing methanal with ammonia. It is used as a fuel for Hodgkin, Dorothy Mary Crowfoot camping stoves, in vulcanizing rubber, and (1910–94) British x-ray crystallographer as a urinary disinfectant. It can be nitrated Dorothy Hodgkin determined the structure to make the high explosive cyclonite. of many complex organic molecules. She won the 1964 Nobel Prize for chemistry hexamine See hexamethylenetetramine. for this work. Together with Charles Bunn, she published the structure of penicillin in hexane (C6H14) A liquid alkane ob- 1949. She then started work on the struc- tained from the light fraction of crude oil. ture of vitamin B12 and found its structure The principal use of hexane is in gasoline in 1956. This work made use of early elec- and as a solvent. tronic computers. In 1969 she determined the structure of insulin. hexanedioic acid (adipic acid; HOOC- (CH2)4COOH) A colorless crystalline Hoffmann, Roald (1937– ) Polish- organic dicarboxylic acid that occurs in born American chemist, Hoffmann is best rosin. It is used in the manufacture of known for his collaboration with Robert NYLON. WOODWARD in the mid 1960s which led to the formulation of the Woodward–Hoff- hexanoate A salt or ester of hexanoic mann rules. These rules stated which acid. chemical reactions can take place in terms of molecular orbitals. Woodward and hexanoic acid (caproic acid; CH3- Hoffmann were largely concerned with or- (CH2)4COOH) An oily carboxylic acid ganic reactions but their rules apply more found (as glycerides) in cow’s milk and generally. They summarized their results in some vegetable oils. the book Conservation of Orbital Symme- try (1970). Hoffmann shared the 1979 hexose A SUGAR that has six carbon Nobel Prize for chemistry with Kenichi atoms in its molecules. FUKUI for this work. Hoffmann has also done a great deal to popularize chemistry hexose monophosphate shunt See in books and television programmes. pentose phosphate pathway. Hofmann, August Wilhelm von hexyl group The group C5H11CH2–, (1818–92) German organic chemist. Hof- having a straight chain of carbon atoms. mann was one of the most influential or-

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Hofmann degradation

ganic chemists of the 19th century. He was HOMO See frontier orbital. particularly influential in Britain and Ger- many. Much of his work was concerned homocyclic compound A compound with the constituents of coal tar, particu- containing a ring made up of the same larly aniline and phenol. He discovered or atoms. Benzene is an example of a homo- investigated a number of compounds, in- cyclic compound. Compare heterocyclic cluding quaternary ammonium salts. He compound. also discovered the reaction known as Hof- mann degradation, which consists of treat- homogeneous Relating to a single ing an amide with bromine and alkali to phase. A homogeneous mixture, for in- give an amine with one fewer carbon atom. stance, consists of only one phase. In ho- He was one of the first people to investi- mogeneous catalysis, the catalyst has the gate formaldehyde. In 1858 he obtained same phase as the reactants. the dye magenta by reacting carbon tetrachloride with aniline. homologous series A group of organic compounds possessing the same functional Hofmann degradation A method of group and having a regular structural pat- preparing primary amines from acid tern so that each member of the series dif- amides. The amide is refluxed with aque- fers from the next one by a fixed number of ous sodium hydroxide and bromine: atoms. The members of a homologous se- → RCONH2 + NaOH + Br2 ries can be represented by a general for- RCONHBr + NaBr + H O 2 mula. For example, the homologous series RCONHBr + OH– → RCON–Br + H O 2 of alkane alcohols CH OH, C H OH, RCON–Br → R–N = C = O + Br– 3 2 5 C H OH, …, has a general formula RNCO + 2OH– → RNH + CO 2– 3 7 2 2 C H OH. Each member differs by CH The reaction is a ‘degradation’ in the n 2n+1 2 from the next. Any two successive mem- sense that a carbon atom is removed from bers of a series are called homologs. the amide chain. It is named for the Ger- man chemist August Wilhelm von Hof- See homologous series. mann. homologs

Hofmann’s method A method for- homolysis See homolytic fission. merly used for determining the vapor density of volatile liquids. A known weight of homolytic fission (homolysis) The sample is introduced into a mercury breaking of a covalent bond so that one barometer tube, which is surrounded by a electron from the bond is left on each frag- heating jacket. The volume of vapor can ment. Two free radicals result: ′→ ′ thus be read off directly, the temperature is RR R• + R • known, and the pressure is obtained by Compare heterolytic fission. taking the atmospheric pressure minus the mercury height in the barometer (with cor- homopolymer See polymerization. rections for the density of mercury at higher temperatures). The method’s only host–guest chemistry A branch of advantage is that it may be used for sam- supramolecular chemistry in which a mo- ples that decompose at their normal boil- lecular structure acts as a ‘host’ to hold an ing point. ion or molecule (the ‘guest’). The guest may be coordinated to the host or may be holoenzyme A catalytically active com- trapped by its structure. For example, cal- plex made up of an apoenzyme and a coen- ixarenes are compounds with cup-shaped zyme. The former is responsible for the molecules that may accept guest molecules. specificity of the holoenzyme whilst the lat- See also crown ether; supramolecular ter determines the nature of the reaction. chemistry.

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hydrochlorofluorocarbon

HPLC High-performance liquid chro- ammonia with sodium chlorate(I) or by the matography; a sensitive analytical tech- gas phase reaction of ammonia with chlo- nique, similar to gas-liquid chromatog- rine. Hydrazine is a weak base, forming raphy but using a liquid carrier. The carrier salts (e.g. N2H4.HCl) with strong acids and is specifically choosen for the particular is also a powerful reducing agent. With substance to be detected. aldehydes and ketones it forms HYDRA- ZONES. Hückel, Erich Armand Arthur Joseph (1896–1980) German physical hydrazone A type of organic compound and theoretical chemist. Hückel has two containing the C:NNH2 group, formed by main claims to fame. His first is the theory the reaction between an aldehyde or ke- of electrolytes which Peter Debye and he tone and hydrazine (N H ). Derivatives of produced in 1923 and which gives a good 2 4 hydrazine were formerly used to produce description of the electrical and thermody- crystalline products, which have sharp namic properties of dilue electrolytes. His second major work was the application of melting points that can be used to charac- quantum mechanics to aromatic molecules terize the original aldehyde or ketone. such as benzene. He used molecular orbital Phenylhydrazine (C6H5NH.NH2), for in- theory to show that in the benzene mol- stance, produces phenylhydrazones. ecule the electrons in the pi orbitals are spread out directly above and below the hydride A compound of hydrogen. Ionic ring of carbon atoms, thus making the mol- hydrides are formed with highly elec- ecule more stable than it would be if one tropositive elements and contain the H– ion had alternating double and single bonds. (hydride ion). Non-metals form covalent Hückel started this work in 1930 and soon hydrides, as in methane (CH4) or silane extended it to predict the Hückel rule (SiH4). The boron hydrides are electron- which states that a molecule is aromatic if deficient covalent compounds. Many tran- it has (4n + 2) pi electrons. sition metals absorb hydrogen to form interstitial hydrides. Hückel rule See aromatic compound. hydrobromic acid (HBr) A colorless humectant A hygroscopic substance liquid produced by adding hydrogen bro- used to maintain moisture levels. Glycerol, mide to water. It shows the typical proper- mannitol, and sorbitol are commonly used ties of a strong acid and it is a strong in foodstuffs, tobacco, etc. reducing agent.

Hund’s rule A rule that states that the hydrocarbon Any compound contain- electronic configuration in degenerate or- ing only the elements carbon and hydro- bitals will have the minimum number of gen. Examples are the alkanes, alkenes, paired electrons. alkynes, and aromatics such as benzene hybrid orbital See orbital. and naphthalene.

hydrate A compound coordinated with hydrochloric acid (HCl) A colorless water molecules. When water is bound up fuming liquid made by adding hydrogen in a compound it is known as water of chloride to water. Dissociation into ions is crystallization. extensive and hydrochloric acid shows the typical properties of a strong acid. Hy- hydration The solvation of such species drochloric acid is used in making dyes, as ions in water. drugs, and photographic materials.

hydrazine (N2H4) A colorless liquid hydrochlorofluorocarbon See halo- that can be prepared by the oxidation of carbon.

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hydrocyanic acid

hydrocyanic acid (prussic acid; HCN) A ecules. The hydrogen bond is represented highly poisonous weak acid formed when as a dotted line: hydrogen cyanide gas dissolves in water. Xδ– – Hδ+ ...... Xδ– – Hδ+ … Its salts are cyanides. Hydrogen cyanide is The length of a hydrogen bond is charac- used in making acrylic plastics. teristically 0.15–0.2 nm. Hydrogen bonding may lead to the formation of dimers hydrofluoric acid (HF) A colorless liq- (for example, in carboxylic acids) and is uid produced by dissolving hydrogen fluo- used to explain the anomalously high boil- ride in water. It is a weak acid, but will ing points of H2O and HF. Hydrogen dissolve most silicates and hence can be bonding is important in many biochemical used to etch glass. As the interatomic dis- systems. It occurs between bases in the tance in HF is relatively small, the H–F chains of DNA. It also occurs between bond energy is very high and hydrogen flu- C=O and N–H groups in PROTEINS, where oride is not a good proton donor. It does, it is responsible for maintaining the sec- however, form hydrogen bonds. ondary structure.

hydrofluorocarbon See halocarbon. hydrogen bromide (HBr) A colorless sharp-smelling gas that is very soluble in hydrogen A colorless gaseous element. water. It is produced by direct combination Hydrogen has some similarities to both the of hydrogen and bromine in the presence of alkali metals (group 1) and the halogens a platinum catalyst or by the reaction of (group 17), but is not normally classified in phosphorus tribromide with water. It dis- any particular group of the periodic table. solves in water to give HYDROBROMIC ACID. It is the most abundant element in the Uni- hydrogencarbonate (bicarbonate) A verse and the ninth most abundant element salt containing the ion –HCO . in the Earth’s crust and atmosphere (by 3 mass). It occurs principally in the form of hydrogen chloride (HCl) A colorless water and petroleum products; traces of gas that has a strong irritating odor and molecular hydrogen are found in some nat- fumes strongly in moist air. It is prepared ural gases and in the upper atmosphere. by the action of concentrated sulfuric acid Symbol: H; m.p. 14.01 K; b.p. 20.28 K; on sodium chloride. The gas is made in- –3 ° d. 0.089 88 kg m (0 C); p.n. 1; r.a.m. dustrially by burning a stream of hydrogen 1.0079. in chlorine. It is not particularly reactive but will form dense white clouds of ammo- hydrogenation The reaction of a com- nium chloride when mixed with ammonia. pound with hydrogen. In organic chem- It is very soluble in water and ionizes al- istry, hydrogenation usually refers to the most completely to give HYDROCHLORIC addition of hydrogen to multiple bonds, ACID. Hydrogen chloride is used in the often with the aid of a catalyst. Unsatu- manufacture of organic chlorine com- rated natural liquid vegetable oils can be pounds, such as polyvinyl chloride (PVC). hydrogenated to form saturated semisolid fats – a reaction used in making types of hydrogen cyanide See hydrocyanic margarine. See Bergius process. acid.

hydrogen bond An intermolecular hydrogen fluoride (HF) A colorless liq- bond between molecules in which hydro- uid produced by the reaction of concen- gen is bound to a strongly electronegative trated sulfuric acid on calcium fluoride. It element. Bond polarization by the elec- produces toxic corrosive fumes and dis- tronegative element X leads to a positive solves readily in water to give HYDROFLUO- δ δ charge on hydrogen X ––H +; this hy- RIC ACID. Hydrogen fluoride is atypical of drogen can then interact directly with the hydrogen halides as the individual H–F electronegative elements of adjacent mol- units are associated into much larger units,

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hydroxyl group

forming zigzag chains and rings. This is drogen sulfide, which is removed before caused by hydrogen bonds that form be- supply to the consumer. tween the hydrogen and the highly elec- – tronegative fluoride ions. Hydrogen hydrogensulfite (bisulfite, HSO3 )An fluoride is used extensively as a catalyst in acidic salt or ester of sulfurous acid the petroleum industry. (H2SO3), in which only one of the acid’s hydrogen atoms has been replaced by a hydrogen ion A positively charged hy- metal or organic radical. An example is drogen atom, H+, i.e. a proton. Hydrogen sodium hydrogensulfite, NaHSO3. ions are produced by all acids in water, in which they are hydrated to hydroxonium hydrolysis A reaction between a com- + (hydronium) ions, H3O . See acid; pH. pound and water. An example is the hydrolysis of an ESTER to give a carboxylic hydrogen peroxide (H2O2) A colorless acid and an alcohol: syrupy liquid, usually used in solution in CH COOC H + H O ˆ CH COOH water. Although it is stable when pure, on 3 2 5 2 3 + C H OH contact with bases such as manganese(IV) 2 5 oxide it gives off oxygen, the manga- hydron The positive ion H+. The name nese(IV) oxide acting as a catalyst: is used when the isotope is not relevant, i.e. 2H O → 2H O + O 2 2 2 2 a hydron could be a proton, deuteron, or Hydrogen peroxide can act as an oxi- triton. dizing agent, converting iron(II) ions to iron(III) ions, or as a reducing agent with Water attracting. See potassium manganate(VII). It is used as a hydrophilic bleach and in rocket fuel. The strength of lyophilic. solutions is usually given as volume strength – the volume of oxygen (dm3) at hydrophobic Water repelling. See STP given by decomposition of 1 dm3 of lyophobic. the solution. hydroquinone See benzene-1,4-diol. – hydrogensulfate (bisulfate, HSO4 )An hydrosol A colloid in aqueous solution. acidic salt or ester of sulfuric acid (H2SO4), in which only one of the acid’s hydrogen atoms has been replaced by a metal or or- hydroxide A compound containing the – ganic radical. An example is sodium hy- ion OH or the group –OH. drogensulfate, NaHSO4. hydroxonium ion See hydrogen ion. hydrogen sulfide (sulfuretted hydrogen, hydroxybenzene See phenol. H2S) A colorless very poisonous gas with an odor of bad eggs. Hydrogen sulfide is prepared by reacting hydrochloric acid hydroxybenzoate See salicylate. with iron(II) sulfide. It is tested for by mixing with lead nitrate, with which it gives a hydroxybenzoic acid See salicylic acid. black precipitate. Its aqueous solution is weakly acidic. Hydrogen sulfide reduces 2-hydroxypropanoic acid See lactic iron(III) chloride to iron(II) chloride, form- acid. ing hydrochloric acid and a yellow precipitate of sulfur. Hydrogen sulfide hydroxyl group A group (–OH) con- precipitates insoluble sulfides, and is used taining hydrogen and oxygen, characteris- in qualitative analysis. It burns with a blue tic of alcohols and phenols, and some flame in oxygen to form sulfur(IV) oxide hydroxides. It should not be confused with and water. Natural gas contains some hy- the hydroxide ion (OH–).

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hygroscopic

hygroscopic Describing a substance hyperfine structure See fine structure. that absorbs moisture from the atmosphere. See also deliquescent. hypertonic solution A solution that has a higher osmotic pressure than some hyperconjugation The interaction of sigma bonds with pi bonds. It is sometimes other solution. Compare hypotonic solu- described in terms of resonance structures tion. of the type: C H CH → C H CH –H+ 6 5 3 6 5 2 hypotonic solution A solution that has to explain the interaction of the methyl group with the pi electrons of the benzene a lower osmotic pressure than some other ring in methylbenzene (toluene). solution. Compare hypertonic solution.

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IAA (indole acetic acid) A naturally oc- as the imido group, and it can form part of curring auxin. See auxin. a ring in cyclic imides.

ideal gas (perfect gas) See gas laws. imido group See imide.

ideal solution A hypothetical solution imine An organic compound containing that obeys RAOULT’S LAW. the group C=N–, in which there is a double bond between the carbon and the nitrogen. ignis fatuus (will-o’-the-wisp) A light A general formula for imines is 1 2 3 1 2 3 sometimes seen over marshy ground. It is R R C=N–R , where R , R , and R are caused by methane produced by rotting hydrocarbon groups or hydrogen. They vegetation, which is ignited by the presence can be made by the reaction of aldehydes of small amounts of spontaneously flam- and ketones with primary amines. For example, propanone (acetone; CH3COCH3) mable phosphine (PH3). with ethylamine (C2H6NH2): CH COCH + C H NH → ignition temperature The lowest tem- 3 3 2 6 2 (CH ) C=N–C H + H O perature to which a given substance can be 3 2 2 6 2 The reaction is acid-catalyzed. When heated before it ignites (without the appli- ammonia is used the imine contains the cation of a spark or flame). It is often called C=N–H group: the autoignition temperature. Compare → CH3COCH3 + NH3 flash point. (CH3)2C=N–H + H2O Most imines are unstable and can be de- imide An organic compound containing tected only in solution unless R1, R2, or R3 the group –CO.NH.CO–, i.e. a –NH group are aryl groups. Related compounds such attached to two carbonyl groups. Simple as OXIMES, HYDRAZONES, and SEMICAR- imides have the general formula BAZONES, in which the nitrogen is attached R1.CO.NH.CO.R2, where R1 and R2 are to an electronegative group, are more sta- alkyl or aryl groups. The group is known ble and can be isolated. Intermediates in

3 3 R H + R N N

C C R1 R2 R1 R2

imine iminium ion

Imine

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iminium ion

which an imine adds a proton to form a indole acetic acid (IAA) A naturally oc- positive ion (e.g. R1R2C=NR3H+) are curring auxin. See auxin. known as iminium ions. inductive effect The effect in which iminium ion See imine. substituent atoms or groups in an organic compound can attract (–I) or push away imino group See imine. electrons (+I), forming polar bonds. Elec- tron-attracting groups include –NO2, iminourea See guanidine. –CN, –COOH, and the halogens. Electron- releasing groups include –OH, –NH2, immiscible Describing two or more liq- –OR, and R, where R is an alkyl group. In- uids that will not mix, such as oil and ductive effects can influence the reactivity water. After being shaken together and left of other parts of a molecule. For example, to stand immiscible liquids form separate an electron-attracting group substituted on layers. a benzene ring withdraws electrons and makes the ring less susceptible to electrophilic substitution. An electron-releas- indene (C9H8) A colorless flammable hydrocarbon. It has a benzene ring fused to ing group makes the ring more susceptible. a five-membered ring. infrared (IR) Electromagnetic radiation indicator A compound that reversibly with longer wavelengths than visible radia- changes color depending on the presence tion. The wavelength range is approxi- µ or absence of a chemical substance. In mately 0.7 m to 1 mm. Many materials acid–base indicators the color depends on transparent to visible light are opaque to the pH of the solution in which it is dis- infrared, including glass. Rock salt, quartz, germanium, or polyethene prisms and solved. Methyl orange and phenolph- lenses are suitable for use with infrared. In- thalein are examples of acid–base frared radiation is produced by movement indicators. Redox titrations require either of charges on the molecular scale; i.e. by specific indicators, which detect one of the vibrational or rotational motion of mol- components of the reaction (e.g. starch for ecules. Infrared spectroscopy is of particu- iodine, potassium thiocyanate for Fe3+) or lar importance in organic chemistry and true redox indicators in which the transi- absorption spectra are used extensively in tion potential of the indicator between ox- identifying compounds. Certain bonds be- idized and reduced forms is important. The tween pairs of atoms (C–C, C=C, C=O, transition potential of a redox indicator is etc.) have characteristic vibrational fre- analogous to the transition pH in acid– quencies, which correspond to bands in the base systems. Complexometric titrations infrared spectrum. Infrared spectra are require indicators that complex with metal thus used in finding the structures of new ions and change color between the free organic compounds by indicating the pres- state and the complex state. ence of certain groups. They are also used to ‘fingerprint’ and thus identify known indigo (C16H10N2O2) A blue organic compounds. At shorter wavelengths, in- dye that occurs (as a glucoside) in plants of frared absorption corresponds to transi- the genus Indigofera. It is a derivative of in- tions between rotational energy levels, and dole, and is now made synthetically.

indole (benzpyrrole; C8H7N) A colorless solid organic compound that occurs in coal tar and various plants, and is the basis of indigo and of several plant hormones. NH The indole molecule has a benzene ring fused to a pyrrole ring. Indole

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iodine

can be used to find the dimensions of mol- intermediate bond A form of covalent ecules (by their moment of inertia). bond that also has an ionic or electrovalent character. See polar bond. Ingold, Sir Christopher Kelk (1893– 1970) British organic chemist. Ingold de- intermolecular force A force of attrac- voted his career to understanding the tion between molecules, as distinguished mechanisms of organic reactions in terms from a force within the molecule (a chemi- of the electrons of the molecules con- cal bond). Forces of attraction between cerned. For example, he postulated the molecules are the result of interactions be- concept of mesomerism in 1926 to explain tween dipoles. See hydrogen bond; Van der how a molecule could exist as a hybrid of Waals force. two possible structures. Ingold was particularly concerned with the mechanisms of internal energy Symbol: U The energy elimination and substitution reactions. In- of a system that is the total of the kinetic gold summarized his work in the classic and potential energies of its constituent book Structure and Mechanisms in Or- particles (e.g. atoms and molecules). If the ganic Chemistry, the second edition of temperature of a substance is raised, by which was published in 1969. transferring energy to it, the internal energy increases (the particles move faster). inhibitor A substance that slows down Similarly, work done on or by a system re- the rate of a chemical reaction. sults in an increase or decrease in the internal energy. The relationship between heat, inner Describing a ring compound that work, and internal energy is given by the is formed, or regarded as formed, by one first law of thermodynamics. Sometimes part of a molecule reacting with another. the internal energy of a system is loosely For example, a LACTAM is an inner amide spoken of as ‘heat’ or ‘heat energy’. and a LACTONE is an inner ester. Strictly, this is incorrect; heat is the transfer of energy as a result of a temperature inorganic chemistry The branch of difference. chemistry concerned with elements other than carbon and with the preparation, inversion A change in which a com- properties, and reactions of their compound is converted from one optical iso- pounds. Certain simple carbon compounds mer to the other. See Walden inversion. are treated in inorganic chemistry, including carbon oxides, carbon disulfide, car- invert sugar See sucrose. bon halides, hydrogen cyanide, and certain simple salts, such as the cyanides, cyanates, in vitro Literally ‘in glass’; describing carbonates, and hydrogencarbonates. experiments or techniques performed in laboratory apparatus rather than in the liv- insertion reaction A reaction in which ing organism. Cell tissue cultures and in an atom or group is inserted between two vitro fertilization (to produce ‘test-tube ba- other groups. See carbene. bies’) are examples. Compare in vivo.

insoluble Describing a compound that in vivo Literally ‘in life’; describing has a very low solubility (in a specified sol- processes that occur within the living or- vent). ganism. Compare in vitro.

intermediate 1. A compound that re- iodide See halide. quires further chemical treatment to produce a finished industrial product. iodine A dark-violet volatile solid el- 2. A transient chemical entity in a complex ement belonging to the HALOGENS (group reaction. 17, formerly VIIA, of the periodic table). It See also precursor. occurs in seawater and is concentrated by

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iodoethane

various marine organisms in the form of io- ionic product The product of concen- dides. Significant deposits also occur in the trations: + – form of iodates. A large number of organic KW = [H ][OH ] iodine compounds are known. in water as a result of a small amount of self-ionization: ˆ + – iodoethane (ethyl iodide; C2H5I) A col- H2O H + OH orless liquid haloalkane made by reaction of ethanol with iodine in the presence of ionic radius A measure of the effective red phosphorus. radius of an ion in a compound. For an isolated ion, the concept is not very meaning- iodoform See triiodomethane. ful, since the ion is a nucleus surrounded by an ‘electron cloud’. Values of ionic radii iodoform reaction See triiodomethane. can be assigned, however, based on the distances between ions in crystals. iodomethane (methyl iodide; CH3I) A liquid haloalkane made by reaction of ionic strength For an ionic solution a methanol with iodine in the presence of red quantity can be introduced that empha- phosphorus. sizes the charges of the ions present: ½Σ 2 I = imiz i ion An atom or molecule that has a neg- where m is the molality and z the ionic ative or positive charge as a result of losing charge. The summation is continued over or gaining one or more electrons. See also all the different ions in the solution, i. ionization. ionization The process of producing ion exchange A process that takes place ions. There are several ways in which ions in certain insoluble materials that contain may be formed from atoms or molecules. ions capable of exchanging with ions in the In certain chemical reactions ionization oc- surrounding medium. Zeolites, the first curs by transfer of electrons; for example, ion-exchange materials, were used for sodium atoms and chlorine atoms react to water softening. These have largely been form sodium chloride, which consists of replaced by synthetic resins made of an sodium ions (Na+) and chloride ions (Cl–). inert backbone material, such as Certain molecules can ionize in solution; polyphenylethene, to which ionic groups acids, for example, form hydrogen ions as are weakly attached. If the ions exchanged in the reaction → + 2– are positive, the resin is a cationic resin. An H2SO4 2H + SO4 anionic resin exchanges negative ions. When The ‘driving force’ for ionization in a all available ions have been exchanged (e.g. solution is solvation of the ions by mol- sodium ions replacing calcium ions) the ecules of the solvent. H+, for example, is material can be regenerated by passing solvated as a hydroxonium (hydronium) + concentrated solutions (e.g. sodium chlo- ion, H3O . ride) through it. The calcium ions are then Ions can also be produced by ionizing replaced by sodium ions. Ion-exchange radiation; i.e. by the impact of particles or techniques are used for a vast range of pu- photons with sufficient energy to break up rification and analytical purposes. molecules or detach electrons from atoms: A → A+ + e–. Negative ions can be formed ionic bond See electrovalent bond. by capture of electrons by atoms or molecules: A + e– → A–. ionic crystal A crystal composed of ions of two or more elements. The positive and ionization energy See ionization poten- negative ions are arranged in definite pat- tial. terns and are held together by electrostatic attraction. Sodium chloride is a typical ex- ionization potential (IP; Symbol: I) The ample. energy required to remove an electron

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isomerism

from an atom (or molecule or group) in the which conversion to products is com- gas phase, i.e. the energy required for the plete; i.e. there is little or no back re- process: action. M → M+ + e– It gives a measure of the ability of metals to isoenzyme (isozyme) An ENZYME that form positive ions. The second ionization occurs in different structural forms within potential is the energy required to remove a single species. The isomeric forms all two electrons and form a doubly charged have the same molecular weight but differ- ion: ing structural configurations and proper- M → M2+ + 2e– ties. Large numbers of different enzymes Ionization potentials stated in this way are known to have isomeric forms; for ex- are positive; often they are given in elec- ample, lactate dehydrogenase has five tronvolts. Ionization energy is the energy forms. Variations in the isoenzyme consti- required to ionize one mole of the sub- tution of individuals can be distinguished stance, and is usually stated in kilojoules by electrophoresis. per mole (kJ mol–1). In chemistry, the terms ‘second’, ‘third’, isocyanide See isonitrile. etc., ionization potentials are usually used for the formation of doubly, triply, etc., isocyanide test (carbylamine reaction) charged ions. However, in spectroscopy A test for the primary amine group in or- and physics, they are often used with a dif- ganic compounds. The sample is warmed ferent meaning. The second ionization po- with trichloromethane in an alcoholic solu- tential is the energy to remove the second tion of potassium hydroxide. If a primary least strongly bound electron in forming a amine is present the resulting isocyanide singly charge ion. For lithium (ls22s1) it (RNC) has a characteristic smell of bad would refer to removal of a 1s electron to onions (and is very toxic): → produce an excited ion with the configura- CHCl3 + 3KOH + RNH2 RNC + 1 1 tion 1s 2s . 3KCl + 3H2O

ionizing radiation Radiation of suffi- isoelectronic Describing compounds ciently high energy to cause IONIZATION. It that have the same number of electrons. may be short-wavelength electromagnetic For example, carbon monoxide (CO) and radiation (ultraviolet, x-rays, or gamma nitrogen (N2) are isoelectronic. rays) or streams of particles. isoleucine See amino acid. ion pair A positive ion and a negative ion in close proximity in solution, held isomer See isomerism. by the attractive force between their charges. isomerism The existence of two or more chemical compounds with the same mo- IP See ionization potential. lecular formulae but different structural formulae or different spatial arrangements IR See infrared. of atoms. The different forms are known as isomers. For example, the compound iron A transition element occurring in C4H10 may be butane (with a straight chain many ores, especially the oxides (hematite of carbon atoms) or 2-methyl propane and magnetite) and carbonate. Iron is pre- (CH3CH(CH3)CH3, with a branched sent in a number of bioinorganic com- chain). pounds, notably HEMOGLOBIN. Structural isomerism is the type of isomerism in which the structural formulae of irreversible change See reversible change. the compounds differ. There are two main types. In one the isomers are different types irreversible reaction A reaction in of compound. An example is the com-

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isonitrile

CH3 H2 C CH3 C H C C C C H3 H2 H3 H3

butane methylpropane (n-butane) (isobutane)

Isomer: isomers differing in carbon skeleton

O H2 C C C H C H3 3 H3 OH

dimethyl ether ethanol

Isomer: isomers differing in the nature of the functional group

H2 C OH C H C C C C H3 H2 H3 H3

propan-1-ol propan-2-ol (n-propanol) (isopropanol)

Isomer: isomers differing in the position of a functional group

H2 H C C C H3C C C CH3

CH3

propan-1-yne propan-2-yne

Isomer: isomers differing in the position of a multiple bond. See also illustration at alkene.

pounds ethanol (C2H5OH) and methoxy- arrangement of the groups in space. There methane (CH3OCH3), both having the for- are two types of stereoisomerism. mula C2H6O but different functional Cis-trans isomerism (or geometrical iso- groups. In the other type of structural isomerism) occurs when there is restricted ro- merism, the isomers differ because of the tation about a bond between two atoms. position of a functional group in the mol- Groups attached to each atom may be on ecule. For example, the primary alcohol the same side of the bond (the cis isomer) propan-1-ol (CH3CH2CH2OH) and the or opposite sides (the trans isomer). See secondary alcohol propan-2-ol (CH3- also E–Z convention. CH(OH)CH3) are isomers; both have the Optical isomerism occurs when the molecular formula C3H7OH. compound has no plane of symmetry and Stereoisomerism occurs when two com- can exist in left- and right-handed forms pounds with the same molecular formulae that are mirror images of each other. Such and the same groups differ only in the molecules have an asymmetric atom – i.e.

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isotope

Cl Cl Cl H

CC CC

H H H Cl

cis-dichloroethene trans-dichloroethene

Isomer: cis–trans isomerism in an alkene

CH3 OH CH3

NC NC

C2H6 C2H6 OH

E-methylethylketone oxime Z-methylethylketone oxime

Isomer: E–Z isomerism in an oxime

H O H H O C2H5

C C C C

C2H5 C H C2H5 2 5 H cis-diethyl epoxide trans-diethyl epoxide

Isomer: cis–trans isomerism in a ring compound

one attached to four different groups – thermal equilibrium with its surroundings. called a chiral center. Molecules that are For example, a cylinder of gas in contact mirror images of each other are more prop- with a constant-temperature box may be erly called enantiomers. Stereoisomers that compressed slowly by a piston. The work are not mirror images are called diastereo- done appears as energy, which flows into isomers. ANOMERS are examples of dia- the reservoir to keep the gas at the same stereoisomers. See also optical activity. temperature. Isothermal changes are con- trasted with adiabatic changes, in which no isonitrile (isocyanide) An organic com- energy enters or leaves the system, and the pound of the formula R–NC. temperature of the system changes. In practice no process is perfectly isothermal and isoprene See methylbuta-1,3-diene. none is perfectly adiabatic, although some can approximate in behavior to one of isopropanol Propan-2-ol. See propanol. these ideals.

isotactic polymer See polymerization. isotones Two or more nuclides that have the same neutron numbers but differ- isotherm A line on a chart or graph join- ent proton numbers. ing points of equal temperature. See also isothermal change. isotonic Describing solutions that have the same osmotic pressure. isothermal change A process that takes place at a constant temperature. Through- isotope One of two or more species of out an isothermal process, the system is in the same element differing in their mass

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isotopic mass

numbers because of differing numbers of ters; stable isotopes can also be used, and neutrons in their nuclei. The nuclei must detected by a mass spectrum. have the same number of protons (an el- Isotopes are also used in kinetic studies ement is characterized by its proton num- to investigate the mechanism of a particu- ber). Isotopes of the same element have lar reaction. For example, if the bond be- very similar properties because they have tween two atoms X–Y is broken in the the same electron configuration, but differ rate-determining step, and Y is replaced by slightly in their physical properties. An un- a heavier isotope of the element, Y*, then stable isotope is termed a radioactive iso- the reaction rate will be slightly lower with tope or radioisotope the Y* present. This difference in rate, Isotopes of elements are useful in chem- known as a kinetic isotope effect, is significant only for reactions in which the rate- istry for studies of the mechanisms of chem- determining step involves breaking a bond ical reactions. A standard technique is to to hydrogen (or deuterium) because the label one of the atoms in a molecule by using deuterium atom has twice the mass of the an isotope of the element. It is then possible hydrogen atom. to trace the way in which this atom behaves throughout the course of the reaction. For isotopic mass (isotopic weight) The mass example, in the esterification reaction: number of a given isotope of an element. ′ ˆ ′ ROH + R COOH H2O + R COOR it is possible to find which bonds are bro- isotopic number The difference be- ken by using a labeled oxygen atom. If the tween the number of neutrons in an atom reaction is performed using 18O in the al- and the number of protons. cohol it is found that this nuclide appears in the ester, showing that the C–OH bond isotopic weight See isotopic mass. of the acid is broken in the reaction. In labeling, radioisotopes are detected by coun- isozyme See isoenzyme.

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Jones oxidation The reaction in which joule Symbol: J The SI unit of energy a secondary alcohol is oxidized to a ketone and work, equal to the work done when using sodium chromate(VII) (Na2Cr2O7) in the point of application of a force of one dilute sulfuric acid. The reaction involves newton moves one meter in the direction of the formation of an intermediate chromate action of the force. 1 J = 1 N m. The joule ester of the type R2HC–O–CrO3H. is the unit of all forms of energy.

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kairomone A chemical messenger emit- behave like a substance with double bonds ted by an individual of a species and caus- Kekulé put forward the idea in 1872 that ing a response in an individual of another there is oscillation between two structures species. This may be detrimental to the that are isomers. This view was justified producer of the kairomone, for example about 60 years later by Linus PAULING many parasites are attracted to their hosts using quantum mechanics. by an excreted kairomone. See also pheromone. kelvin Symbol: K The SI base unit of thermodynamic temperature. It is defined Karrer, Paul (1889–1971) Swiss or- as the fraction 1/273.16 of the thermody- ganic chemist. Karrer was famous for his namic temperature of the triple point of work on vitamins and vegetable pigments. water. Zero kelvin (0 K) is absolute zero. In 1930 he determined the structure of One kelvin is the same as one degree on the carotene. In 1931 he synthesized vitamin Celsius scale of temperature. The unit is A, having worked out the structure. Karrer named for the British physicist Lord Kelvin was aware of the similarity between the (William Thomson; 1824–1907). two molecules. He subsequently determined the structures of each and synthe- Kendrew, Sir John Cowdery (1917–97) sized riboflavin (vitamin B ) in 1937 and 2 British biochemist. Kendrew is renowned tocopherol (vitamin E) in 1938. Karrer for having determined the structure of the shared the 1937 Nobel Prize for chemistry protein molecule myoglobin. He did so with Norman HAWORTH for ‘the constitu- using x-ray crystallography. He was one of tion of carotenoids, flavins, and vitamins A the first people to use electronic computers and B’. Karrer wrote an influential textbook entitled Textbook of Organic Chem- as an aid in analyzing the data produced by istry (1927). x-ray diffraction. He was able to determine the structure of myoglobin by 1960. He katharometer See gas chromatography. shared the 1962 Nobel Prize for chemistry with Max PERUTZ, his colleague at the Lab- Kekulé, Friedrich August von oratory for Molecular Biology, Cam- Stradonitz (1829–96) German organic bridge. chemist. Kekulé was the founder of the concept of structure in organic chemistry keratin One of a group of fibrous insol- but is probably best remembered for the uble sulfur-containing proteins (scleropro- ring structure of benzene. In 1858, inde- teins) found in ectodermal cells of animals, pendently of Archibald Couper, Kekulé as in hair, horns, and nails. Leather is al- put forward the idea that carbon is a most pure keratin. There are two types: α quadrivalent element which can combine keratins and β keratins. The former have a with other carbon atoms. In 1865 he pro- coiled structure, whereas the latter have a posed the idea that the structure of benzene beta pleated sheet structure. is a hexagonal ring of carbon atoms. In response to objections that benzene does not kerosene See petroleum.

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kilogram

H OH O CC ˆ CC H H enol form keto form

Keto–enol tautomerization

ketal See acetal. 1. Undergo addition reactions with hydrogen cyanide and hydrogensulfite (bisul- keten See ketene. fite) ions. 2. Undergo condensation reactions with ketene (keten) A member of a group of hydroxylamine, hydrazine, and their de- organic compounds, general formula rivatives. R2C:CO, where R is hydrogen or an or- 3. Are reduced to (secondary) alcohols. ganic radical. The simplest member is the They are not, however, easily oxidized. colorless gas ketene, CH2:CO; the other Strong oxidizing agents give a mixture of ketenes are colored because of the presence carboxylic acids. They do not react with of the double bonds. Ketenes are unstable Fehling’s solution or Tollen’s reagent, and and react with other unsaturated com- do not easily polymerize. pounds to give cyclic compounds. ketone body One of a group of organic keto–enol tautomerism A type of tau- substances formed in fat metabolism, tomerism in which a compound containing mainly in the liver. Examples are aceto- the –CH2–CO– group (the keto form) acetic acid and acetone. Ketone bodies are interconverts with one containing the major fuel source for resting skeletal –CH=CH(OH)– (the enol) by hydrogen muscle. If the body has little or no carbo- migration. hydrate as a respiratory substrate, ketosis occurs, in which more ketone bodies are keto form See keto–enol tautomerism. produced than the body can use.

ketohexose A ketose sugar with six car- ketopentose A ketose SUGAR with five bon atoms. See sugar. carbon atoms.

ketone A type of organic compound ketose A SUGAR containing a keto- with the general formula RCOR, having (=CO) or potential keto- group. two alkyl or aryl groups bound to a carbonyl group. They are made by oxidizing kieselguhr A siliceous deposit formed secondary alcohols (just as aldehydes are by diatoms, used as an absorbent, filter, made from primary alcohols). Simple ex- and filler. amples are propanone (acetone, 3 CH3COCH3) and butanone (methyl ethyl kilo- Symbol: k A prefix denoting 10 . 3 ketone, CH3COC2H5). For example, 1 kilometer (km) = 10 The chemical reactions of ketones are meters (m). similar in many ways to those of ALDEHY- DES. The carbonyl group is polarized, with kilogram (kilogramme; symbol: kg) The positive charge on the carbon and negative SI base unit of mass, equal to the mass of charge on the oxygen. Thus nucleophilic the international prototype of the kilo- addition can occur at the carbonyl group. gram, which is a piece of platinum–iridium Ketones thus: kept at Sèvres in France.

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kilogramme

kilogramme An alternative spelling of Klug, (Sir) Aaron (1926– ) Lithuan- kilogram. ian-born British molecular biologist distinguished for his determination of the kilowatt-hour Symbol: kWh A unit of structure of transfer RNA and for his work energy, usually electrical, equal to the en- on three-dimensional structure of com- ergy transferred by one kilowatt of power plexes of proteins and nucleic acids. He in one hour. It has a value of 3.6 × 106 was awarded the Nobel Prize for chemistry joules. in 1982 for his development of crystallo- graphic electron microscopy and his work kinetic energy See energy. on the structures of biologically important nucleic acid–protein complexes. kinetic isotope effect See isotope. Kolbe, Adolph Wilhelm Hermann kinetics A branch of physical chemistry (1818–84) German organic chemist. Kolbe concerned with the study of rates of chem- made many important contributions to the ical reactions and the effect of physical development of organic chemistry. These conditions that influence the rate of reac- included the synthesis of ethanoic acid tion, e.g. temperature, light, concentration, from completely inorganic materials, the etc. The measurement of these rates under formation and hydrolysis of nitriles, the different conditions gives information on electrolysis of solutions of fatty acid salts the mechanism of the reaction, i.e. on the and the synthesis of salicylic acid from phe- sequence of processes by which reactants nol and carbon dioxide. In his later years are converted into products. his influence had a negative effect on the development of chemistry because he op- Kipp’s apparatus An apparatus for the posed concepts such as structure. production of a gas from the reaction of a Kolbe electrolysis The electrolysis of liquid on a solid. It consists of three globes, sodium salts of carboxylic acids to prepare the upper globe being connected via a wide alkanes. The alkane is produced at the tube to the lower globe. The upper globe is anode after discharge of the carboxylate the liquid reservoir. The middle globe con- anion and decomposition of the radical: tains the solid and also has a tap at which RCOO– → RCOO• + e– the gas may be drawn off. When the gas is → RCOO• R• + CO2 drawn off the liquid rises from the lower → R• + RCOO• R – R + CO2 globe to enter the middle globe and reacts and with the solid, thereby releasing more gas. 2R• → R – R When turned off the gas released forces the As the reaction is a coupling reaction, liquid back down into the lower globe and only alkanes with an even number of car- up into the reservoir, thus stopping the re- bon atoms in the chain can be prepared in action. this way. It is named for the German chemist Adolph Kolbe. Kjeldahl’s method A method used for the determination of nitrogen in organic Kornberg, Arthur (1918– ) Ameri- compounds. The nitrogenous substance is can biochemist. Kornberg conducted reconverted to ammonium sulfate by boiling search on enzymes at the start of his career. with concentrated sulfuric acid (often with In this period he clarified the chemical re- a catalyst such as CuSO4) in a specially de- actions leading to certain enzymes. In 1956 signed long-necked Kjeldahl flask. The he discovered an enzyme, which he called mixture is then made alkaline and the am- DNA polymerase, that catalyses the monia distilled off into standard acid for process of forming polynucleotides from measurement by titration. It is named for nucleoside triphosphates. This enabled him the Danish chemist Johan Kjeldahl to synthesize short DNA molecules, start- (1849–1900). ing from a DNA template and triphosphate

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Kroto, Sir Harold Walter

bases. He shared the 1959 Nobel Prize for Krebs cycle (tricarboxylic acid cycle; TCA medicine for this work with Severo Ochoa, cycle; citric acid cycle) A complex and who found the enzyme that catalyses the almost universal cycle of reactions in formation of RNA. which the acetyl group of acetyl CoA is oxidized to carbon dioxide and water, with Kossel, Karl Martin Leonhard Al- the production of large amounts of energy. brecht (1853–1927) German chemist It is the final common pathway for the ox- noted for his discovery of adenine, cyto- idation of carbohydrates, fatty acids, and sine, thymine, and uracil as breakdown amino acids. It requires oxygen, and in eu- products of nucleic acids. He also dis- karyotes occurs in the mitochondrial ma- covered histone and histidine. He was trix. awarded the Nobel Prize for physiology or 2-carbon acetate reacts with 4-carbon medicine in 1910 in recognition of his oxaloacetate to form 6-carbon citrate, work on cell chemistry. which is then decarboxylated to reconsti- tute oxaloacetate. Some ATP is produced Krebs, (Sir) Hans Adolf (1900–81) by direct coupling with cycle reactions, but German-born British biochemist renowned most production is coupled to the electron- for his work on metabolism and, in partic- transport chain via the generation of re- ular, for the discovery of the cyclic meta- duced coenzymes, NADH and FADH2. See bolic pathways named after him. He was electron-transport chain. awarded the Nobel Prize for physiology or medicine in 1953 for his discovery of the Kroto, Sir Harold Walter (1939– ) TCA cycle. The prize was shared with F. A. British chemist. In the mid-1980s Harold Lipmann. Kroto heard that the American chemist

glycolysis In the Krebs cycle acetyl (from pyruvate) is broken down to CO2 and H. The H is held in NADH (from + pyruvate reduction of NAD ). The NADH then is reoxidized in an electron- transport chain, with production of acetyl CoA ATP. CoA

2H oxaloacetate citrate

malate isocitrate

CO2 + 2H H2O fumarate α-oxoglutarate

succinate succinyl CO + 2H CoA 2

ATP ADP

Krebs cycle

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Kroto, Sir Harold Walter

Richard Smalley was using laser bombard- hexagons. Kroto called this molecule buck- ment as a new technique to produce clus- minsterfullerene after its resemblance to ters of atoms. When Kroto visited Smalley the designs of the architect Buckminster he persuaded him to bombard graphite Fuller. The name of the molecule is usually with a laser beam. When Smalley did so he abbreviated to fullerene. Such molecular found not only the chains that Kroto had structures are frequently called ‘bucky been expecting but a molecule with 60 car- balls’. Kroto and Smalley shared the 1996 bon atoms. Kroto and Smalley correctly Nobel Prize for chemistry with Robert postulated that this molecule is a poly- Curl for their parts in the discovery of C60 hedron in which the faces are pentagons or molecules.

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label A stable or radioactive ISOTOPE lactate A salt or ester of lactic acid. used to investigate a chemical reaction. La- beling is a common method of investigat- lactic acid (2-hydroxypropanoic acid) A ing chemical reaction mechanisms. A colorless liquid carboxylic acid: classic example is the hydrolysis of an ester CH3CH(OH)COOH to give a carboxylic acid and an alcohol, as See optical activity. in: → H2O + CH3COOCH3 CH3COOH + lactim See lactam. CH3OH It is possible to investigate the mechanism lactone A type of organic compound by using an ester enriched with the isotope containing the group –O.CO– as part of a 18O. If the 18O is on the oxygen attached to ring in the molecule. A lactone can be re- the carbonyl, it is found that it ends up in garded as formed from a compound with the alcohol rather than the acid: an alcohol (–OH) group on one end of the 18 → H2O + CH3CO OH CH3COOH + chain and a carboxylic acid (–COOH) 18 CH3 OH group on the other. The lactone then re- This, and similar experiments using labeled sults from reaction of the –OH group with water and labeling of the carbonyl oxygen, the –COOH group; i.e. it is an internal help to establish the mechanism. ester. Certain radioisotopes such as tritium 3 14 ( H) and C have been used as labels but lactose (milk sugar; C12H22O11)A it is now more usual to use deuterium (2H), SUGAR found in milk. It is a disaccharide 13C, and 18O, which can be detected by composed of glucose and galactose units. mass spectroscopy. Ladenburg benzene An incorrect lactam A type of organic compound structure for BENZENE in which the six car- containing the –NH.CO– group as part of bon atoms are at the corners of a triangu- a ring in the molecule. Lactams can be re- lar prism. It is named for Albert Ladenburg garded as formed from a straight-chain (1842–1911). The actual compound, compound that has an amino group known as prismane, was synthesized in (–NH2) at one end of the molecule and a 1973. carboxylic acid group (–COOH) at the other; i.e. from an amino acid. The reac- lake A pigment formed by absorbing an tion of the amine group with the carboxylic organic dyestuff on an inorganic oxide, hy- acid group, with elimination of water, droxide, or salt. leads to the cyclic lactam, which is thus an internal (or inner) amide. Lactams can lamellar compound A compound with exist in an alternative tautomeric form in a crystal structure composed of thin plates which the hydrogen atom has migrated or layers. Silicates form many compounds from the nitrogen onto the O of the car- with distinct layers. Typical examples are bonyl. This, the lactim form, contains the talc (Mg3(OH)2Si4O10) and pyrophyllite group –N=C(OH)–. (Al2(OH)2Si4O10). 131 iranchembook.ir/edu

lanolin

lanolin A yellowish viscous substance appearance but may be colorless, orange, obtained from wool fat. It contains choles- or brown. Its function is obscure, but may terol and terpene compounds, and is used be involved in wound healing as well as a in cosmetics, in ointments, and in treating repository for excretory substances. Com- leather. mercial rubber comes from the latex of the rubber plants Ficus elastica and Hevea Lapworth, Arthur (1872–1941) British brasiliensis.Opium comes from alkaloids organic chemist. In the early part of his ca- found in the latex of the opium poppy. reer Lapworth investigated the structure of camphor and related compounds. How- lattice A regular three-dimensional ever, his most significant contribution to arrangement of points. A lattice is used to chemistry was his work, starting in 1920, describe the positions of the particles on the mechanisms of organic molecules in (atoms, ions, or molecules) in a crystalline terms of the electrons in the molecules. For solid. The lattice structure can be exam- example, he was one of the first people to ined by x-ray diffraction techniques. emphasize that ionization can occur in organic molecules and that parts of these lauric acid See dodecanoic acid. molecules can have electrical charges, either on a permanent basis or when the re- law of conservation of energy See action is occurring. Similar views on conservation of energy; law of. describing organic reactions in terms of electrons were developed by Sir Christo- law of conservation of mass See con- pher INGOLD and Sir Robert ROBINSON. servation of mass; law of. laser An acronym for Light Amplifica- law of constant composition See con- tion by Stimulated Emission of Radiation. stant proportions; law of. A laser device produces high-intensity, monochromatic, coherent beams of light. law of constant proportions See con- In the laser process the molecules of a sample (such as ruby doped with Cr3+ ions) are stant proportions; law of. promoted to an excited state. As the sample is in a cavity between two reflective law of definite proportions See defi- surfaces, when a molecule emits sponta- nite proportions; law of. neously, the photon so generated ricochets backward and forward. In this way other law of equivalent proportions See molecules are stimulated to emit photons equivalent proportions, law of. of the same energy. If one of the reflective surfaces is partially transmitting this radia- law of mass action See mass action; tion can be tapped. law of.

latent heat The heat evolved or ab- law of reciprocal proportions See sorbed when a substance changes its phys- equivalent proportions, law of. ical state, e.g. the latent heat of fusion is the heat absorbed when a substance changes laws of chemical combination See from a solid to a liquid. chemical combination; laws of.

latex A liquid found in some flowering LDL See low-density lipoprotein. plants contained in special cells or vessels called laticifers (or laticiferous vessels). It is leaching The washing out of a soluble a complex variable substance that may material from an insoluble solid using a contain terpenes (e.g. rubber), resins, tan- solvent. This is often carried out in batch nins, waxes, alkaloids, sugar, starch, en- tanks or by dispersing the crushed solid in zymes, crystals, etc. It is often milky in a liquid.

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lignin

lead-free fuel Vehicle fuel that contains L-form See optical activity. none of the anti-knock agent LEAD TETRA- ETHYL. Liebig, Justus von (1803–73) German organic chemist. Lieberg was one of the lead tetraethyl (tetraethyl lead; most influential chemists of the 19th cen- Pb(CH2CH3)4) A poisonous liquid that is tury. The early part of his career was de- insoluble in water but soluble in organic voted to classical organic chemistry. After solvents. It is manufactured by the reaction 1840 he mostly devoted himself to bio- of an alloy of sodium and lead with chemistry and the application of chemistry 1-chloroethane. The product is obtained to agriculture. In 1826 he made his first by steam distillation. Lead tetraethyl was significant discovery when, together with formerly extensively used as an additive in his long-standing colleague Friedrich WÖH- internal-combustion engine fuel to increase LER, he discovered that silver fulminate and its octane number and thus prevent pre- silver cyanate have the same chemical for- ignition (knocking). mula. In this way, Liebig and Wöhler discovered isomerism. Liebig and Wöhler leaving group The group leaving a mol- discovered the benzoyl radical in 1832. ecule in a substitution or elimination reac- When Liebig moved into biochemistry his tion. The nature of the leaving group is an tendency towards dogmatism became even important factor in the progress of the re- more pronounced than it had been previ- action. ously. As a result, he became involved in many acrimonious controversies. He was a Le Chatelier’s principle If a system is prolific writer of books and papers. at equilibrium and a change is made in the conditions, the equilibrium adjusts so as to Liebig condenser A simple type of lab- oppose the change. The principle can be oratory condenser. It consists of a straight applied to the effect of temperature and glass tube, in which the vapor is con- pressure on chemical reactions. A good ex- densed, with a surrounding glass jacket ample is the Haber process for synthesis of through which cooling water flows. It is ammonia: named for the German chemist Justus von ˆ N2 + 3H2 2NH3 Liebig. The ‘forward’ reaction → N2 + 3H2 2NH3 ligand A molecule or ion that forms a is exothermic. Thus, reducing the tempera- coordinate bond to a metal atom or ion in ture displaces the equilibrium towards pro- a complex. duction of NH3 (as this tends to increase temperature). Increasing the pressure also light (visible radiation) A form of elec- favors formation of NH3, because this tromagnetic radiation able to be detected leads to a reduction in the total number of by the human eye. Its wavelength range is molecules (and hence pressure). The princi- between about 400 nm (far red) and about ple is named for the French chemist Henri 700 nm (far violet). The boundaries are not Louis Le Chatalier (1850–1936), who dis- precise because individuals vary in their covered it in 1884. ability to detect extreme wavelengths; this ability also declines with age. leucine See amino acid. lignin One of the main structural ma- levo-form See optical activity. terials of vascular plants. With cellulose it is one of the main constituents of wood. levorotatory See optical activity. Lignified tissues include sclerenchyma and xylem. Lignin is deposited during sec- Lewis acid See acid. ondary thickening of cell walls. The degree of lignification varies but values of 25– Lewis base See acid. 30% lignin and 50% cellulose are average.

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lignite

It is a complex variable polymer, derived such as chloroplasts and mitochondria; (2) from sugars via aromatic alcohols. Phenyl- compartmentalizing units for metaboli- propane (C6–C3) units are linked in various cally active proteins localized in mem- ways by oxidation reactions during poly- branes; (3) a store of chemical energy and merization. carbon skeletons; and (4) primary transport systems of nonpolar material through lignite (brown coal) The poorest grade biological fluids. There are also the more of coal, containing up to 45% carbon and physiologically specific lipid hormones, with a high moisture content. e.g. the steroid hormones and lipid vitamins. The simple lipids include neutral lipids ligroin A mixture of hydrocarbons ob- or glycerides, which are esters of glycerol tained from petroleum, used as a general and fatty acids, and the waxes, which are solvent. It has a boiling range of 80 to esters of long-chain monohydric alcohols 120°C. See petroleum. and fatty acids. Compound lipids have one of the fatty limiting step See rate-determining step. acid parts replaced, such that complete hydrolysis gives only two fatty acids; the line spectrum A SPECTRUM composed of phospholipids, which are particularly im- a number of discrete lines corresponding to portant examples. single wavelengths of emitted or absorbed radiation. Line spectra are produced by lipoic acid A sulfur-containing fatty atoms or simple (monatomic) ions in gases. acid found in a wide variety of natural ma- Each line corresponds to a change in elec- terials. It is an essential as a coenzyme for tron orbit, with emission or absorption of certain dehydrogenase enzymes, notably radiation. pyruvate dehydrogenase, which catalyzes the dehydrogenation of pyruvic acid to linoleic acid (C17H31COOH) An unsat- form acetyl-CoA. Lipoic acid is classified urated carboxylic acid that occurs in LIN- with the water-soluble B vitamins and has SEED OIL and other plant oils. It contains not yet been demonstrated to be required two double bonds. in the diet of higher animals.

linolenic acid (C17H29COOH) A liquid lipopolysaccharide A conjugated polyunsaturated carboxylic acid that occurs in saccharide in which the noncarbohydrate LINSEED OIL and other plant oils. It contains part is a lipid. Lipopolysaccharides are a three double bonds. constituent of the cell walls of certain bacteria. linseed oil An oil extracted from the seeds of flax (linseed). It hardens on expo- lipoprotein Any conjugated protein sure to air (it is a drying oil) because it con- formed by the combination of a protein tains linoleic acid, and is used in enamels, with a lipid. In the blood of humans and paints, putty, and varnishes. See linoleic other mammals, cholesterol, triglycerides, acid. and phospholipids associate with various plasma proteins to form lipoproteins. lipid A collective term used to describe a These are particles with diameters in the re- group of substances in cells characterized gion of 7.5–70 nm, and are placed in sev- by their solubility in organic solvents such eral classes. The largest lipoproteins in this as ether and benzene, and their absence of range are the very low-density lipoproteins solubility in water. The group is rather het- (VLDLs), which are formed in the liver erogeneous in terms of both function and and contain up to about 20% choles- structure. They encompass the following terol. Low-density lipoproteins (LDLs) are broad bands of biological roles: (1) basic formed in plasma from VLDLs and contain structural units of cellular membranes and over 50% cholesterol. LDLs transport cho- cytologically distinct subcellular bodies lesterol from the liver to peripheral tissues,

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Lonsdale, Dame Kathleen

and an excess of LDLs in the blood is a being in excess. Lithium tetrahydridoalu- factor in the development of fatty arterial minate reacts violently with water. It is a deposits and cardiovascular disease. High- powerful reducing agent, reducing ketones density lipoproteins (HDLs), with about and carboxylic acids to their correspond- 20% cholesterol, are the smallest of the ing alcohols. In inorganic chemistry it is plasma lipoproteins, and apparently func- used in the preparation of hydrides. tion in transporting cholesterol from tissues to the liver. litmus A natural pigment that changes color when in contact with acids and alka- liquefied natural gas (LNG) Liquid lis; above a pH of 8.3 it is blue and below methane, obtained from natural gas and a pH of 4.5 it is red. Thus it gives a rough used as a fuel. See methane; natural gas. indication of the acidity or basicity of a solution; because of its rather broad range of liquefied petroleum gas (LPG) A mix- color change it is not used for precise work. ture of liquefied hydrocarbon gases Litmus is used both in solution and as lit- (mainly propane) extracted from petro- mus paper. leum, used as a fuel for internal combustion engines and for heating. See lixiviation The process of separating petroleum; propane. soluble components from a mixture by washing them out with water. liquid The state of matter in which the particles of a substance are loosely bound LNG See liquefied natural gas. by intermolecular forces. The weakness of these forces permits movement of the par- localized bond A bond in which the ticles and consequently liquids can change electrons contributing to the bond remain their shape within a fixed volume. The liq- between the two atoms concerned, i.e. the uid state lacks the order of the solid state. bonding orbital is localized. The majority Thus, amorphous materials, such as glass, of bonds are of this type. Compare delo- in which the particles are disordered and calized bond. can move relative to each other, can be classed as liquids. lone pair A pair of valence electrons having opposite spin that are located to- liquid–liquid extraction See solvent gether on one atom, i.e. are not shared as extraction. in a covalent bond. Lone pairs occupy similar positions in space to bond pairs and ac- liter Symbol: l A unit of volume now de- count for the shapes of molecules. A fined as 10–3 meter3; i.e. 1000 cm3. The molecule with a lone pair can donate the milliliter (ml) is thus the same as the cubic pair of electrons to an electron acceptor, centimeter (cm3). However, the name is not such as H+ or a metal ion, to form coordi- recommended for precise measurements nate bonds. because the liter was formerly defined as the volume of one kilogram of pure water long period See period. at 4°C and standard pressure. On this definition, one liter is the same as 1000.028 Lonsdale, Dame Kathleen (1903–71) cubic centimeters. British x-ray crystallographer. Dame Kath- leen Lonsdale was an early pioneer of x-ray lithium aluminum hydride See lithium crystallography. In 1929 she showed the tetrahydridoaluminate(III). hexagonal ring nature of the hexamethyl- benzene molecule. In 1931 she investigated lithium tetrahydridoaluminate(III) the structure of hexachlorobenzene. This (lithium aluminum hydride; LiAlH4)A was the first investigation in which Fourier white solid produced by action of lithium analysis had been used for an organic mol- hydride on aluminum chloride, the hydride ecule. She subsequently investigated many

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low-density lipoprotein

problems including thermal motion in particularly for large molecules, such as crystals, the magnetic susceptibility of crys- polymers. See also Raoult’s law. tals and the composition of bladder stones. She reviewed her research in her book Lowry–Brønsted theory See acid. Crystals and X-Rays (1948). She was also the editor of the first three volumes of the LPG See liquefied petroleum gas. International Tables for X-Ray Crystallog- raphy (1952, 1959, 1962). LSD See lysergic acid diethylamide.

low-density lipoprotein (LDL) A lumen Symbol: lm The SI unit of lumi- spherical particle, typically about 20–25 nous flux, equal to the luminous flux emit- nm in diameter, that is found in blood ted by a point source of one candela in a plasma and transports cholesterol to tissue solid angle of one steradian. 1 lm = 1 cd sr. cells. It is bounded by a single layer of phospholipid and free cholesterol, which luminescence The emission of radiation encloses a core of cholesterol esterified to a from a substance in which the particles long-chain fatty acid. Embedded in the sur- have absorbed energy and gone into ex- face layer is a single large protein, called cited states. They then return to lower en- apo-B, which assists in binding of the LDL ergy states with the emission of to cell-surface receptors. LDLs are taken electromagnetic radiation. If the lumines- into cells by receptor-mediated endocyto- cence persists after the source of excitation sis. The cholesterol is incorporated into cell is removed it is called phosphorescence: if membranes or stored as lipid droplets. not, it is called fluorescence. High concentrations of LDLs in the blood have been associated with an increased risk lutein The commonest of the xantho- of atherosclerosis (‘hardening of the arter- phyll pigments. It is found in green leaves ies’). and certain algae, e.g. the Rhodophyceae. See photosynthetic pigments. lowering of vapor pressure A colligative property of solutions in which the lux Symbol: lx The SI unit of illumina- vapor pressure of a solvent is lowered as a tion, equal to the illumination produced by solute is introduced. When both solvent a luminous flux of one lumen falling on a and solute are volatile the effect of increas- surface of one square meter. 1 lx = 1 lm ing the solute concentration is to lower the m–2. partial vapor pressure of each component. When the solute is a solid of negligible lyophilic Solvent attracting. When the vapor pressure the lowering of the vapor solvent is water, the word hydrophilic is pressure of the solution is directly propor- often used. The terms are applied to: tional to the number of species introduced 1. Ions or groups on a molecule. In aque- rather than to their nature and the propor- ous or other polar solutions ions or tionality constant is regarded as a general polar groups are lyophilic. For example, solvent property. Thus the introduction of the –COO– group on a soap is the the same number of moles of any solute lyophilic (hydrophilic) part of the mol- causes the same lowering of vapor pres- ecule. sure, if dissociation does not occur. If the 2. The disperse phase in colloids. In solute dissociates into two species on dis- lyophilic colloids the dispersed particles solution the effect is doubled. The kinetic have an affinity for the solvent, and the model for the lowering of vapor pressure colloids are generally stable. Compare treats the solute molecules as occupying lyophobic. part of the surface of the liquid phase and thereby restricting the escape of solvent lyophobic Solvent repelling. When the molecules. The effect can be used in the solvent is water, the word hydrophobic is measurement of relative molecular masses, used. The terms are applied to:

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lysine

1. Ions or groups on a molecule. In aque- lysergic acid diethylamide (LSD) A ous or other polar solvents, the lyopho- synthetic organic compound that has phys- bic group will be nonpolar. For iological effects similar to those produced example, the hydrocarbon group on a by alkaloids in certain fungi. Even small soap molecule is the lyophobic (hy- quantities, if ingested, produce hallucina- drophobic) part. tions and extreme mental disturbances. 2. The disperse phase in colloids. In The initials LSD come from the German lyophobic colloids the dispersed parti- form of the chemical’s name, Lysergic- cles are not solvated and the colloid is Saure-Diathylamide. easily solvated. Gold and sulfur sols are examples. Compare lyophilic. lysine See amino acid.

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macromolecular crystal A crystal maleic acid See butenedioic acid. composed of atoms joined together by covalent bonds that form giant three-dimen- malic acid (2-hydroxybutanedioic acid; sional or two-dimensional networks. HCOOCH2CH(OH)COOH) A color- Diamond is an example of a macromolecu- less crystalline CARBOXYLIC ACID found in lar crystal. unripe fruits. It tastes of apples and is used in food flavorings. macromolecule A large molecule; e.g. a natural or synthetic polymer. malonic acid See propanedioic acid.

magic acid See superacid. maltose (C12H22O11)A SUGAR found in germinating cereal seeds. It is a disaccha- magnetic quantum number See atom. ride composed of two glucose units. Mal- tose is an important intermediate in the magnetism The study of the nature and enzyme hydrolysis of starch. It is further cause of magnetic force fields, and how dif- hydrolyzed to glucose. ferent substances are affected by them. Magnetic fields are produced by moving mannitol (HOCH2(CHOH)4CH2OH) A charge – on a large scale (as with a current soluble hexahydric alcohol that occurs in in a coil, forming an electromagnet), or on many plants and fungi. It is used in medicines and as a sweetener (particularly in the small scale of the moving charges in the foods for diabetics). It is an isomer of sor- atoms. It is generally assumed that the bitol. Earth’s magnetism and that of other plan- ets, stars, and galaxies have the same cause. mannose (C H O ) A simple SUGAR Substances may be classified on the 6 12 6 found in many polysaccharides. It is an basis of how samples interact with fields. aldohexose, isomeric with glucose. Different types of magnetic behavior result from the type of atom. Diamagnetism, manometer A device for measuring which is common to all substances, is due pressure. A simple type is a U-shaped glass to the orbital motion of electrons. Para- tube containing mercury or other liquid. magnetism is due to electron spin, and a The pressure difference between the arms property of materials containing unpaired of the tube is indicated by the difference in electrons. It is particularly important in heights of the liquid. transition-metal chemistry, in which the complexes often contain unpaired elec- Markovnikoff’s rule A rule that pre- trons. Magnetic measurements can give in- dicts the quantities of the products formed formation about the bonding in these when an acid (HA) adds to the double complexes. Ferromagnetism, the strongest bond in an alkene. If the alkene is not sym- effect, also involves electron spin and the metrical two products may result; for in- alignment of magnetic moments in do- stance (CH3)2C:CH2 can yield either mains. (CH3)2HCCH2A or (CH3)2ACCH3. The 138 iranchembook.ir/edu

mass spectrometer

rule states that the major product will be determines its gravitational attraction for the one in which the hydrogen atom at- other masses. The SI unit of mass is the taches itself to the carbon atom with the kilogram. larger number of hydrogen atoms. In the example above, therefore, the major prod- mass action, law of The principle that, uct is (CH3)2ACCH3. at constant temperature, the rate of a The Markovnikoff rule is explainable if chemical reaction is directly proportional the mechanism is ionic. The first step is ad- to the active mass of the reactants, the ac- dition of H+ to one side of the double bond, tive mass being taken as the concentration forming a carbonium ion. The more stable for a reaction in solution or the partial form of carbonium ion will be the form in pressure for a gas reaction. For the reaction which the positive charge appears on the A + B → products, the law of mass action carbon atom with the largest number of states that: alkyl groups – thus the hydrogen tends to rate = k[A][B] attach itself to the other carbon. The posi- where [A] represents the concentration of tive charge is partially stabilized by the A, [B] the concentration of B, and k is a electron-releasing (inductive) effect of the constant dependent on the particular reac- alkyl groups. tion. The interpretation of active mass as Additions of this type do not always concentration or partial pressure is only follow the Markovnikoff rule. Under cer- valid if there is no interaction or interfer- tain conditions the reaction may involve ence between the reacting molecules. In the free radicals H• and A•, in which case general the concentration has to be multi- the opposite (anti-Markovnikoff) effect plied by an ACTIVITY COEFFICIENT in order occurs. to obtain the actual active mass. See activity coefficient. marsh gas Methane produced in marshes by decomposing vegetation. mass number See nucleon number.

Martin, Archer John Porter (1910– ) mass spectrometer An instrument for British biochemist. Martin is best known producing ions and analyzing them accord- for the development of paper chromatog- ing to their charge/mass ratio. The earliest raphy with Richard SYNGE, starting in experiments by the British physicist J. J. 1941. Martin and Synge won the 1952 Thomson (1856–1940) used a stream of Nobel Prize for chemistry for this work. In positive ions from a discharge tube, which the method they used a drop of the mixture were deflected by parallel electric and mag- which is being analyzed is placed at one of netic fields at right angles to the beam. the ends of a strip of filter paper. This is Each type of ion formed a parabolic trace then soaked in a solvent which carries the on a photographic plate (a mass spectro- components of the mixture at different graph). The design was improved by the rates as it moves along the strip. The posi- British chemist Francis William Aston tions of the components are revealed by (1877–1945). In modern instruments, the spraying the strip with a reagent which ions are usually produced by ionizing a gas causes the components to change color. with electrons. The positive ions are accel- Martin and Synge were able to identify erated out of this ion source into a high- amino acids in a protein this way, using vacuum region. Here, the stream of ions is ninhydrin to record the positions of the deflected and focused by a combination of amino acid on the strip. electric and magnetic fields, which can be varied so that different types of ion fall on mass Symbol: m A measure of the quan- a detector. In this way, the ions can be an- tity of matter in an object. Mass is deter- alyzed according to their mass, giving a mined in two ways: the inertial mass of a mass spectrum of the material. Mass spec- body determines its tendency to resist trometers are used for accurate measure- change in motion; the gravitational mass ments of relative atomic mass and for

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mass spectrum

analysis of isotope abundance. They can melting point The temperature at also be used to identify compounds and an- which a solid is in equilibrium with its liq- alyze mixtures. An organic compound uid phase at standard pressure and above bombarded with electrons forms a number which the solid melts. This temperature is of fragment ions. (Ethane (C2H6), for in- always the same for a particular solid. Ion- + + + stance, might form CH , C2H5 , CH2 , ically bonded solids generally have much etc.) The relative proportions of different higher melting points than those in which types of ions may be used to find the struc- the forces are covalent or intermolecular. ture of new compounds. The characteristic spectrum can also identify compounds by membrane A thin pliable sheet of tissue comparison with standard spectra. or other material acting as a boundary. The membrane may be either natural (as in mass spectrum See mass spectrometer. cells, skin, etc.) or synthetic modifications of natural materials (cellulose derivatives matrix A continuous solid phase in or rubbers). In many physicochemical which particles of a different solid phase studies membranes are supported on are embedded. porous materials, such as porcelain, to provide mechanical strength. Membranes are mechanism A step-by-step description generally permeable to some degree. of the events taking place in a chemical re- Membranes can be prepared to permit action. It is a theoretical framework ac- the passage of other molecules and micro- counting for the fate of bonding electrons molecular material. Because of permeabil- and illustrates which bonds are broken and ity effects, concentration differences at a membrane give rise to a whole range of which are formed. For example, in the membrane-equilibrium studies, of which chlorination of methane to give chloro- osmosis, dialysis, and ultrafiltration are ex- methane: amples. See also semipermeable mem- step 1 brane. Cl:Cl → 2Cl• step 2 → menaquinone See vitamin K. Cl• + CH4 HCl + CH3• step 3 → Mendius reaction The reduction of the CH3• + Cl:Cl CH3Cl + Cl• cyanide group to a primary amine group See also nucleophilic substitution. using sodium in alcohol: → RCN + 2H2 RCH2NH2 mega- Symbol: M A prefix denoting It is a method of increasing the chain length 6 10 . For example, 1 megahertz (MHz) = of compounds in ascending a homologous 6 10 hertz (Hz). series of compounds.

melamine (triaminotriazine; C3N3(NH2)3) mer See polymer. A white solid organic compound whose molecules consist of a six-membered hete- mercaptan See thiol. rocyclic ring of alternate carbon and nitrogen atoms with three amino groups meso-form See optical activity. attached to the carbons. Condensation polymerization with methanal or other mesomerism See resonance. aldehydes produces melamine resins, which are important thermosetting plas- meta- 1. Designating a benzene com- tics. pound with substituents in the 1,3 positions. The position on a benzene ring that melting (fusion) The process by which a is two carbon atoms away from a sub- solid is converted into a liquid by heat or stituent is the meta position. This was used pressure. in the systematic naming of benzene deriv-

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methanal

atives. For example, meta-dinitrobenzene metastable species An excited state of (or m-dinitrobenzene) is 1,3-dinitroben- an atom, ion, or molecule, that has a rela- zene. tively long lifetime before reverting to the 2. Certain inorganic acids regarded as ground state. Metastable species are inter- formed from an anhydride and water are mediates in some chemical reactions. named meta acids to distinguish them from the more hydrated ortho acids. For exam- metastable state A condition of a sys- ple, H2SiO3 (SiO2 + H2O) is metasilicic tem or body in which it appears to be in acid; H4SiO4 (SiO2 + 2H2O) is orthosilicic stable equilibrium but, if disturbed, can acid. settle into a lower energy state. For exam- See also ortho-; para-. ple, supercooled water is liquid below 0°C (at standard pressure). When a small crys- metabolic pathway See metabolism. tal of ice or dust (for example) is introduced, rapid freezing occurs. metabolism The biochemical reactions that take place in cells. The molecules tak- meter Symbol: m The SI base unit of ing part in these reactions, either as a reac- length, defined as the distance traveled by tant or a product, are termed metabolites. light in vacuum in 1/(2.99 792 458 × 108) Some are synthesized within the organism second. This definition was adopted in itself, whereas others have to be taken in as 1983 to replace the 1967 definition of a food. Metabolic reactions characteristi- length equal to 1 650 763.73 wavelengths cally occur in small steps that together in vacuum corresponding to the transition make up a metabolic pathway. They in- between the levels 2p10 and 5d5 of the 86Kr volve the breaking down of molecules to atom. provide energy (catabolism) and the building up of more complex molecules and structures from simpler molecules (an- CH abolism). 2 O O metabolite See metabolism.

metaldehyde See ethanal. H C CH 2 2 metallic bond A bond formed between O atoms of a metallic element in its zero oxi- Methanal: methanal trimer dation state and in an array of similar atoms. The outer electrons of each atom are regarded as contributing to an ‘electron methanal (formaldehyde; HCOH) A gas’, which occupies the whole crystal of colorless gaseous aldehyde. It is manufac- the metal. It is the attraction of the positive tured by the oxidation of methanol (500°C atomic cores for the negative electron gas and a silver catalyst): → that provides the strength of the metallic 2CH3OH + O2 2HCOH + 2H2O bond. The compound is used in the manufacture of UREA–FORMALDEHYDE RESINS. A solution metallocene A SANDWICH COMPOUND in of methanal (40%) in water is called for- which a metal atom or ion is coordinated malin. It is extensively used as a preserva- to two cyclopentadienyl ions. Ferrocene tive for biological specimens. (Fe(C5H5)2) is the commonest example. If an aqueous solution of methanal is evaporated a polymer – polymethanal – is metalloid Any of a class of chemical el- formed: ements that are intermediate in properties –O–CH2–O–CH2–O–CH2– between metals and nonmetals. Examples This was formerly called paraformalde- are germanium, arsenic, and tellurium. hyde. If methanal is distilled from acidic

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methanal trimer

solutions a cyclic methanal trimer methylaniline See toluidine. (C3O3H6) is produced. methylated spirits Ethanol to which is methanal trimer See methanal. added methanol (about 9.5%), pyridine (about 0.5%), and a blue dye. The ethanol

methane (CH4) A gaseous alkane. Nat- is denatured in this way so that it can be ural gas is about 99% methane and this sold without excise duty for use as a fuel provides an important starting material for and solvent. the organic-chemicals industry. Methane can be chlorinated directly to produce the methylation A reaction in which a more reactive chloromethanes, or it can be methyl group (CH3–) is introduced into a compound. ‘reformed’ by partial oxidation or using steam to give mixtures of carbon oxides methylbenzene (toluene; C H CH )A and hydrogen. Methane is the first member 6 5 3 colorless liquid hydrocarbon, similar to of the homologous series of alkanes and benzene both in structure and properties. the simplest organic compound. As methylbenzene is much less toxic than benzene it is more widely used, especially methanoate (formate) A salt or ester of as a solvent. Large quantities are required methanoic acid. for the manufacture of TNT (trinitrotoluene). methanoic acid (formic acid; Methylbenzene can be obtained by the HCOOH) A liquid carboxylic acid made fractional distillation of coal tar or synthe- by the action of sulfuric acid on sodium sized from methylcyclohexane (a con- methanoate (NaOOCH). It is a strong re- stituent of some crude oils): → ducing agent. Methanoic acid is the sub- C6H11CH3 C6H5CH3 + 3H2 stance responsible for the stings of ants and A catalyst of aluminum and molybdenum nettles. oxides is employed at high temperatures and pressures. methanol (methyl alcohol; wood alcohol; See bromomethane. CH3OH) A colorless liquid alcohol, methyl bromide which is used as a solvent and in the manufacture of methanal (formaldehyde) for 2-methylbuta-1,3-diene (isoprene; CH2- the plastics and drugs industries. Methanol CH:CH2) A colorless unsaturated liquid was originally produced from the distilla- hydrocarbon, which occurs in terpenes and natural rubber. Is is used to make synthetic tion of wood. Now it is manufactured by rubber. the catalytic oxidation of methane from natural gas. methyl chloride See chloromethane. methionine See amino acids. methyl cyanide (acetonitrile; CH3CN) A pleasant-smelling poisonous colorless The group CH O–. methoxy group 3 liquid organic NITRILE. It is a polar solvent, widely used for dissolving both inorganic methyl acetate See methyl ethanoate. and organic compounds.

methyl alcohol See methanol. methylene See carbene.

(CH NH ) A colorless methylamine 3 2 methylene group The group :CH2. flammable gas that smells like ammonia. It is the simplest primary amine, used for methyl ethanoate (methyl acetate; making herbicides and other organic chem- CH3COOCH3) A colorless liquid ester icals. with a fragrant odor, used as a solvent.

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Michaelis kinetics

methyl ethyl ketone See butanone. c.g.s. units, and m.k.s. units are all scientific metric systems of units. methyl group The group CH3–. mho See siemens. methyl 2-hydroxybenzoate See methyl salicylate. micelle An aggregate of molecules in a COLLOID. methyl iodide See iodomethane. Michaelis constant Symbol: Km For an methyl methacrylate (CH2:CCH3- enzyme-catalyzed reaction obeying MICH- COOCH3) The methyl ester of AELIS KINETICS under steady-state condi- methacrylic acid, used to make acrylic tions (when the reaction intermediates polymers such as Plexiglas (polymethyl- have reached a steady concentration): methacrylate). [ES] = [E][S]/Km where [ES] is the concentration of en- methyl orange An acid–base indicator zyme–substrate complex, [E] is the concen- that is red in solutions below a pH of 3 and tration of enzyme, [S] is the concentration yellow above a pH of 4.4. As the transition of substrate, and Km is the Michaelis con- range is clearly on the acid side, methyl or- stant. ange is suitable for the titration of an acid Km gives the concentration of substrate with a moderately weak base, such as at which half the active sites are filled and sodium carbonate. also gives an indication of the strength of the enzyme–substrate complex if the rate methylphenol (cresol; HOC6H4CH3)A of product formation is much slower than compound with both methyl and hydroxyl the rate of dissociation of the enzyme– groups substituted onto the benzene ring. substrate complex into enzyme and sub- There are three isomers, with the methyl strate. If this is the case then a high Km in- group in the 2–, 3–, and 4– positions, re- dicates weak binding of the complex and a spectively. A mixture of the isomers can be low Km indicates strong binding. obtained from coal tar. It is used as a ger- micide (known as Lysol). Michaelis kinetics (Michaelis–Menten kinetics) A simple and useful model of methyl red An acid–base indicator that the kinetics of enzyme-catalyzed reactions. is red in solutions below a pH of 4.2 and It assumes the formation of a specific en- yellow above a pH of 6.3. It is often used zyme–substrate complex. Many enzymes for the same types of titration as methyl or- obey Michaelis kinetics and a plot of reac- ange but the transition range of methyl red tion velocity (V) against substrate concen- is nearer neutral (pH7) than that of methyl tration [S] gives a characteristic curve orange. The two molecules are structurally showing that the rate increases quickly at similar. first and then levels off to a maximum value. When substrate concentration is methyl salicylate (oil of wintergreen; low, the rate of reaction is almost propor- methyl 2-hydroxybenzoate; C8H8O3) The tional to substrate concentration. When methyl ester of SALICYLIC ACID, which oc- substrate concentration is high, the rate is curs in certain plants. It is absorbed at a maximum, Vmax, and independent of through the skin and used medicinally to substrate concentration. The Michaelis relieve rheumatic symptoms. It is also used constant Km is the concentration of sub- in perfumes and as a flavoring agent in var- strate at half the maximum rate and can be ious foods. determined experimentally by measuring reaction rate at varying substrate concen- metric system A system of units based trations. Different types of inhibition can on the meter and the kilogram and using also be distinguished in this way. Allosteric multiples and submultiples of 10. SI units, enzymes do not obey Michaelis kinetics.

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micro-

micro- Symbol: µ A prefix denoting mineral acid An inorganic acid, espe- 10–6. For example, 1 micrometer (µm) = cially an acid used commercially in large 10–6 meter (m). quantities. Examples are hydrochloric, nitric, and sulfuric acids. micron Symbol: µ A unit of length equal to 10–6 meter. mirror image A shape that is identical to another except that its structure is re- microwaves A form of electromagnetic versed as if viewed in a mirror. If an object radiation, ranging in wavelength from is not symmetrical it cannot be superim- about 1 mm (where it merges with in- posed on its mirror image. For example, frared) to about 120 mm (bordering on the left hand is the mirror image of the radio waves). Microwaves are produced by right hand. See chirality. various electronic devices; they are often carried over short distances in tubes of rec- miscible Denoting combinations of sub- tangular section called waveguides. Spectra stances that, when mixed, give rise to only in the microwave region can give informa- one phase; i.e. substances that dissolve in tion on the rotational energy levels of cer- each other. tain molecules. See also electromagnetic radiation. mixed indicator A mixture of two or more indicators so as to decrease the pH migration 1. The movement of an range or heighten the color change, etc. atom, group, or double bond from one po- Two or more substances form- sition to another in a molecule. mixture ing a system in which there is no chemical 2. The movement of ions in an electric bonding between the two. In homogeneous field. mixtures (e.g. solutions or mixtures of gases) the molecules of the substances are milk sugar See lactose. mixed, and there is only one phase. In heterogeneous mixtures (e.g. gunpowder or Miller, Stanley Lloyd (1930– ) certain alloys) different phases can be dis- American chemist. Stanley Miller is fa- tinguished. Mixtures differ from chemical mous for an experiment, the results of compounds in that: which he published in 1953, concerning 1. The chemical properties of the compo- the possible origin of life. Miller, who was nents of a mixture are the same as those then a graduate student of Harold Urey, of the pure substances. mixed water vapour, ammonia, methane 2. The mixture can be separated by physi- and hydrogen in a flask so as to simulate cal means (e.g. distillation or crystalliza- the early atmosphere of the Earth and then tion) or mechanically. put a powerful electric discharge through it 3. The proportions of the components can to simulate lightning. He discovered that vary. Some mixtures (e.g. certain solu- after a short time organic molecules of bio- tions) can only vary in proportions be- logical interest, including some simple tween definite limits. amino acids, were formed. Since amino acids are the ‘building blocks’ for proteins m.k.s. system A system of units based it has been suggested that this experiment on the meter, the kilogram, and the second. may give a clue as to the origin of life on It formed the basis for SI units. Earth. mmHg (millimeter of mercury) A for- milli- Symbol: m A prefix denoting 10–3. mer unit of pressure defined as the pressure For example, 1 millimeter (mm) = 10–3 that will support a column of mercury one meter (m). millimeter high under specified conditions. It is equal to 133.322 4 Pa, and is almost millimeter of mercury See mmHg. identical to the torr.

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molecule

moiety A part of a molecule; for exam- molecularity The total number of react- ple, the sugar moiety in a nucleoside. ing molecules in the individual steps of a chemical reaction. Thus, a unimolecular molal concentration See concentra- step has molecularity 1, a bimolecular step tion. 2, etc. Molecularity is always an integer, whereas the order of a reaction need not molar 1. Denoting a physical quantity necessarily be so. The molecularity of a re- divided by the amount of substance. In al- action gives no information about the most all cases the amount of substance will mechanism by which it takes place. be in moles. For example, volume (V) divided by the number of moles (n) is molar molecular orbital See orbital. volume Vm = v/n. 2. Denoting a solution that contains one molecular sieve A substance through mole of solute per cubic decimeter of sol- which molecules of a limited range of sizes vent. can pass, enabling volatile mixtures to be separated. Zeolites and other metal alu- molarity A measure of the concentra- minum silicates can be manufactured with tion of solutions based upon the number of pores of constant dimensions in their mo- molecules or ions present, rather than on lecular structure. When a sample is passed the mass of solute, in any particular vol- through a column packed with granules of ume of solution. The molarity (M) is the this material, some of the molecules enter number of moles of solute in one cubic these pores and become trapped. The re- decimeter (litre). Thus a 0.5M solution of mainder of the mixture passes through the hydrochloric acid contains 0.5 × (1 + interstices in the column. The trapped mol- 35.5)g HCl per dm3 of solution. ecules can be recovered by heating. Mo- lecular-sieve chromatography is widely mole Symbol: mol The SI base unit of used in chemistry and biochemistry labora- amount of substance, defined as the tories. A modified form of molecular sieve amount of substance that contains as many is used in gel filtration. The sieve is a con- elementary entities as there are atoms in tinuous gel made from a polysaccharide. In 0.012 kilogram of 12C. The elementary en- this case, molecules larger than the largest tities may be atoms, molecules, ions, elec- pore size are totally excluded from the col- trons, photons, etc., and they must be umn. specified. The amount of substance is proportional to the number of entities, molecular-sieve chromatography See the constant of proportionality being the molecular sieve. Avogadro number. One mole contains 6.022 045 × 1023 entities. One mole of molecular spectrum The absorption or an element with relative atomic mass A emission SPECTRUM that is characteristic of has a mass of A grams (this mass was for- a molecule. Molecular spectra are usually merly called one gram-atom of the el- band spectra. ement). molecular weight See relative molecu- molecular crystal A crystal in which lar mass. molecules, as opposed to atoms, occupy lattice points. Because the forces holding molecule A particle formed by the com- the molecules together are weak, molecular bination of atoms in a whole-number ratio. crystals have low melting points. When the A molecule of an element (combining molecules are small, the crystal structure atoms are the same, e.g. O2) or of a com- approximates to a close-packed arrange- pound (different combining atoms, e.g. ment. HCl) retains the properties of that element or compound. Thus, any quantity of a molecular formula See formula. compound is a collection of many identical

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mole fraction

molecules. Molecular sizes are characteris- mordant An inorganic compound used tically 10–10 to 10–9 m. to fix dye in cloth. The mordant (e.g. alu- Many molecules of natural products are minum hydroxide or chromium salts) is so large that they are regarded as giant precipitated in the fibers of the cloth, and molecules (macromolecules); they may the dye then absorbs in the particles. contain thousands of atoms and have complex structural formulae that require very morphine An alkaloid present in opium advanced techniques to identify. See also (from the poppy Papaver somniferum). It formula; relative molecular mass. is used for the relief of severe pain. The drug heroin is a derivative. mole fraction The number of moles of a mother liquor The solution remaining given component in a mixture divided by after the formation of crystals. the total number of moles present of all the components. The mole fraction of compo- MSG See monosodium glutamate. nent A is nA/(nA + nB + nC + …) mucopolysaccharide See glycosamino- where nA is the number of moles of A, etc. glycan.

Molisch’s test See alpha-naphthol test. mucoprotein See proteoglycan.

monobasic acid An acid that has only multicenter bond A two-electron bond one acidic hydrogen. Ethanoic acid, formed by the overlap of orbitals from CH3COOH, is an example of a monobasic more than two atoms (usually 3). The acid. See also dibasic acid. bridging in diborane is believed to take place by overlap of an sp3 hybrid orbital monochlorobenzene See chloroben- from each boron atom with the 1s orbital zene. on the hydrogen atom. This multicenter bond is called a two-electron three-center monohydric alcohol see alcohol. bond. See also electron-deficient compound. monomer The molecule, group, (or compound) from which a dimer, trimer, or multidentate ligand A LIGAND that pos- POLYMER is formed. sesses at least two sites at which it can coordinate. monosaccharide A SUGAR that cannot be hydrolyzed to simpler carbohydrates of multiple bond A bond between two atoms involving more than one pair of elec- smaller carbon content. Glucose and fruc- trons; i.e. a double bond or a triple bond. tose are examples. This additional bonding arises from overlap of atomic orbitals that are perpendicu- monosodium glutamate (MSG) A lar to the internuclear axis and gives rise to white crystalline solid compound, made an increase in electron density above and from soya-bean protein. It is a sodium salt below this axis. Such bonds are called pi of glutamic acid (see amino acid) used as a bonds. (The bond along the axis is a sigma flavor enhancer, particularly in Chinese bond.) food. Monosodium glutamate can cause an allergic reaction in certain people. multiple proportions, law of Pro- posed by Dalton in 1804, the principle that monoterpene See terpene. when two elements A and B combine to form more than one compound, the monovalent (univalent) Having a va- weights of B that combine with a fixed lence of one. weight of A are in small whole-number ra-

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myoglobin

tios. For example, in dinitrogen oxide, systematic name is 2,2′-dichlorodiethyl

N2O, nitrogen monoxide, NO, and dini- sulfide. trogen tetroxide, N2O4, the amounts of nitrogen combined with a fixed weight of mutarotation A change in the optical rotation of a solution with time, caused by oxygen are in the ratio 4:2:1. the conversion of one optical isomer into another. See optical activity. multiple-range indicator See universal indicator. myoglobin A globular protein formed of a heme group and a single polypeptide mustard gas ((CH2ClCH2)2S) A poiso- chain. It occurs in muscle tissue, where it nous vesicant gas used as a war gas. The acts as an oxygen store.

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NH2 structures known as ‘bucky tubes’, which N have a structure similar to that of buck- N minsterfullerene. Interest has been shown in nanotubes as possible microscopic N N O probes in experiments, as semiconductor materials, and as a component of compos- HO OP CH2 O ite materials. Nanotubes can also be produced by joining amino acids to give tubular polypeptide structures. See also O buckminsterfullerene. OH OH

CONH2 naphtha (solvent naphtha) A mixture of PO OH hydrocarbons obtained from coal and pe- + troleum. It has a boiling range of OCH NO 2 70–160°C and is used as a solvent and as a raw material for making various other organic chemicals. OH OH naphthalene (C10H8) A white crys- NAD talline solid with a distinctive smell of

NH2 NAD (nicotinamide adenine dinucleo- N tide) A derivative of nicotinic acid that N acts as a coenzyme in electron-transfer re- N N actions (e.g. the electron-transport chain). O Its role is to carry hydrogen atoms; the re- - OPO CH O duced form is written NADH. 2

NADP Nicotinamide adenine dinu- O cleotide phosphate; a coenzyme similar in O OH PO O- its action to NAD. The reduced form is - written NADPH, which acts as an electron O donor in many synthetic reactions. CONH2 OOP - –9 + nano- Symbol: n A prefix denoting 10 . OCHO N For example, 1 nanometer (nm) = 10–9 2 meter (m).

nanotube A tubular structure with a di- OH OH ameter of a few nanometers (1nm = 10–9 m). Examples of nanotubes are the carbon NADP

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Newman projection

mothballs. Naphthalene is found in both other volatile hydrocarbons (ethane, the middle- and heavy-oil fractions of propane, and butane). crude oil and is obtained by fractional crystallization. It is used in the manufacture of neighboring-group participation An benzene-1,2-dicarboxylic anhydride (ph- effect in an organic reaction in which thalic anhydride) and thence in the produc- groups close to the point at which reaction tion of plastics and dyes. occurs affect the rate of reaction or stereo- The structure of naphthalene is ‘ben- chemistry of the products in some way. zene-like’, having two six-membered rings fused together. The reactions are charac- neoprene A type of synthetic RUBBER teristic of AROMATIC COMPOUNDS. made by polymerization of 2-chlorobuta- 1,2,-diene (H2C:CHCCl:CH2). It is more nascent hydrogen A particularly reac- resistant to oil, solvents, and temperature tive form of hydrogen, which is believed to than natural rubbers. exist briefly between its generation (e.g. by the action of dilute acid on magnesium) neutralization The stoichiometric reac- and its appearance as bubbles of normal tion of an acid and a base in volumetric hydrogen gas. It is thought that part of the analysis. The neutralization point or end free energy of the production reaction re- point is detected with indicators. mains with the hydrogen molecules for a short time. Nascent hydrogen may be used neutron diffraction A method of struc- to produce the hydrides of phosphorus, ar- ture determination used for solids, liquids, senic, and antimony, which are not readily and gases that makes use of the quantum formed from ordinary hydrogen. mechanical wave nature of neutrons. Ther- mal neutrons with average kinetic energies Natta, Giulio (1903–79) Italian of about 0.025eV have a wavelength of chemist. The early part of Natta’s career about 0.1 nanometer, making them suit- was devoted to x-ray crystallography and able for investigating the structure of mat- catalysts. In 1938 he started research on ter at the atomic level. synthetic rubbers. In 1953 he developed Since a neutron has a nonzero magnetic methods for using catalysts to produce moment, it interacts both with nuclear polymers which had been initiated by Karl magnetic moments and with the magnetic ZIEGLER to form polypropene. In 1954 he moments of unpaired electrons. This prop- found that polymers produced in this way erty is particularly useful in identifying the are very specific and regular in their stere- positions of hydrogen atoms in a molecule. ochemistry. This meant that these poly- These positions are difficult to establish mers have desirable properties such as high using x-rays because x-rays interact with melting points and high strength. After electrons, and hence are scattered weakly 1954 he continued to investigate this type by hydrogen atoms. Protons scatter neu- of polymerization. Natta and Ziegler trons strongly, and the positions of protons shared the 1963 Nobel Prize for chemistry can readily be determined by neutron dif- for their work on polymers. fraction.

Natta process A method for the manu- neutron number Symbol: N The num- facture of isotactic polypropene using ber of neutrons in the nucleus of an atom; Ziegler catalysts. It is named for the Italian i.e. the nucleon number (A) minus the pro- chemist Giulio Natta. See Ziegler process. ton number (Z).

natural gas Gas obtained from under- Newman projection A type of projec- ground deposits and often associated with tion FORMULA in which the molecule is sources of petroleum. It contains a high viewed along a bond between two of its proportion of methane (about 85%) and atoms. See illustration at conformation.

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newton

newton Symbol: N The SI unit of force, with the evolution of oxides of nitrogen equal to the force needed to accelerate one (the composition of the mixture of the ox- kilogram by one meter second–2. 1 N = 1 kg ides depends on the temperature and on the m s–2. It is named for Sir Isaac Newton concentration of the nitric acid used). (1642–1727). Some nonmetals (e.g. sulfur and phosphorus) react to produce oxyacids. Organic niacin See nicotinic acid. substances (e.g. sawdust and ethanol) react violently, but the more stable aromatic nicotinamide adenine dinucleotide compounds, such as benzene and toluene, See NAD. can be converted to NITRO COMPOUNDS in controllable reactions. nicotinic acid (niacin) One of the water- soluble B-group of vitamins. Its deficiency nitrile (cyanide) A type of organic com- in man causes pellagra. Nicotinic acid pound containing the –CN group. Nitriles functions as a constituent of two coen- are colorless liquids with pleasant smells. zymes, NAD and NADP, which operate as They can be prepared by refluxing an or- hydrogen and electron transfer agents and ganic halogen compound with an alcoholic play a vital role in metabolism. See also solution of potassium cyanide: vitamin B complex. R+Cl + KCN → RCN + KCl Alternatively it is possible to dehydrate an ninhydrin A colorless organic com- amide using a dehydrating agent such as pound that gives a blue coloration with phosphorus(V) oxide: → amino acids. It is used as a test for amino RCONH2 – H2O RCN acids, in particular to show the positions of Nitriles can be hydrolyzed to give the spots of amino acids in paper chromatog- amide. Another reaction is hydrogenation raphy. to give amines: → RCN + 2H2 RCH2NH2 nitrate A salt or ester of nitric acid. nitrile rubber A copolymer of butadi- nitration A reaction introducing the ene and propenonitrile (acrylonitrile; nitro (–NO2) group into an organic com- CH2=CHCN). It is a useful type of rubber pound. Nitration of aromatic compounds because of its resistance to oil and solvents. is usually carried out using a mixture of concentrated nitric and sulfuric acids, al- nitrobenzene (C6H5NO2) A yellow or- though the precise conditions differ from ganic oil obtained by refluxing benzene compound to compound. The attacking with a mixture of concentrated nitric and + species is NO2 (the nitryl ion), and the re- sulfuric acids. The reaction is a typical elec- action is an example of electrophilic substi- trophilic substitution on the benzene ring + tution. by the nitryl ion (NO2 ).

nitric acid (HNO3) A colorless fuming nitrocellulose See cellulose trinitrate. corrosive liquid that is a strong acid. Nitric acid can be made in a laboratory by the dis- nitro compound A type of organic tillation of a mixture of an alkali metal ni- compound containing the nitro (–NO2) trate and concentrated sulfuric acid. group attached to an aromatic ring. Nitro Commercially it is prepared by the cat- compounds can be prepared by nitration alytic oxidation of ammonia and is sup- using a mixture of concentrated nitric and plied as concentrated nitric acid, which sulfuric acids. They can be reduced to aro- contains 68% of the acid and is often col- matic amines: → ored yellow by dissolved oxides of nitro- RNO2 + 3H2 RNH2 + 2H2O gen. They can also undergo further substitution Nitric acid is a strong oxidizing agent. on the benzene ring. The nitro group di- Most metals are converted to their nitrates rects substituents into the 3 position.

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normal solution

+ nitrogen The first element of group 15 nitryl ion (nitronium ion) The ion NO2 , (formerly group VA) of the periodic table; occurring in NITRATION reactions. a very electronegative element existing in the uncombined state as gaseous diatomic NMR See nuclear magnetic resonance. N2 molecules. The nitrogen atom has the electronic configuration [He]2s22p3. It is nonbenzenoid aromatic See aromatic typically nonmetallic and its bonding is compound. primarily by polarized covalent bonds. With electropositive elements the nitride nonessential amino acid See amino ion N3– may be formed. It is present in acid. many organic compounds including amines, amides, nitriles, and nitro com- nonlocalized bond See delocalized pounds. bond. Nitrogen has two isotopes; 14N, the common isotope, and 15N (natural abun- nonmetal Any of a class of chemical el- dance 0.366%), which is used as a label in ements. Non-metals lie in the top right- mass spectrometric studies. hand region of the periodic table. They are Symbol: N; m.p. –209.86°C; b.p. electronegative elements with a tendency –195.8°C; d. 1.2506 kg m–3 (0°C); p.n. 7; to form covalent compounds or negative r.a.m. 14. ions. They have acidic oxides and hydroxides. nitrogenous base A basic compound in which a nitrogen atom can accept a proton. nonpolar compound A compound The term is used especially for the cyclic that has molecules with no permanent di- ring compounds adenine, guanine, cyto- pole moment. Examples of nonpolar com- sine, thymine, and uracil, which occur in pounds are hydrogen, tetrachloromethane, nucleic acids. See also quaternary ammo- and carbon dioxide. nium compound. nonpolar solvent See solvent. nitroglycerine (glyceryl trinitrate) A highly explosive substance used in dyna- noradrenaline See norepinephrine. mite. It is obtained by treating glycerol (1,2,3-trihydroxypropane) with a mixture norepinephrine (noradrenaline) A cat- of concentrated nitric and sulfuric acids. It echolamine, secreted as a hormone by the is not a nitro compound, but a nitrate ester adrenal medulla, that regulates heart mus- CH2(NO3)CH(NO3)CH2(NO3) cle, smooth muscle, and glands. It causes narrowing of arterioles and hence raises nitro group The group –NO2; the func- blood pressure. It is also secreted by nerve tional group of nitro compounds. endings of the sympathetic nervous system in which it acts as a neurotransmitter. In nitronium ion See nitryl ion. the brain, levels of norepinephrine are related to mental function; lowered levels nitrophenols (C6H4(OH)NO2) Organic lead to mental depression. compounds formed directly or indirectly by the nitration of phenol. Three isomeric normality The number of gram equiva- forms are possible. The 2 and 4 isomers are lents per cubic decimeter of a given solu- produced by the direct nitration of phenol tion. and can be separated by steam distillation, the 2 isomer being steam volatile. The 3 normal solution A solution that con- isomer is produced from nitrobenzene by tains one gram equivalent weight per liter formation of 1,3-dinitrobenzene, conver- of solution. Values are designated by the sion to 3-nitrophenylamine, and thence by symbol N, e.g. 0.2N, N/10, etc. Because diazotization to 3-nitrophenol. there is not a clear definition of equivalent

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NTP

weight suitable for all reactions, a solution nucleon number (mass number) Sym- may have one value of normality for one bol: A The number of nucleons (protons reaction and another value in a different re- plus neutrons) in an atomic nucleus. action. Because of this it is now more usual to use the molar solution notation. nucleophile An electron-rich ion or molecule that takes part in an organic re- NTP See STP. action. The nucleophile can be a negative ion (Br–, CN–) or a molecule with a lone nuclear magnetic resonance (NMR) A pair of electrons (NH3, H2O). The nucle- method of investigating nuclear spin. In an ophile attacks positively charged parts of external magnetic field the nucleus can molecules, which usually arise from the have certain quantized energy states, corre- presence of an electronegative atom else- sponding to certain orientations of the spin where in the molecule. Compare electrophile. magnetic moment. Hydrogen nuclei, for instance, can have two energy states, and nucleophilic addition A class of reac- transitions between the two occur by ab- tion involving the addition of a small mol- sorption of radiofrequency radiation. In ecule to the double bond in an unsaturated chemistry, this is the basis of a spectro- organic compound. The initial part of the scopic technique for investigating the reaction is attack by a nucleophile and the structure of molecules. In the basic tech- unsaturated bond must contain an elec- nique, radiofrequency radiation is fed to a tronegative atom, which creates an elec- sample and the magnetic field is changed tron-deficient area in the molecule. slowly. Absorption of the radiation is de- Nucleophilic addition is a characteristic re- tected when the difference between the nu- action of aldehydes and ketones where po- clear levels corresponds to absorption of a larization of the C=O carbonyl causes a quantum of radiation. This difference de- positive charge on the carbon. This is the pends slightly on the electrons around the site at which the nucleophile attacks. Addi- nucleus – i.e. the position of the atom in the tion is often followed by the subsequent molecule. The difference in frequency of elimination of a different small molecule, absorption caused by the distribution of particularly water. See condensation reac- electrons is known as a chemical shift. tion. Thus a different absorption frequency is seen for each type of hydrogen atom. In nucleophilic substitution A reaction ethanol, for example, there are three fre- involving the substitution of an atom or quencies, corresponding to hydrogen group of atoms in an organic compound atoms on the CH , the CH , and the OH. with a nucleophile as the attacking sub- 3 2 stituent. Since nucleophiles are electron- The intensity of absorption also depends rich species, nucleophilic substitution on the number of hydrogen atoms (3:2:1). occurs in compounds in which a strongly NMR spectroscopy is a powerful method electronegative atom or group leads to a of finding the structures of organic com- dipolar bond. The electron-deficient center pounds. It is most often used to detect hy- can then be attacked by the electron-rich drogen atoms but certain other nuclei can nucleophile causing the electronegative 13 be investigated (e.g. C). atom or group to be displaced. In general terms: nucleic acids Organic acids whose mol- R–Le + Nu– → R–Nu + Le– ecules consist of chains of alternating sugar where Nu– represents the incoming nucle- and phosphate units, with nitrogenous ophile and Le– represents the LEAVING bases attached to the sugar units. They GROUP. occur in the cells of all organisms. In DNA There are two possible mechanisms for the sugar is deoxyribose; in RNA it is ri- nucleophilic substitution. In the SN1 (sub- bose. See DNA; RNA. stitution, nucleophilic, monomolecular) re-

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nucleotide

X X + – CCl C +Cl Y ZYZ

OH– OH–

X X COH HO C Y Y Z Z

Nucleophilic substitution: the SN1 reaction

X X – – OH C Cl HO... C ... Cl Y ZYZ

X HO C +Cl– Y Z

Nucleophilic substitution: the SN2 reaction

action the molecule first forms a carbo- intermediates and so favor the SN1 nium ion; for example: mechanism. → + – RCH2Cl RCH2 + Cl There is a difference in the two mecha- The nucleophile then attaches itself to this nisms in that, for an optically active reac- carbonium ion: tant, the SN1 mechanism gives a racemic + – → RCH2 + OH RCH2OH mixture of products, whereas an SN2 In the SN2 (substitution, nucleophilic, mechanism gives an optically active prod- bimolecular) reaction the nucleophile ap- uct (see illustration). proaches as the other group leaves, form- See also substitution reaction. ing a transition state in which the carbon has five attached groups. nucleoside A molecule consisting of a The preferred mechanism depends on purine or pyrimidine base linked to a several factors: sugar, either ribose or deoxyribose. ADENO- 1. The stability of the intermediate in the SINE, CYTIDINE, GUANOSINE, THYMIDINE, and SN1 mechanism. URIDINE are common nucleosides. 2. Steric factors affecting the formation of the transition state in the SN2 mecha- nucleotide The compound formed by nism. condensation of a nitrogenous base (a 3. The solvent in which the reaction oc- purine, pyrimidine, or pyridine) with a curs: polar solvents will stabilize polar sugar (ribose or deoxyribose) and phos-

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nucleus

H H O O

NN(CH2)6 CC(CH2)4 H H HO OH

1,6-diaminohexane hexanedioic acid

HN (CH2)6 NH C (CH2)4 C NH (CH2)6

O O

Nylon: formation of nylon by a condensation reaction

phoric acid. The coenzymes NAD and A nucleus is depicted by a symbol indi- FAD are dinucleotides (consisting of two cating nucleon number (mass number), linked nucleotides) while the nucleic acids proton number (atomic number), and el- 2 3 are polynucleotides (consisting of chains of ement name. For example, 1 1 Na repre- many linked nucleotides). sents a nucleus of sodium having 11 protons and mass 23, hence there are (23 – nucleus The compact positively charged 11) = 12 neutrons. center of an atom made up of one or more nucleons (protons and neutrons) around nuclide A nuclear species with a given which is a cloud of electrons. The density number of protons and neutrons; for ex- of nuclei is about 1015 kg m–3. The number ample, 23Na, 24Na, and 24Mg are all differ- of protons in the nucleus defines the el- ent nuclides. Thus: 2 3 ement, being its proton number (or atomic 1 1 Na has 11 protons and 12 neutrons 2 4 number). The nucleon number, or atomic 1 1 Na has 11 protons and 13 neutrons 2 4 mass number, is the sum of the protons and 1 2 Mg has 12 protons and 12 neutrons neutrons. The simplest nucleus is that of a The term is applied to the nucleus and hydrogen atom, 1H, being simply one pro- often also to the atom. ton (mass 1.67 × 10–27 kg). The most mas- sive naturally occurring nucleus is 238U of nylon A type of synthetic polymer 92 protons and 146 protons (mass 4 × linked by amide groups –NH.CO–. Nylon 10–25 kg, radius 9.54 × 10–15 m). Only cer- polymers can be made by copolymeriza- tain combinations of protons and neutrons tion of a molecule containing two amine form stable nuclei. Others undergo sponta- groups with one containing two carboxylic neous decay. acid groups.

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OAA See oxaloacetic acid. value of 100 and heptane is given a value 0. The performance of a fuel is compared occlusion 1. The process in which small with a mixture of these hydrocarbons. amounts of one substance are trapped in the crystals of another; for example, pock- octet A stable shell of eight electrons in ets of liquid occluded during crystallization an atom. The completion of the octet gives from a solution. rise to particular stability and this is the 2. Absorption of a gas by a solid; for ex- basis of the Lewis octet theory, thus: ample, the occlusion of hydrogen by palla- 1. The rare gases have complete octets and dium. are chemically inert. 2. The bonding in small covalent molecules octadecanoic acid (stearic acid; is frequently achieved by the central CH3(CH2)16COOH) A solid carboxylic atom completing its octet by sharing acid present in fats and oils as the glyc- electrons with surrounding atoms, e.g. eride. CH4, H2O. 3. The ions formed by electropositive and octadecenoic acid (oleic acid) A natu- electronegative elements are generally rally occurring unsaturated carboxylic acid those with a complete octet, e.g. Na+, present (as glycerides) in fats and oils: Ca2+, O2–, Cl–. CH3(CH2)7CH:CH(CH2)7COOH The naturally occurring form is cis-9- ohm Symbol: Ω The SI unit of electrical octadecenoic acid. resistance, equal to a resistance that passes a current of one ampere when there is an octane (C8H18) A liquid alkane ob- electric potential difference of one volt tained from the light fraction of crude oil. across it. 1 Ω = 1 V A–1. Formerly, it was Octane and its isomers are the principal defined in terms of the resistance of a col- constituents of gasoline, which is obtained umn of mercury under specified condi- as the refined light fraction from crude oil. tions. The unit is named for the German See also octane rating. physicist Georg Ohm (1787–1854).

octane number See octane rating. oil Any of various viscous liquids. Min- eral oils are mainly composed of mixtures octane rating (octane number) A rating of hydrocarbons (see petroleum). Natural for the performance of gasoline in internal- oils secreted by plants and animals are ei- combustion engines. The octane rating ther mixtures of esters and terpenes (see es- measures the freedom from ‘knocking’ – sential oil) or are GLYCERIDES of fatty acids. i.e. preignition of the fuel in the engine. This depends on the relative proportions of oil of wintergreen See methyl salicy- branched-chain and straight-chain hydro- late. carbons present. High proportions of branched-chain alkanes are better in high- oil shale A sedimentary rock that in- performance engines. In rating fuels, 2,2,4- cludes in its structure 30–60% of organic trimethylpentane (isooctane) is given a matter, mainly in the form of bitumen.

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oleate COOH COOH

C C CH3 OH CH3 H

H OH

(R)-lactic acid (S)-lactic acid

Optical activity: enantiomers of lactic acid

COOH COOH COOH H OH HO H H OH

C C C

H OH HO H H OH COOH COOH COOH

D-form L-form meso-form

Optical activity: tartaric acid

Heated in the absence of air it produces an onium ion An ion formed by addition oily substance resembling petroleum, of a proton (H+) to a molecule. The hydro- + which is rich in nitrogen and sulfur com- nium (or hydroxonium) ion (H3O ) and + pounds. ammonium ion (NH4 ) are examples.

oleate A salt or ester of oleic acid; i.e. an Oparin, Alexandr Ivanovich (1894– octadecenoate. 1980) Russian biochemist. Oparin was one of the first people to put forward a theory olefin See alkene. of the origin of life on Earth. In 1922 he postulated that life originated in the seas, oleic acid See octadecenoic acid. with there being many suitable organic molecules being present there, that there is oligomer A polymer formed from a rel- a large supply of external energy and that atively few monomer molecules. See poly- life is characterized by a high degree of mer; polymerization. order. He expounded his views in more de- tail in his book The Origin of Life on Earth oligopeptide See peptide. (1936). His ideas stimulated a great deal of further work such as the experiment of oligosaccharide A carbohydrate formed Stanley MILLER. of a small number of monosaccharide units (up to around 20). See sugar. optical activity The ability of certain compounds to rotate the plane of POLAR- one-pot synthesis A synthesis of a IZATION of plane-polarized light when the chemical compound in which the reactants light is passed through them. Optical activ- form the required product in a single reac- ity can be observed in crystals, gases, tion mixture. liquids, and solutions. The amount of rota-

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optical activity H

C H2N R

COOH

The corn rule for absolute configuration of alpha amino acids

1 1

C C

32 23

R–configuration S–configuration

Corn rule

tion depends on the concentration of the levorotatory compounds l or (–). A mix- active compound. ture of the two isomers in equal amounts Optical activity is caused by the interac- does not show optical activity. Such a mix- tion of the varying electric field of the light ture is sometimes called the (±) or dl-form, with the electrons in the molecule. It occurs a racemate, or a racemic mixture when the molecules are asymmetric – i.e. Optical isomers have identical physical they have no plane of symmetry. Such mol- properties (apart from optical activity) and ecules have a mirror image that cannot be cannot be separated by fractional crystal- superimposed on the original molecule. lization or distillation. Their general chem- The two forms of the molecule are optical ical behavior is also the same, although isomers or enantiomers. (Stereoisomers they do differ in reactions involving other that are not mirror images of each other optical isomers. Many naturally occurring are called diastereoisomers.) In organic substances are optically active (only one compounds this usually means that the optical isomer exists naturally) and bio- molecule contains a carbon atom attached chemical reactions occur only with the nat- to four different groups, forming a chiral ural isomer. For instance, the natural form center. One isomer will rotate the polar- of glucose is d-glucose and living organ- ized light in one sense and the other by the isms cannot metabolize the l-form. same amount in the opposite sense. Such The terms ‘dextrorotatory’ and ‘levoro- isomers are described as dextrorotatory or tatory’ refer to the effect on polarized light. levorotatory, according to whether they A more common method of distinguishing rotate the plane to the ‘right’ or ‘left’ re- two optical isomers is by their D-form spectively (rotation to the left is clockwise (dextro-form) or L-form (levo-form). This to an observer viewing the light coming to- convention refers to the absolute structure ward the observer). Dextrorotatory com- of the isomer according to specific rules. pounds are given the symbol d or (+) and Sugars are related to a particular configu-

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optical isomer

ration of glyceraldehyde (2,3-dihydroxy- optical rotation Rotation of the plane propanal). For alpha amino acids the corn of polarization of plane-polarized light by rule is used: the structure of the acid an optically active substance. RC(NH2)(COOH) H is drawn with H at the top; viewed from the top the groups orbit The path of an electron as it moves spell CORN in a clockwise direction for all around the nucleus in an atom. D-amino acids (i.e. the clockwise order is orbital A region around an atomic nu- –COOH,R,NH2). The opposite is true for L-amino acids. Note that this convention cleus in which there is a high probability of refers to absolute configuration, not to op- finding an electron. The modern picture of the atom according to quantum mechanics tical activity: D-alanine is dextrorotatory does not have electrons moving in fixed el- but D-cystine is levorotatory. liptical orbits. Instead, there is a finite An alternative is the R-S convention for probability that the electron will be found showing configuration. There is an order in any small region at all possible distances of priority of attached groups based on the from the nucleus. In the hydrogen atom the proton number of the attached atom: probability is low near the nucleus, in- I, Br, Cl, SO3H, OCOCH3, OCH3, OH, creases to a maximum, and falls off to in- NO2, NH2, COOCH3, CONH2, COCH3, finity. It is useful to think of a region in CHO, CH2OH, C6H5, C2H5, CH3, H space around the nucleus – in the case of Hydrogen has the lowest priority (see hydrogen the region within a sphere – CIP system). The chiral carbon is viewed within which there is a high chance of find- such that the group of lowest priority is ing the electron. Each of these, called an hidden behind it. If the other three groups atomic orbital, corresponds to a subshell are in descending priority in a clockwise di- and can ‘contain’ a single electron or two rection, the compound is R-. If descending electrons with opposite spins. Another way priority is anticlockwise it is S-. of visualizing an orbital is as a cloud of The existence of a carbon atom bound electron charge (the average distribution to four different groups is not the strict with time). condition for optical activity. The essential Similarly, in molecules the electrons point is that the molecule should be asym- move in the combined field of the nuclei metric. Inorganic octahedral complexes, and can be assigned to molecular orbitals. for example, can show optical isomerism. In considering bonding between atoms it is It is also possible for a molecule to contain useful to treat molecular orbitals as formed asymmetric carbon atoms and still have by overlap of atomic orbitals. a plane of symmetry. One structure of It is possible to calculate the shapes and energies of atomic and molecular orbitals tartaric acid has two parts of the mol- by quantum theory. The shapes of atomic ecule that are mirror images, thus having a orbitals depend on the orbital angular mo- plane of symmetry. This (called the meso- mentum (the subshell). For each shell there form) is not optically active. See also reso- is one s orbital, three p orbitals, five d or- lution. bitals, etc. The s orbitals are spherical, the p orbitals each have two lobes; d orbitals optical isomer See optical activity. have more complex shapes, typically with four lobes. optical rotary dispersion (ORD) The Molecular orbitals are formed by over- phenomenon in which the amount of rota- lap of atomic orbitals, and again there are tion of plane-polarized light by an optically different types. If the orbital is completely active substance depends on the wave- symmetrical about an axis between the nu- length of the light. Plots of rotation against clei, it is a sigma orbital. This can occur, wavelength can be used to give informa- for instance, by overlap of two s orbitals, tion about the molecular structure of opti- as in the hydrogen atom, or two p orbitals cally active compounds. with their lobes along the axis. However,

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order z z z

y y y

x x x

dxy orbital dz2 orbital s orbital z z z

y y y

x x x

px orbital py orbital pz orbital

Orbital: atomic orbitals

two p orbitals overlapping at right angles maining p orbital is at right angles to the to the axis form a different type of molecu- plane, and can form pi bonds. Finally, sp lar orbital – a pi orbital – with regions hybridization may occur, giving two or- above and below the axis. Pi orbitals are bitals in a line. More complex types of hy- also formed by overlap of d orbitals. Each bridization, involving d orbitals, explain molecular orbital can contain a pair of the geometries of inorganic complexes. electrons, forming a sigma bond or pi The combination of two atomic orbitals bond. A double bond, for example the in fact produces two molecular orbitals. bond in ethene, is a combination of a sigma One – the bonding orbital – has a concen- bond and a pi bond. The triple bond in tration of electron density between the nu- ethyne is one sigma bond and two pi clei, and thus tends to hold the atoms bonds. together. The other – the antibonding or- Hybrid orbitals are atomic orbitals bital – has slightly higher energy and tends formed by combinations of s, p, and d to repel the atoms. If both atomic orbitals atomic orbitals, and are useful in describ- are filled, the two molecular orbitals are ing the bonding in compounds. There are also filled and cancel each other out – there various types. In carbon, for instance, the is no net bonding effect. If each atomic or- electron configuration is 1s22s22p2. Car- bital has one electron, the pair occupies the bon, in its outer (valence) shell, has one lower energy bonding orbital, producing a filled s orbital, two filled p orbitals, and net attraction. one ‘empty’ p orbital. These four orbitals may hybridize (sp3 hybridization) to act as order The sum of the indices of the con- four equal orbitals arranged tetrahedrally, centration terms in the expression that de- each with one electron. In methane, each termines the rate of a chemical reaction. hybrid orbital overlaps with a hydrogen s For example, in the expression: orbital to form a sigma bond. Alterna- rate = k[A]x[B]y tively, the s and two of the p orbitals may x is called the order with respect to A, y the hybridize (sp2 hybridization) and act as order with respect to B, and (x + y) the three orbitals in a plane at 120°. The re- order overall. The values of x and y are not

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ATOMIC ORBITALS HYBRID ORBITALS SHAPE

sp +

one s one p two sp3 linear

sp2 +

one s two p three sp2 planar

sp3 +

one s three p four sp3 tetrahedral

Orbital: hybrid orbitals

s orbitals sigma

px orbitals sigma

C z z

pz orbitals pi

Orbital: molecular orbitals

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osmotic pressure

necessarily equal to the coefficients of A ortho- 1. Designating a benzene com- and B in the molecular equation. Order is pound with substituents in the 1,2 posi- an experimentally determined quantity de- tions. The position next to a substituent is rived without reference to any equation or the ortho position on the benzene ring. mechanism. Fractional orders do occur. This was used in the systematic naming of For example, in the reaction: benzene derivatives. For example, ortho- → CH3CHO CH4 + CO dinitrobenzene (or o-dinitrobenzene) is 1.5 the rate is proportional to [CH3CHO] 1,2-dinitrobenzene. i.e. it is of order 1.5. 2. Certain acids, regarded as formed from an anhydride and water, were named ortho organic acid An organic compound acids to distinguish them from the less hy- that can release hydrogen ions (H+) to a drated meta acids. For example, H4SiO4 base, such as a carboxylic acid or a phenol. (from SiO2 + 2H2O) is orthosilicic acid; See acid; carboxylic acid; phenol. H2SiO3 (SiO2 + H2O) is metasilicic acid. 3. Designating the form of a diatomic mol- organic base An organic compound ecule in which both nuclei have the same that can function as a base. Organic bases spin direction; e.g. orthohydrogen, orth- are typically amines that gain H+ ions. See odeuterium. amine. See also meta-; para-.

organic chemistry The chemistry of osmium(IV) oxide (osmium tetroxide; compounds of carbon. Originally the term OsO4) A volatile yellow crystalline solid organic chemical referred to chemical com- with a penetrating odor, used as an oxidizing agent and, in aqueous solution, as a pounds present in living matter, but now it catalyst for organic reactions. covers any carbon compound with the exception of certain simple ones, such as the osmium tetroxide See osmium(IV) carbon oxides, carbonates, cyanides, and oxide. cyanates. These are generally studied in inorganic chemistry. The vast numbers of osmosis Systems in which a solvent is synthetic and natural organic compounds separated from a solution by a SEMIPERME- exist because of the ability of carbon to ABLE MEMBRANE approach equilibrium by form chains of atoms (catenation). Other solvent molecules on the solvent side of the elements are involved in organic com- membrane migrating through it to the so- pounds: principally hydrogen and oxygen lution side; this process is called osmosis but also nitrogen, halogens, sulfur, and and always leads to dilution of the solu- phosphorus. tion. The phenomenon is quantified by measurement of the osmotic pressure. The organometallic compound An organic process of osmosis is of fundamental im- compound containing a carbon–metal portance in transport and control mecha- bond. Tetraethyl lead, (C2H5)4Pb, is an ex- nisms in biological systems; for example, ample of an organometallic compound, plant growth and general cell function. See formerly used as an additive in petrol. osmotic pressure.

ornithine cycle (urea cycle) The se- osmotic pressure Symbol: π The pres- quence of enzyme-controlled reactions by sure that must be exerted on a solution to which urea is formed as a breakdown prevent the passage of solvent molecules product of amino acids. It occurs in cells of into it when the solvent and solution are the liver. The amino acid ornithine is com- separated by a semipermeable membrane. bined with ammonia (from amino acids) The osmotic pressure is therefore the pres- and carbon dioxide, forming another sure required to maintain equilibrium be- amino acid, arginine, which is then split tween the passage of solvent molecules into urea (which is excreted) and ornithine. through the membrane in either direction

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Ostwald’s dilution law

and thus prevent the process of osmosis requiring reaction and itself is a step to- proceeding. The osmotic pressure can be wards the formation of citric acid in a measured by placing the solution, con- reaction involving pyruvate ion and coen- tained in a small perforated thimble cov- zyme A. ered by a semipermeable membrane and fitted with a length of glass tubing, in a oxidant An oxidizing agent. In rocket beaker of the pure solvent. Solvent mol- fuels, the oxidant is the substance that pro- ecules pass through the membrane, dilut- vides the oxygen for combustion (e.g. ing the solution and thereby increasing the liquid oxygen or hydrogen peroxide). volume on the solution side and forcing the solution to rise up the glass tubing. The oxidative metabolism See aerobic res- process continues until the pressure ex- piration. erted by the solvent molecules on the membrane is balanced by the hydrostatic oxidative phosphorylation The pro- pressure of the solution in the tubing. A duction of ATP from phosphate and ADP sample of the solution is then removed and as electrons are transferred along the its concentration determined. Osmosis is a electron-transport chain from NADH or colligative property; therefore the method FADH2 to oxygen. Most of the NADH and can be applied to the determination of rel- FADH2 is formed in the mitochondrial ma- ative molecular masses, particularly for trix by the Krebs cycle and fatty acid oxi- large molecules, such as proteins, but it is dation. Oxidative phosphorylation occurs restricted by the difficulty of preparing in mitochondria and is the main source of good semipermeable membranes. ATP in aerobes. See electron-transport As the osmotic pressure is a colligative chain. property it is directly proportional to the molar concentration of the solute if the oxidation An atom, an ion, or a mol- temperature remains constant; thus π is ecule is said to undergo oxidation or to be proportional to the concentration n/V, oxidized when it loses electrons. The where n is the number of moles of solute, process may be effected chemically, i.e. by and V the solvent volume. The osmotic reaction with an oxidizing agent, or electri- pressure is also proportional to the ab- cally, in which case oxidation occurs at the solute temperature. Combining these two proportionalities gives πV = nCT, which anode. For example, → + – has the same form as the gas equation, PV 2Na + Cl2 2Na + 2Cl = nRT, and experimental values of C are where chlorine is the oxidizing agent and similar to those for R, the universal gas sodium is oxidized, and – 2+ → constant. This gives considerable support 4CN + 2Cu C2N2 + 2CuCN 2+ – to the kinetic theory of colligative proper- where Cu is the oxidizing agent and CN ties. is oxidized. The oxidation state of an atom is indi- Ostwald’s dilution law See dissocia- cated by the number of electrons lost or ef- tion constant. fectively lost by the neutral atom, i.e. the oxidation number. The oxidation number oxalate A salt or ester of ethanedioic of a negative ion is negative. The process of acid (oxalic acid). See ethanedioic acid. oxidation is the converse of reduction. See also redox. oxalic acid See ethanedioic acid. oxidizing agent See oxidation. oxaloacetic acid (OAA) A water- soluble carboxylic acid, structurally related oxime A type of organic compound con- to fumaric acid and maleic acid. Oxalo- taining the C:NOH group, formed by reac- acetic acid forms part of the Krebs cycle, tion of an ALDEHYDE or KETONE with it is produced from L-malate in an NAD- hydroxylamine (NH2OH). The reaction is 162 iranchembook.ir/edu

ozonolysis

a condensation reaction, in which a mol- ble molecule trioxygen (OZONE), O3, which ecule of water is lost. is formed by passing an electric discharge through oxygen gas. oxo process A method of manufactur- Oxygen occurs in three natural isotopic ing aldehydes by passing a mixture of car- forms, 16O (99.76%), 17O (0.0374%), 18O bon monoxide, hydrogen, and alkanes (0.2039%); the rarer isotopes are used in over a cobalt catalyst at high pressure (100 detailed studies of the behavior of oxygen- atmospheres and 150°C). The aldehydes containing groups during reactions (tracer can subsequently be reduced to alcohols, studies). making the process a useful source of alco- Symbol: O; m.p. –218.4°C; b.p. hols of high molecular weight. –182.962°C; d. 1.429 kg m–3 (0°C); p.n. 8; r.a.m. 15.9994. 2-oxopropanoic acid See pyruvic acid.

ozone (trioxygen; O3) A poisonous, oxyacid An acid in which the replace- blue-colored allotrope of oxygen made by able hydrogen atom is part of a hydroxyl passing oxygen through a silent electric group, including carboxylic acids, phenols discharge. Ozone is unstable and decom- and inorganic acids such as phosphoric(V) poses to oxygen on warming. It is a strong acid and sulfuric(VI) acid. See acid. oxidizing agent. It is present in the upper layers of the atmosphere, where it screens oxygen A colorless odorless diatomic the Earth from harmful short-wave ultravi- gas; the first member of group 16 (formerly VIA) of the periodic table. It has the elec- olet radiation. There is concern that the tronic configuration [He]2s22p4 and its ozone layer is possibly being depleted by chemistry involves the acquisition of elec- fluorocarbons and other compounds pro- trons to form either the di-negative ion, duced by industry. O2–, or two covalent bonds. In each case the oxygen atom attains the configuration ozonide See ozonolysis. of the rare gas neon. Oxygen is the most plentiful element in the Earth’s crust ac- ozonolysis The addition of ozone (O3) counting for over 40% by weight. It is pre- to alkenes and the subsequent hydrolysis of sent in the atmosphere (20%) and is a the ozonide into hydrogen peroxide and a constituent of the majority of minerals and mixture of carbonyl compounds. The car- rocks (e.g. sandstones, SiO2, carbonates, bonyl compounds can be separated and CaCO3, aluminosilicates) as well as the identified, which in turn, identifies the major constituent of the sea. Oxygen is an groups and locates the position of the dou- essential element for almost all living ble bond in the original alkene. Ozonolysis things. Elemental oxygen has two forms: was formerly an important analytical tech- the diatomic molecule O2 and the less sta- nique.

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palmitate A salt or ester of palmitic solvent, which rises up to the base line and acid. beyond by capillary action. The components within the sample mixture dissolve in palmitic acid See hexadecanoic acid. this mobile phase and are carried up the paper. However, the paper holds a quan- pantothenic acid (vitamin B5) One of tity of moisture and some components will the water-soluble B-group of vitamins. have a greater tendency than others to dis- Sources of the vitamin include egg yolk, solve in this moisture rather than in the kidney, liver, and yeast. As a constituent of mobile phase. In addition, some compo- coenzyme A, pantothenic acid is essential nents will preferentially cling to the surface for several fundamental reactions in me- of the paper. Therefore, as the solvent tabolism. A deficiency results in symptoms moves through the paper, certain compo- affecting a wide range of tissues; the over- nents will be left behind and components in all effects include fatigue, poor motor co- the mixture will become separated from ordination, and muscle cramps. each other. When the solvent has almost reached the top of the paper, the paper is removed and quickly dried. The paper is developed to locate the positions of colorless fractions by spraying with a suitable chemical, e.g. ninhydrin, or by exposure to ultraviolet ra- developing diation. The components are identified by chamber comparing the distance they have traveled up the paper with standard solutions that have been run simultaneously, or by com- paper strip puting an RF VALUE. A simplified version of paper chromatography uses a piece of filter mobile phase paper.

spotted samples para- 1. Designating a benzene compound with substituents in the 1,4 posi- Paper chromatography tions. The position on a benzene ring directly opposite a substituent is the para paper chromatography A form of position. This was used in the systematic CHROMATOGRAPHY widely used for the naming of benzene compounds. For exam- analysis of mixtures. Paper chromatogra- ple, para-dinitrobenzene (or p-dinitroben- phy usually employs a specially produced zene) is 1,4-dinitrobenzene. paper as the stationary phase. A base line is 2. Designating the form of a diatomic mol- marked in pencil near the bottom of the ecule in which both nuclei have opposite paper and a small sample of the mixture is spin directions; e.g. parahydrogen, para- spotted onto it using a capillary tube. The deuterium. paper is then placed vertically in a suitable See also meta-, ortho-.

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Pauling, Linus Carl

paraffin 1. See petroleum. named for the French mathematician 2. See alkane. Blaise Pascal (1623–62).

paraffin oil See petroleum. Pasteur, Louis (1822–95) French chemist and biologist. Pasteur is famous for his in- paraffin wax A solid mixture of hydro- vestigations on stereochemistry, fermenta- carbons obtained from petroleum. See also tion, and vaccines. In his early work he wax. discovered the phenomenon of optical isomers and invented methods for separating paraformaldehyde See methanal. such isomers. In 1860 he postulated that optical isomers existed because of the paraldehyde See ethanal. arrangements of atoms in the molecules. This idea was a major stimulus to the de- partial ionic character The electrons velopment of structural chemistry. His of a covalent bond between atoms or work on fermentation led to the process groups with different electronegativities known as pasteurization, which uses ele- will be polarized towards the more elec- vated temperatures to kill organisms that tronegative constituent; the magnitude of spoil milk or wine. He also established the this effect can be measured by the ionic role of micro-organisms such as yeast in character of the bond. When the effect is fermentation. small the bond is referred to simply as a polar bond and is adequately treated using Pauli exclusion principle See exclu- DIPOLE MOMENTS; as the effect grows the sion principle. theoretical treatment requires other contributions to ionic character. Pauling, Linus Carl (1901–94) Ameri- can chemist. Pauling was one of the great- partial pressure In a mixture of gases, est scientists of the twentieth century. His the contribution that one component early work was on determining the struc- makes to the total pressure. It is the pres- ture of complex minerals such as molyb- sure that the gas would have if it alone denite by x-ray diffraction. In 1928–29 this were present in the same volume. See Dal- led to Pauling’s rules governing the struc- ton’s law. ture of complex minerals. Pauling was a major pioneer in the application of quan- particulate matter (PM) An airborne tum mechanics to chemical bonding. In pollutant consisting of small particles of 1931 he published a classic paper entitled silicate, carbon, or large polyaromatic hy- The Nature of the Chemical Bond that ex- drocarbons (PAHs). These pollutants are plained how a chemical bond is formed often referred to as particulates. They are from a pair of electrons. Pauling also intro- classified according to size; e.g. PM10 is duced the concept of hybridization to ex- particulate matter formed of particles less plain the chemical bonding of the carbon than 10 µm diameter. atom. Pauling also considered partially ionic bonds. Pauling put together his ideas particulates See particulate matter. about chemical bonding in his book The Nature of the Chemical Bond, the first edi- partition coefficient If a solute dis- tion of which was published in 1939. In the solves in two nonmiscible liquids, the par- mid-1930s Pauling turned his attention to tition coefficient is the equilibrium ratio of molecules of biological interest and was the concentration in one liquid to the con- one of the founders of molecular biology. centration in the other liquid. Together with Robert Corey, he showed that many proteins have helical shapes. He pascal Symbol: Pa The SI unit of pres- also worked on sickle-cell anemia. He co- sure, equal to a pressure of one newton per authored the book Introduction to Quan- square meter (1 Pa = 1 N m–2). The unit is tum Mechanics (1935) and wrote the

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PCB

influential books General Chemistry is an alternative to glycolysis and is much (1948) and college Chemistry (1950). Paul- more active in adipose tissue (where large ing was awarded the 1954 Nobel Prize for amounts of NADPH are consumed) than chemistry. In the 1950s he became con- in skeletal muscle. cerned with nuclear weapons, particularly their testing in the atmosphere. This led to pentyl group (amyl group) The group his winning the 1962 Nobel Prize for CH3CH2CH2CH2CH2– peace.

PCB See polychlorinated biphenyl. R H O R pectic substances Polysaccharides that, together with hemicelluloses, form the matrix of plant cell walls. They serve to ce- C N C C ment the cellulose fibers together. Fruits are a rich source. They are principally made from the H H group of sugar acids known as uronic Peptide linkage acids. Pectic acids, the basis of the other pectic substances, are soluble unbranched chains of α-1,4 linked galacturonic acid peptide A compound formed by linkage units (derived from the sugar galactose). of two or more amino-acid groups. Pep- The acid is precipitated as insoluble cal- tides can be formed by reaction of the car- cium or magnesium pectate in the middle boxylic acid group on one amino acid with lamella of plant cells. Pectinic acids are the amino group on another amino acid, slightly modified pectic acids. Under suit- with elimination of water. The amino acids able conditions pectinic acids and pectins are joined together by a bond of the type form gels with sugar and acid. Pectins are –CO–NH–, known as a peptide linkage. used commercially as gelling agents, e.g. in Simple peptides consisting of a small num- jams. Insoluble pectic substances are ber of amino-acid units (less than about termed protopectin and this is the most im- 10) are known as oligopeptides and are portant group in normal cell walls. Pro- designated as dipeptides, tripeptides, etc., topectin is hydrolyzed to soluble pectin by according to the number of amino-acid pectinase in ripening fruits, changing the units present. Peptides with large numbers fruit consistency. of amino-acid units are called POLYPEP- TIDES. pectin See pectic substances. peptide linkage See peptide. pentane (C5H12) A straight-chain alkane obtained by distillation of crude oil. percentage composition A way of expressing the composition of a chemical pentanoic acid (valeric acid; CH3(CH2)3- compound in terms of the percentage (by COOH) A colorless liquid carboxylic mass) of each of the elements that make it acid, used in making perfumes. up. It is calculated by dividing the mass of each element (taking into account the num- pentose A SUGAR that has five carbon ber of atoms present) by the relative mo- atoms in its molecules. lecular mass of the whole molecule. For example, methane (CH4) has a relative mo- pentose phosphate pathway (hexose lecular mass of 16 and its percentage com- monophosphate shunt) A pathway of position is 12/16 = 75% carbon and (4 × glucose breakdown in which pentoses are 1)/16 = 25% hydrogen. produced, in addition to reducing power (NADPH) for many synthetic reactions. It perfect gas (ideal gas) See gas laws.

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Perkin, Sir William Henry

pericyclic reaction A type of CON- Mendeléev (1834–1907), this law stated CERTED REACTION in which the TRANSITION that the properties of the elements are a pe- STATE is cyclic and can be regarded as riodic function of their atomic weights: if formed by movement of electrons in a cir- arranged in order of increasing atomic cle, from one bond to an adjacent bond. It weight then elements having similar prop- is usual to distinguish three types of peri- erties occur at fixed intervals. Certain ex- cyclic reaction: ceptions or gaps in the table lead to the Cycloadditions. In these reactions a conju- view that the nuclear charge is a more gated diene adds to a double bond to form characteristic function, thus the modern a ring. This involves the formation of two statement of the periodic law is that the new sigma bonds. The DIELS–ALDER REAC- physical and chemical properties of el- TION is a cycloaddition. The reverse reac- ements are a periodic function of their pro- tion, in which a ring containing a double ton number. bond breaks to a diene and a compound containing a double bond, is also a peri- periodic table A table of the elements cyclic reaction. arranged in order of increasing proton Sigmatropic rearrangements. In these reac- number to show similarities in chemical tions a sigma bond breaks and another is behavior between elements. Horizontal formed. rows of elements are called periods. Across Electrocyclic reactions. In these reactions a a period there is a general trend from ring is formed across the ends of a conju- metallic to nonmetallic behavior. Vertical gated system of double bonds. The reverse columns of related elements are called reaction, in which a ring containing two groups. Down a group there is an increase double bonds breaks by movement of elec- in atomic size and in electropositive (metal- trons in a cycle, is also an electrocyclic re- lic) behavior. action. Originally the periodic table was Pericyclic reactions are often treated using arranged in eight groups with the alkali FRONTIER ORBITAL theory or the WOOD- metals as group I, the halogens as group WARD–HOFFMANN RULES. VII, and the rare gases as group 0. The transition elements were placed in a block period One of the horizontal rows in the in the middle of the table. Groups were conventional periodic table. Each period split into two sub-groups. For example, represents the elements arising from pro- group I contained the main-group el- gressive filling of the outer shell (i.e. the ad- ements, Li, Na, K, Rb, Cs, in subgroup IA dition of one extra electron for each new and the subgroup IB elements Cu, Ag, Au. element), the elements being arranged in The system was confusing because there order of ascending proton number. In a was a difference in usage for subgroups strict sense hydrogen and helium represent and a current form of the table has 18 one period but convention refers to the el- groups. ements lithium to neon (8 elements) as the See also group; period. first short period (n = 2), and the elements sodium to argon (8 elements) as the second Perkin, Sir William Henry (1838– short period (n = 3). With entry to the n = 1907) British organic chemist. Perkin be- 4 level there is filling of the 4s, then back came famous for his discovery of mauve, filling of the 3d, before the 4p are filled. the first synthetic dye. This discovery orig- Thus this set contains a total of 18 elec- inated in 1856 when Perkin attempted to trons (potassium to krypton) and is called synthesize quinine. He did not succeed in a long period. The next set, rubidium to doing so. However, when he used chromic xenon, is similarly a long period. acid to oxidize toluidine he obtained a dark precipitate. When he repeated the experi- periodic law The law upon which the ment using aniline he again obtained a modern periodic table is based. Enunciated dark precipitate. He found that adding al- in 1869 by the Russian chemist Dmitri cohol to this precipitate produced a bright

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peroxide

purple solution. He also found that this of hydrocarbons, mainly pentane and bright purple substance was a dye which hexane, used as a solvent. Note that it is did not fade readily in light. This synthetic not an ether. dye was called mauve. Perkin patented the process for the manufacture of mauve and pH The logarithm to base 10 of the rec- set up a factory for its production. This iprocal of the hydrogen-ion concentration venture was so successful that he was able of a solution. In pure water at 25°C the to retire in 1874 and devote his life to sci- concentration of hydrogen ions is 1.00 × entific research. His son William Henry 10–7 mol l–1, thus the pH equals 7 at neu- Perkin (1860–1929) also became an emi- trality. An increase in acidity increases the nent organic chemist. value of [H+], decreasing the value of the pH below 7. An increase in the concentra- peroxide 1. A compound containing tion of hydroxide ion [OH–] proportion- the –O–O– group. Organic peroxides tend ately decreases [H+], therefore increasing to be unstable and form free radicals. They the value of the pH above 7 in basic solu- are used to initiate free-radical polymeriza- tions. pH values can be obtained approxi- tion reactions. mately by using indicators. More precise – – 2. An oxide containing the O–O ion. measurements use electrode systems. The

15 term ‘pH’ is short for ‘potential of hydro- peta- Symbol: P A prefix denoting 10 . gen’.

petrochemicals Chemicals that are ob- phase One of the physically separable tained from crude oil or from natural gas. parts of a chemical system. For example, a mixture of ice (solid phase) and water petrol See petroleum. (liquid phase) consists of two phases. A system consisting of only one phase is said petroleum A mixture of hydrocarbons to be homogeneous. A system consisting of formed originally from marine animals and more than one phase is said to be heteroge- plants, found beneath the ground trapped neous. between layers of rock. It is obtained by drilling (also called crude oil). Different A graphical representa- oilfields produce petroleum with differing phase diagram compositions. The mixture is separated tion of the state in which a substance will into fractions by fractional distillation in a occur at a given pressure and temperature. vertical column. The main fractions are: The lines show the conditions under which Diesel oil (gas oil) in the range 220–350°C, more than one phase can coexist at equilibrium. For one-component systems (e.g. consisting mainly of C13–C25 hydrocarbons. It is used as fuel in diesel engines. water) the point at which all three phases Kerosene (paraffin) in the range can coexist at equilibrium is called the ° triple point and is the point on the graph at 160–250 C, consisting mainly of C11 and which the pressure–temperature curves in- C12 hydrocarbons. It is a fuel both for domestic heating and jet engines. tersect. Gasoline (petrol) in the range 40–180°C, 1. (carbolic acid; hydroxyben- consisting mainly of C5–C10 hydrocarbons. phenol It is used as motor fuel and as a raw ma- zene; C6H5OH) A white crystalline solid terial for making other chemicals. used to make a variety of other organic Refinery gas, consisting of C1–C4 gaseous compounds. It is usually made using the hydrocarbons. CUMENE PROCESS or the RASHIG PROCESS. In addition lubricating oils and paraffin 2. A type of organic compound in which at wax are obtained from the residue. The least one hydroxyl group is bound directly black material left is bitumen tar. to one of the carbon atoms of an aromatic ring. Phenols do not show the behavior petroleum ether A flammable mixture typical of alcohols. In particular they are

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Phillips process

→ more acidic because of the electron-with- C6H5C2H5 C6H5CH:CH2 + H2 drawing effect of the aromatic ring. Phenol ionizes in water: phenyl group The group C6H5–, de- → – + C6H5OH C6H5O + H rived from benzene. The preparation of phenol itself is by fus- ing the sodium salt of the sulfonic acid with phenylhydrazone See hydrazone. sodium hydroxide: → C6H5SO2.ONa + 2NaOH C6H5ONa phenylmethanol (benzyl alcohol; C6H5- + Na2SO3 + H2O CH2OH) An aromatic primary alcohol The phenol is then liberated by sulfuric used as a solvent. Phenylmethanol is syn- acid: thesized by the CANNIZZARO REACTION, → 2C6H5ONa + H2SO4 2C6H5OH + which involves the simultaneous oxidation Na2SO4 and reduction of benzenecarbaldehyde Reactions of phenol include: (benzaldehyde) by refluxing in an aqueous 1. Replacement of the hydroxyl group with solution of sodium hydroxide: a chlorine atom using phosphorus(V) → 2C6H5CHO C6H5CH2OH + chloride: C H COOH → 6 5 ROH RCl Benzoic acid is the other product. 2. Reaction with acyl halides to form esters Phenylmethanol undergoes the reac- of carboxylic acids: tions characteristic of alcohols, especially 1 2 → 2 1 R OH + R COCl R COOR those in which the formation of a stable 3. Reaction with haloalkanes under alka- carbonium ion as an intermediate line conditions to give mixed alkyl–aryl (C H CH +) enhances the reaction. Substi- ethers: 6 5 2 tution onto the benzene ring is also possi- R1OH + R2Cl → R1OR2 ble; the –CH OH group directs into the 2- In addition phenol can undergo further 2 or 4-position by the donation of electrons substitution on the benzene ring. The hy- to the ring. droxyl group directs other substituents into the 2- and 4-positions. phenyl methyl ketone (acetophenone; C H COCH ) A colorless sweet-smelling phenolphthalein An acid–base indica- 6 5 3 organic liquid, which solidifies below tor that is colorless in acid solutions and ° becomes red if the pH rises above the tran- 20 C. It is used as a solvent for methyl and sition range of 8–9.6. It is used as the indi- ethyl cellulose plastics. cator in titrations for which the end point lies clearly on the basic side (pH > 7), e.g. 3-phenylpropenoic acid (cinnamic acid) oxalic acid or potassium hydrogentartrate A white pleasant-smelling crystalline car- against caustic soda. boxylic acid, C6H5CH:CHCOOH. It occurs in amber but can be synthesized and is phenoxy resin A type of thermoplastic used in perfumes and flavorings. resin made by condensation of phenols. pheromone A substance that is excreted phenylalanine See amino acid. by an animal and causes a response in other animals of the same species (e.g. sex- phenylamine See aniline. ual attraction, development). Compare kairomone.

phenylethene (styrene; C6H5CHCH2) A liquid hydrocarbon used as the starting Phillips process A method for the man- material for the production of polystyrene ufacture of high-density polyethene using a and some synthetic rubbers. The manufac- catalyst of chromium(III) oxide on a pro- ture of phenylethene is by dehydrogenation moter of silica and alumina. The reaction of ethyl benzene using various metal oxide conditions are 150°C and 30 atm pressure. catalysts: See also Ziegler process.

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phosgene

phosgene See carbonyl chloride. and other effectors that control important cellular functions, e.g. glycogenolysis, in- phosphate A salt or ester of a phos- sulin secretion, the aggregation of platelets, phorus(V) oxoacid, especially one of and smooth muscle contraction. phosphoric(V) acid, H3PO4. Polymeric phosphates occur containing P–O–P phosphonic acid (phosphorous acid; bridges. H3PO3) A colorless deliquescent solid that can be prepared by the action of water phosphide A compound of phosphorus on phosphorus(III) oxide or with a more electropositive element. phosphorus(III) chloride. It is a dibasic – acid producing the anions H2PO3 and 2– phosphine (phosphorus(III) hydride; PH3) HPO3 in water. The acid and its salts are A colorless gas that is slightly soluble in slow reducing agents. water. It has a characteristic fishy smell. It + can be made by reacting water and calcium phosphonium ion The ion PH4 de- phosphide or by the action of yellow phos- rived from phosphine. phorus on a concentrated alkali. Phosphine usually ignites spontaneously in air be- phosphoprotein A conjugated protein cause of contamination with diphosphine. formed by the combination of protein with phosphate groups. Casein is an example. phospholipid A lipid with a phosphate group attached by an ester linkage. They phosphoric(V) acid (orthophosphoric are the major class of lipid in all biological acid; H3PO4) A white solid that can be membranes and, together with glycolipids made by reacting phosphorus(V) oxide and cholesterol, are the main structural with water or by heating yellow phospho- components. All membrane phosphlipids, rus with nitric acid. The naturally occur- except sphingosine, are derived from glyc- ring phosphates (orthophosphates, erol and are called glycerophospholipids. M3PO4) are salts of phosphoric(V) acid. They consist of a glycerol backbone with two fatty acid chains esterified to carbons phosphorous acid See phosphonic 1 and 2 and a phosphorylated alcohol acid. esterified at carbon 3. The simplest glyc- erophospholipid is diacylglycerol 3-phos- phosphorescence 1. The absorption of phate or phosphatidate, which has no energy by atoms followed by emission of alcohol part. Only small amounts exist electromagnetic radiation. Phosphores- naturally, but it is a key intermediate in the cence is a type of luminescence, and is dis- biosynthesis of other glycerophospholipids tinguished from fluorescence by the fact with the phosphate being esterified to one that the emitted radiation continues for of several alcohols (serine, ethanolamine, some time after the source of excitation has choline, or glycerol) to form the major been removed. In phosphorescence the ex- membrane glycerophospholipids: phos- cited atoms have relatively long lifetimes phatidylserine, phosphatidylethanolamine, before they make transitions to lower en- phosphatidylcholine, and phosphatidyl- ergy states. However, there is no defined glycerol. Sphingomylein (like glycolipids) time distinguishing phosphorescence from is derived from sphingosine and has a fluorescence. phosphorylcholine esterified to the pri- 2. In general usage the term is applied to mary hydroxyl group of sphingosine. the emission of ‘cold light’ – light produced Another important glycerophospho- without a high temperature. The name lipid is phosphatidylinositol, which is phos- comes from the fact that white phosphorus phorylated by specific kinases to phos- glows slightly in the dark as a result of a phatidylinositol 4,5-bisphosphate. This is a chemical reaction with oxygen. The light key molecule in signal transduction that comes from excited atoms produced di- mediates the action of several hormones rectly in the reaction – not from the heat

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phosphorus(V) oxide

produced. It is thus an example of chemi- main use is in organic chemistry to replace luminescence. There are also a number of a hydroxyl group with a bromine atom. biochemical examples termed biolumines- cence; for example, phosphorescence is phosphorus(III) chloride (phosphorus sometimes seen in the sea from marine or- trichloride; PCl3) A colorless liquid ganisms, or on rotting wood from certain formed from the reaction of phosphorus fungi (known as ‘fox fire’). with chlorine. It is rapidly hydrolyzed by water to phosphonic acid and hydrogen phosphorus A reactive solid nonmetal- chloride. Phosphorus(III) chloride is used lic element; the second element in group 15 in organic chemistry to replace a hydroxyl (formerly VA) of the periodic table. It has group with a chlorine atom. the electronic configuration [Ne]3s23p3 and is therefore formally similar to nitro- phosphorus(V) chloride (phosphorus gen. It is however very much more reactive pentachloride; PCl5) A white easily sub- than nitrogen and is never found in nature limed solid formed by the action of chlo- in the uncombined state. Phosphorus is rine on phosphorus(III) chloride. It is widespread throughout the world; eco- hydrolyzed by water to phosphoric(V) acid nomic sources are phosphate rock and hydrogen chloride. Its main use is as a (Ca3(PO4)2) and the apatites, variously oc- chlorinating agent in organic chemistry to curring as both fluoroapatite replace a hydroxyl group with a chlorine (3Ca3(PO4)2.CaF2) and as chloroapatite atom. (3Ca3(PO4)2CaCl2). Guano formed from the skeletal phosphate of fish in sea-bird phosphorus(III) chloride oxide (phos- droppings is also an important source of phorus trichloride oxide; phosphorus oxy- phosphorus. The largest amounts of phos- chloride, phosphoryl chloride, POCl )A phorus compounds produced are used as 3 colorless liquid that can be obtained by re- fertilizers, with the detergents industry acting phosphorus(III) chloride with oxy- producing increasingly large tonnages of gen or by distilling phosphorus(III) phosphates. Phosphorus is an essential chloride with potassium chlorate. The re- constituent of living tissue and bones, and actions of phosphorus(III) chloride oxide it plays a very important part in metabolic processes and muscle action. are similar to those of phosphorus(III) Symbol: P; m.p. 44.1°C (white) 410°C chloride. The chlorine atoms can be re- (red under pressure); b.p. 280.5°C; r.d. placed by alkyl groups using Grignard 1.82 (white) 2.2 (red) 2.69 (black) (all at reagents or by alkoxo groups using alco- 20°C); p.n. 15; r.a.m. 30.973762. hols. Water hydrolysis yields phosphoric(V) acid. phosphorus(III) bromide (phosphorus phosphorus(III) hydride See phos- tribromide; PBr3) A colorless liquid made by reacting phosphorus with phine. bromine. It is readily hydrolyzed by water to phosphonic acid and hydrogen bromide. phosphorus(V) oxide (phosphorus pen- Phosphorus(III) bromide is important in toxide, P2O5) A white powder that is sol- organic chemistry, being used to replace a uble in organic solvents. It usually exists as hydroxyl group with a bromine atom. P4O10 molecules. Phosphorus(V) oxide can be prepared by burning phosphorus in a phosphorus(V) bromide (phosphorus plentiful supply of oxygen. It readily com- pentabromide; PBr5) A yellow crystalline bines with water to form phosphoric(V) solid that sublimes easily. It can be made acid and is therefore used as a drying agent by the reaction of bromine and phospho- for gases. It is a useful dehydrating agent rus(III) bromide. Phosphorus(V) bromide because it is able to remove the elements of is readily hydrolyzed by water to phos- water from compounds containing oxygen phoric(V) acid and hydrogen bromide. Its and hydrogen

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phosphorus oxychloride

phosphorus oxychloride See phos- photochemistry The branch of chem- phorus(III) chloride oxide. istry dealing with reactions induced by light or ultraviolet radiation. phosphorus pentabromide See phosphorus(V) bromide. photoelectron spectroscopy See photoionization. phosphorus pentachloride See phosphorus(V) chloride. photoemission The emission of photoelectrons by the photoelectric effect or by phosphorus pentoxide See phospho- photoionization. rus(V) oxide. photoionization The ionization of phosphorus tribromide See phospho- atoms or molecules by electromagnetic ra- rus(III) bromide. diation. Photons absorbed by an atom may have sufficient photon energy to free an phosphorus trichloride See phospho- electron from its attraction by the nucleus. rus(III) chloride. The process is M + hv → M+ + e– phosphorus trichloride oxide See As in the photoelectric effect, the radia- phosphorus(III) chloride oxide. tion must have a certain minimum thresh- old frequency. The energy of the phosphorylation The transfer of a photoelectrons ejected is given by W = hv – phosphoryl group –PO(OH) from ATP to 2 I, where I is the ionization potential of the a protein by a protein kinase. Many meta- atom or molecule. Analysis of the energies bolic enzymes are regulated by phosphory- of the emitted electrons gives information lation as are several signal pathways on the ionization potentials of the sub- involved in cell growth. The removal of the stance – a technique known as photoelec- phosphoryl group (dephosphorylation) is tron spectroscopy. brought about by enzymes known as phos- phatases. photolysis A chemical reaction that is phosphoryl chloride See phospho- produced by electromagnetic radiation rus(III) chloride oxide. (light or ultraviolet radiation). Many pho- tolytic reactions involve the formation of photochemical reaction A reaction free radicals. See also flash photolysis. brought about by light or ultraviolet radiation; examples include the bleaching of col- photosynthesis The synthesis of or- ored material, the reduction of silver ganic compounds using light energy ab- halides (in photography), and the photo- sorbed by chlorophyll. With the exception synthesis of carbohydrates. Chemical of a small group of bacteria, organisms changes occur only when the reacting photosynthesize from inorganic materials. atoms or molecules absorb photons of the All green plants photosynthesize as well as appropriate energy to produce excited certain prokaryotes (some bacteria). In species or when the photons have sufficient green plants, photosynthesis takes place in energy to produce free radicals or ions. The chloroplasts, mainly in leaves. Directly or amount of substance that reacts is propor- indirectly, photosynthesis is the source of tional to the quantity of energy absorbed. carbon and energy for all except chemoau- For example, in the reaction between hy- totrophic organisms. The mechanism is drogen and chlorine, it is not the concen- complex and involves two sets of stages: trations of hydrogen or chlorine that light reactions followed by dark reactions. dictate the rate of reaction but the intensity The overall reaction in green plants can be of the radiation. summarized by the equation:

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picric acid

→ CO2 + 4H2O [CH2O] + blue and red light are absorbed in the sur- 3H2O + O2 face layers. They comprise the blue phyco- In the light reactions, light energy is ab- cyanins, which absorb extra orange and sorbed by chlorophyll (and other pig- red light, and the red phycoerythrins, ments), setting off a chain of chemical which absorb green light, enabling red reactions in which water is split and algae to grow at depth in the sea. See also gaseous oxygen evolved. The hydrogen absorption spectrum; photosynthetic pig- from the water is attached to other mol- ments. ecules, and used to reduce carbon dioxide to carbohydrates in the later dark reac- phycocyanin A photosynthetic pig- tions. These involve a complex cycle of re- ment. See phycobilins. actions (the Calvin cycle) in which sugar phosphates are formed. phycoerythrin A photosynthetic pig- See electron-transport chain; photosyn- ment. See phycobilins. thetic pigments.

photosynthetic pigments Pigments that phylloquinone See vitamin K. absorb the light energy required in photosynthesis. They are located in the chloro- physical change A change to a sub- plasts of plants and algae, whereas in most stance that does not alter its chemical prop- photosynthetic bacteria they are located in erties. Physical changes (e.g. melting, thylakoid membranes, typically distributed boiling, and dissolving) are comparatively around the cell periphery. All photosyn- easy to reverse. thetic organisms contain chlorophylls and carotenoids; some also contain phyco- physical chemistry The branch of bilins. Chlorophyll a is the primary pig- chemistry concerned with the physical ment since energy absorbed by this is used properties of compounds and how these directly to drive the light reactions of pho- depend on the chemical bonding. It in- tosynthesis. The chlorophyll a that forms cludes such topics as chemical thermody- the reaction center of photosystem II has namics and electrochemistry. an absorption peak at 680 nm and that of photosystem I at 700 nm. The other pig- physisorption See adsorption. ments (chlorophylls b, c, and d, and the carotenoids and phycobilins) are accessory phytohormone See plant hormone. pigments that pass the energy they absorb on to chlorophyll a. They broaden the pi bond See orbital. spectrum of light used in photosynthesis. See absorption spectrum. pico- Symbol: p A prefix denoting 10–12. For example, 1 picofarad (pF) = 10–12 phthalic acid See benzene-1,2-dicar- farad (F). boxylic acid. picrate A salt of picric acid; metal pi- phthalic anhydride See benzene-1,2- dicarboxylic acid. crates are explosive. See also picric acid.

phycobilins A group of accessory pho- picric acid (2,4,6-trinitrophenol; tosynthetic pigments found in Cyanobacte- C6H2(NO3)3OH) A yellow crystalline ria and red algae. Chemically they are solid made by nitrating phenolsulfonic linear tetrapyrroles in contrast to chloro- acid. It is used as a dye and as an explosive. phyll, which is a cyclic tetrapyrrole. They With aromatic hydrocarbons picric acid absorb light in the middle of the spectrum forms characteristic charge-transfer com- not absorbed by chlorophyll, an important plexes (misleadingly called picrates), used function in algae living under water where in analysis for identifying the hydrocarbon.

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pine-cone oil

pine-cone oil See turpentine. such as plasticizers, fillers, stabilizers, and colorants. See also resin. pi orbital See orbital. plasticizer A substance added to a syn- pipette A device used to transfer a thetic resin to make it more flexible. known volume of solution from one container to another; in general, several sam- platinum A silvery-white malleable duc- ples of equal volume are transferred for tile transition metal. It occurs naturally in individual analysis from one stock solu- Australia and Canada, either free or in as- tion. Pipettes are of two types, bulb sociation with other platinum metals. It is pipettes, which transfer a known and fixed resistant to oxidation and is not attacked volume, and graduated pipettes, which can by acids (except aqua regia) or alkalis. Plat- transfer variable volumes. Pipettes were at inum is used as a catalyst for ammonia ox- one time universally mouth-operated but idation (to make nitric acid), hydrocarbon safety pipettes using a plunger or rubber cracking, and in catalytic converters. It is bulb are now preferred. also used in jewelry. Symbol: Pt; m.p. 1772°C; b.p. 3830 ± pK The logarithm to the base 10 of the 100°C; r.d. 21.45 (20°C); p.n. 78; r.a.m. reciprocal of an acid’s dissociation con- 195.08. stant: log10(1/Ka) platinum black A finely divided black form of platinum produced, as a coating, Planck constant Symbol: h A funda- by evaporating platinum onto a surface in mental constant; the ratio of the energy an inert atmosphere. Platinum-black coat- (W) carried by a photon to its frequency ings are used as absorbents and as cata- (v). A basic relationship in the quantum lysts. theory of radiation is W = hv. The value of h is 6.626 196 × 10–34 J s. The Planck con- Plexiglas (Trademark) A widely-used stant appears in many relationships in acrylic resin, polymethylmethacrylate. which some observable measurement is quantized (i.e. can take only specific dis- PM See particulate matter. crete values rather than any of a range of values). poison 1. A substance that destroys catalyst activity. plane polarization A type of POLARIZA- 2. Any substance that endangers biological TION of electromagnetic radiation in which activity, whether by physical or chemical the vibrations take place entirely in one means. plane. polar Describing a compound with mol- plant hormone (phytohormone) One ecules that have a permanent dipole mo- of a group of essential organic substances ment. Hydrogen chloride and water are produced in plants. They are effective in examples of polar compounds. very low concentrations and control growth and development by their interac- polar bond A covalent bond in which tions. Examples are auxins, gibberellins, the bonding electrons are not shared cytokinins, abscisic acid, and ethylene. equally between the two atoms. A bond between two atoms of different electronega- plastic A substance that can be shaped tivity is said to be polarized in the direction by heat and pressure. Most plastics are of the more electronegative atom, i.e. the made from synthetic POLYMERS, although electrons are drawn preferentially towards some are based on natural materials such the atom. This leads to a small separation as cellulose. In a plastic, the synthetic resin of charge and the development of a bond is usually mixed with other substances dipole moment as in, for example, hydro-

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pollution

gen fluoride, represented as H→F or as example, in water. Such molecules possess δ δ H +–F – (F is more electronegative). a DIPOLE MOMENT. The charge separation is much smaller than in ionic compounds; molecules in polarography An analytical method in which bonds are strongly polar are said to which current is measured as a function of display partial ionic character. The effect potential. A special type of cell is used in of the electronegative element can be trans- which there is a small easily polarizable mitted beyond adjacent atoms, thus the cathode (the dropping mercury electrode) C–C bonds in, for example, CCl3CH3 and and a large non-polarizable anode (refer- CH3CHO are slightly polar. See also di- ence cell). The analytical reaction takes pole moment; intermolecular force. place at the cathode and is essentially a reduction of the cations, which are dis- polarimeter (polariscope) An instru- charged according to the order of their ment for measuring optical activity. See electrode potential values. The data is ex- optical activity. pressed in the form of a polarogram, which is a plot of current against applied voltage. polariscope See polarimeter. As the applied potential is increased a point is reached at which the ion is discharged. polarizability The ease with which an There is a step-wise increase in current, electron cloud is deformed (polarized). In which levels off because of polarization ions, an increase in size or negative charge effects. The potential at half the step height leads to an increase in polarizability. (called the half-wave potential) is used to identify the ion. Most elements can be de- polarization 1. The restriction of the termined by polarography. The optimum vibrations in a transverse wave so that the concentrations are in the range vibration occurs in a single plane. Electro- 10–2–10–4M; modified techniques allow magnetic radiation, for instance, is a trans- determinations in the parts per million verse wave motion. It can be thought of as range. an oscillating electric field and an oscillating magnetic field, both at right angles to polar solvent See solvent. the direction of propagation and at right angles to each other. Usually, the electric pollution Any damaging or unpleasant vector is considered since it is the electric change in the environment that results field that interacts with charged particles from the physical, chemical, or biological of matter and causes the effects. In ‘nor- side-effects of human industrial or social mal’ unpolarized radiation, the electric activities. Pollution can affect the atmos- field oscillates in all possible directions per- phere, rivers, seas, and the soil. pendicular to the wave direction. On re- Air pollution is caused by the domestic flection or on transmission through certain and industrial burning of carbonaceous substances (e.g. Polaroid) the field is con- fuels, by industrial processes, and by car fined to a single plane. The radiation is exhausts. Among recent problems are in- then said to be plane-polarized. If the tip of dustrial emissions of sulfur(IV) oxide caus- the electric vector describes a circular helix ing acid rain, and the release into the as the wave propagates, the light is said to atmosphere of chlorofluorocarbons, used be circularly polarized. in refrigeration, aerosols, etc., has been 2. See polarizability. linked to the depletion of ozone in the stratosphere. Carbon dioxide, produced by polar molecule A molecule in which the burning fuel and by car exhausts, is slowly individual polar bonds are not perfectly building up in the atmosphere, which symmetrically arranged and are therefore could result in an overall increase in the not ‘in balance’. Thus the charge separa- temperature of the atmosphere (green- tion in the bonds gives rise to an overall house effect). Car exhausts also contain charge separation in the molecule as, for carbon monoxide and lead. The former has

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polyamide

not yet reached dangerous levels, but vege- Examples are benzene (C6H6) and methane tation near main roads contains a high pro- (CH4). portion of lead and levels are sufficiently high in urban areas to cause concern about polybasic acid An acid that has two or the effects on children. Lead-free gasoline more replaceable hydrogen atoms. For ex- is widely available. Photochemical smog, ample, phosphorus(V) acid, H3PO4, is trib- caused by the action of sunlight on hydro- asic. carbons and nitrogen oxides from car exhausts, is a problem in several countries. polycarbonate A thermoplastic poly- Catalytic converters reduce harmful emis- mer consisting of polyesters of carbonic sions from car exhausts. acid and dihydroxy compounds. Polycar- Water pollutants include those that are bonates are tough and transparent, used biodegradable, such as sewage effluent, for making soft-drink bottles and electrical which cause no permanent harm if ade- connectors. quately treated and dispersed, as well as those that are nonbiodegradable, such as certain chlorinated hydrocarbon pesticides polychlorinated biphenyl (PCB) A (e.g. DDT) and heavy metals, such as lead, type of compound based on biphenyl copper, and zinc in some industrial efflu- (C6H5C6H5), in which some of the hydro- ents (causing heavy-metal pollution). gen atoms have been replaced by chlorine When these accumulate in the environment atoms. They are used in certain polymers they can become very concentrated in food used for electrical insulators. PCBs are chains. The pesticides DDT, aldrin, and highly toxic and concern has been caused dieldrin are now banned in many coun- by the fact that they can accumulate in the tries. Water supplies can become polluted food chain. by leaching of nitrates from agricultural land. The discharge of waste heat can cause polychloroethene (polyvinyl chloride; thermal pollution of the environment, but PVC) A synthetic polymer made from this is reduced by the use of cooling towers. chloroethene. It is a strong material with a In the sea, oil spillage from tankers and the wide variety of uses. inadequate discharge of sewage effluent are the main problems. polycyclic Describing a compound that Other forms of pollution are noise from has two or more rings in its molecules. aircraft, traffic, and industry and the dis- posal of radioactive waste. polyene An alkene with more than two double bonds in its molecules. polyamide A synthetic polymer in which the monomers are linked by the polyester A synthetic polymer made by group –NH–CO–. Nylon is an example of reacting alcohols with acids, so that the a polyamide. monomers are linked by the group –O–CO–. Synthetic fibers such as Dacron polyamine An aliphatic compound which has two or more amino and/or imino are polyesters. groups. Polyamines are often found associated with DNA and RNA in bacteria and polyethene (polyethylene; polythene) A viruses. This may stabilize the nucleic acid synthetic polymer made from ethene. It is molecule in a way analogous to the action produced in two forms – a soft material of of histones on DNA in eukaryote cells. Ex- low density and a harder, higher density amples of polyamines include spermine, form, which is more rigid. It can be made spermidine, cadaverine, and putrescine. by the ZIEGLER PROCESS and the PHILLIPS PROCESS. polyatomic molecule A molecule that consists of several atoms (three or more). polyethylene See polyethene.

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polysaccharide

HHH HHH HHH HHH

HHH H X H X XXX X H X H isotactic syndiotactic

Polymerization: examples of stereospecific polymers

polyhydric alcohol An alcohol that has way. These are tactic polymers. If one par- several –OH groups in its molecules. ticular group is always on the same side of the chain, the polymer is said to be isotac- polymer A compound in which there tic. If the group alternates in position along are very large molecules made up of re- the chain the polymer is syndiotactic. If peating molecular units (monomers). A there is no regular pattern, the polymer is polymer has a repeated structural unit, atactic. known as a mer. Polymers do not usually Polymerization reactions are also classi- have a definite relative molecular mass, be- fied according to the type of reaction. Ad- cause there are variations in the lengths of dition polymerization occurs when the different chains. They may be natural sub- monomers undergo addition reactions, stances (e.g. polysaccharides and proteins) with no other substance formed. Conden- or synthetic materials (e.g. nylon and poly- sation polymerization involves the elimina- ethene). The two major classes of synthetic tion of small molecules in the formation of polymers are thermosetting (e.g. Bakelite) the polymer. See also cross linkage. and thermoplastic (e.g. polyethene). The former are infusible, and heat may only polymethanal See methanal. make them harder, whereas the latter soften on heating. See also polymerization. polymethylmethacrylate A transparent ACRYLIC RESIN made by polymerizing polymerization The process in which methyl methacrylate, trade name Plexi- one or more compounds react to form a glass. POLYMER. Homopolymers are formed by polymerization of one monomer (e.g. the polypeptide A PEPTIDE composed of a formation of polyethene). Heteropolymers large number of amino-acid units. PRO- or copolymers come from two or more TEINS are polypeptides containing a few monomers (as for nylon). Heteropolymers hundred amino-acid units. may be of different types depending on the arrangement of units. An alternating polypropene (polypropylene) A syn- copolymer of two units A and B has an thetic polymer made from propene. It is arrangement: similar in properties to polyethene, but –A–B–A–B–A–B– stronger and lighter. The propene is poly- A block copolymer has an arrangement in merized by the Ziegler process. which blocks of one monomer alternate with blocks of the other; for example: polypropylene See polypropene. –A–A–A–B–B–B–A–A–A– In a graft copolymer there is a main choice polysaccharide A high-molecular-weight of one monomer (–A–A–A–A–), with short polymer of a monosaccharide (see sugar). side chains of the other monomer attached The polysaccharides contain many re- at regular intervals (–B–B–). peated units in their molecular structures. Stereospecific polymers have the sub- They can be broken down to smaller unit repeated along the chain in a regular polysaccharides, disaccharides, and mono-

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polystyrene

saccharides by hydrolysis or by the appro- of metalloporphyrins are conjugated to priate enzyme. Important polysaccharides proteins to form a number of very impor- are heparin, inulin, starch, glycogen (some- tant molecules, e.g. hemoglobin, myoglo- times known as animal starch), and cellu- bin, and the cytochromes. lose. See also carbohydrates; sugar. potentiometric titration A titration in polystyrene A synthetic polymer made which an electrode is used in the reaction from styrene (phenylethene). Expanded mixture. The end point can be found by polystyrene is a rigid foam used in packing monitoring the electrode potential of this and insulation. during the titration.

polytetrafluoroethene (PTFE) A syn- precipitate A suspension of small parti- thetic polymer made from tetrafluoro- cles of a solid in a liquid formed by a chem- ethene (i.e. CF2:CF2). It is able to withstand ical reaction. high temperatures without decomposing and also has a very low coefficient of fric- precursor A substance from which an- tion, hence its use in non-stick pans, bear- other substance is formed in a chemical re- ings, etc. action.

polythene See polyethene. Prelog, Vladimir (1906–98) Yugoslav- born Swiss organic chemist. The early part polyunsaturated Describing a com- of Prelog’s career was devoted to the struc- pound that has a number of C=C bonds in ture of the alkaloids. This resulted in the its compounds. determination of the structure of anti- malarial alkaloids. He also corrected the polyurethane A synthetic polymer con- formulae for strychnine alkaloids found by taining the group –NH–CO–O– linking the Sir Robert ROBINSON. Prelog subsequently monomers. Polyurethanes are made by became interested in the relation between condensation of isocyanates (–NCO) with stereochemistry and chemical reactivity of alcohols. ring molecules with about 10 members of the ring. He showed that specific stereo- polyvinyl acetate (PVA; polyvinyl chemistry has an important effect on the ethanoate) A thermoplastic polymer course of chemical reactions. Together made by the polymerization of vinyl with Sir Christopher INGOLD he found a ethanoate, CH2:CHOOCH3. It is used as a system for defining chirality of molecules. coating for paper and cloth and as an Prelog shared the 1975 Nobel Prize for adhesive. chemistry with Sir John CORNFORTH for his work on the ‘stereochemistry of organic polyvinyl chloride See polychloroethene. molecules and reactions’.

polyvinyl ethanoate See polyvinyl ac- pressure Symbol: p The pressure on a etate. surface due to forces from another surface or from a fluid is the force acting at 90° to p orbital See orbital. unit area of the surface: pressure = force/area porphyrin Any of several cyclic organic The unit is the pascal (Pa). structures that have the important characteristic property of forming complexes primary alcohol See alcohol. with metal ions. Examples of such metalloporphyrins are the iron porphyrins (e.g. primary amine See amine. heme in hemoglobin) and the magnesium porphyrin, chlorophyll, the photosynthetic primary standard A substance that can pigment in plants. In nature, the majority be used directly for the preparation of stan-

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propenoate

dard solutions without reference to some propanedioic acid (malonic acid; other concentration standard. Primary CH2(COOH)2) A white crystalline di- standards should be easy to purify, dry, ca- basic carboxylic acid. pable of preservation in a pure state, unaf- fected by air or CO2, of a high molecular propane-1,2,3-triol (glycerol; glycerine; weight (to reduce the significance of weigh- CH2(OH)CH(OH)CH2(OH)) A coloring errors), stoichiometric, and readily sol- less viscous liquid obtained as a by-product uble. Any likely impurities should be easily from the manufacture of soap by the reac- identifiable. tion of animal fats with sodium hydroxide. It is used as a solvent and plasticizer. See producer gas (air gas) A mixture of car- also glyceride. bon monoxide (25–30%), nitrogen (50– 55%), and hydrogen (10–15%), prepared propanoate A salt or ester of propanoic by passing air with a little steam through a acid. thick layer of white-hot coke in a furnace or ‘producer’. The air gas is used while still propanoic acid (propionic acid, hot to prevent heat loss and finds uses in C2H5COOH) A colorless liquid car- industrial heating, for example the firing of boxylic acid. retorts and in glass furnaces. Compare water gas. propanol Either of two alcohols: propan-1-ol (CH3CH2CH2OH) and propan-2-ol (CH CH (OH)CH ). Both are projection formula See formula. 3 2 3 colorless volatile flammable liquids. proline See amino acid. propanone (acetone; CH3COCH3)A colorless liquid ketone, used as a solvent promoter (activator) A substance that and in the manufacture of methyl 2- improves the efficiency of a catalyst. It does methyl-propanoate (from which poly- not itself catalyze the reaction but assists methylmethacrylate is produced). Pro- the catalytic activity. panone is manufactured from propene, either by the air-oxidation of propan-2-ol proof A measure of the ethanol content or as a by-product from the CUMENE of intoxicating drinks. In the United States, PROCESS. proof spirit contains 40% ethanol by volume. In the UK, it contains 49.28% of propenal (acrolein; CH2:CHCHO) A ethanol by weight (57.1% by volume at colorless liquid unsaturated aldehyde with ° 16 C). The degree of proof gives the num- a pungent odor. It can be polymerized to ber of parts of proof spirit per 100 parts of make acrylic resins. the total. 100° of proof is 50% by volume, 80° of proof is 0.8 × 50%, etc. propene (propylene; C3H6) A gaseous alkene. Propene is not normally present in propanal (propionaldehyde, C2H5CHO) the gaseous crude-oil fraction but can be A colorless liquid aldehyde. obtained from heavier fractions by catalytic cracking. This is the principal indus- propane (C3H8) A gaseous alkane ob- trial source. Propene is the organic starting tained either from the gaseous fraction of material for the production of propan-2-ol, crude oil or by the cracking of heavier frac- required for the manufacture of propanone tions. The principal use of propane is as a (acetone), and the starting material for the fuel for heating and cooking, since it can be production of polypropene (polypropy- liquefied under pressure, stored in cylin- lene). ders, and transported easily. Propane is the third member of the homologous series of propenoate A salt or ester of propenoic alkanes. acid.

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propenoic acid

propenoic acid (acrylic acid, CH2:CH- The primary structure of a protein is the COOH) An unsaturated liquid car- particular sequence of amino acids present. boxylic acid with a pungent odor. The acid The secondary structure is the way in and its esters are used to make acrylic which this chain is arranged; for example, resins. See acrylic resin. coiled in an ALPHA HELIX or held in beta pleated sheets. The secondary structure is propenonitrile (acrylonitrile, H2C:- held by hydrogen bonds. The TERTIARY CH(CN)) An organic compound from STRUCTURE of the protein is the way in which acrylic-type polymers are produced. which the polypeptide chain is folded into a three-dimensional structure. This may be propenyl group (allyl group) The or- held by cystine bonds and by attractive ganic group, CH2=CH–CH2–, derived by forces between atoms. A protein must have removing one hydrogen atom from the correct tertiary structure in order to propene. function properly and abnormally folded proteins can cause disease, e.g. prion propionaldehyde See propanal. diseases, Alzheimer’s. QUATERNARY STRUC- TURE is the interaction between polypep- propionic acid See propanoic acid. tide chains (or subunits).

propylene See propene. proteoglycan (mucoprotein) A type of glycoprotein consisting of long branched propyl group The group CH3CH2CH2–. heterogeneous chains of glycosaminoglycan molecules linked to a protein core of prosthetic group The nonprotein com- amino acids. Unlike more typical glyco- ponent of a conjugated protein. Thus the proteins, they have a greater carbohy- heme group in hemoglobin is an example drate content, the protein core is rich in of a prosthetic group, as are the coenzyme serine, and they have a higher molecular components of a wide range of enzymes. weight.

protamine One of a group of polypep- protic acid See acid. tides formed from a few amino acids. They are soluble in water, dilute acids, and proton An elementary particle with a bases. On heating they do not coagulate. positive charge (+1.602 192 × 10–19 C) and When protamines are hydrolyzed they rest mass 1.672 614 × 10–27 kg. Protons yield a large proportion of basic amino are nucleons, found in all nuclides. acids, particularly arginine, alanine, and serine. They occur in the sperm of verte- proton number (atomic number) Symbol: brates, packing the DNA into a condensed Z The number of protons in the nucleus form. of an atom. The proton number determines the chemical properties of the element be- protein One of a large number of sub- cause the electron structure, which deter- stances that are important in the structure mines chemical bonding, depends on the and function of all living organisms. Pro- electrostatic attraction to the positively teins are polypeptides; i.e. they are made charged nucleus. up of AMINO ACID molecules joined together by peptide links. Their molecular prussic acid See hydrocyanic acid. weight may vary from a few thousand to several million. About 20 amino acids are pseudoaromatic See aromatic com- present in proteins. Simple proteins con- pound. tain only amino acids. In conjugated proteins, the amino acids are joined to other pseudo-first order Describing a reac- groups. Proteins may consist of one or sev- tion that appears to exhibit first-order ki- eral polypeptide chains. netics under special conditions, even

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pyrolysis

though the ‘true’ order is greater than one. pyrene (C16H10) A solid aromatic com- For example, in the hydrolysis of an ester pound whose molecules consist of four in the presence of a large volume of water, benzene rings joined together. It is carcino- the concentration of water remains ap- genic. proximately constant. The rate of reaction is thus found experimentally to be proportional to the concentration of the ester only (even though it also depends on the amount of water present). Such a reaction is described as ‘bimolecular of the first order’. N pteroylglutamic acid See folic acid. Pyridine PTFE See polytetrafluoroethene. pyridine (C5H5N) An organic liquid of formula C5H5N. The molecules have a N hexagonal planar ring and are isoelectronic N1 7 with benzene. Pyridine is an example of an 3 9 aromatic heterocyclic compound, with the N N H electrons in the carbon–carbon pi bonds and the lone pair of the nitrogen delocal- Purine ized over the ring of atoms. The compound is extracted from coal tar and used as a sol- purine A simple nitrogenous organic vent and as a raw material for organic syn- molecule with a double ring structure. thesis. Members of the purine group include adenine and guanine, which are con- pyridoxine (vitamin B6) One of the stituents of the nucleic acids, and certain water-soluble B-group of vitamins. Good plant alkaloids, e.g. caffeine and theo- sources include yeast and certain seeds (e.g. bromine. wheat and corn), liver, and to a limited extent, milk, eggs, and leafy green vegetables. putrescine An amine, H2N[CH2]4NH2, There is also some bacterial synthesis of the produced from ornithine in decaying meat vitamin in the intestine. Pyridoxine gives or fish. rise to a coenzyme involved in various as- pects of amino acid metabolism. See also PVA See polyvinyl acetate. vitamin B complex.

PVC See polychloroethene. N pyranose A sugar that has a six-membered ring form (five carbon atoms and one N oxygen atom). See also sugar. Pyrimidine

pyrazine (1,4-diazine; C4H4N2) A heterocyclic aromatic compound with a six- pyrimidine A simple nitrogenous or- membered ring containing four carbon ganic molecule whose ring structure is con- atoms and two nitrogen atoms. tained in the pyrimidine bases cytosine, thymine, and uracil, which are constituents pyrazole (1,2-diazole; C3H4N2) A hete- of the nucleic acids, and in thiamine (vita- rocyclic crystalline aromatic compound min B1). with a five-membered ring containing three carbon atoms and two nitrogen atoms. pyrolysis The decomposition of chemi-

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pyrometer

cal compounds by subjecting them to very uid aromatic compound with a five-mem- high temperature. bered ring containing four carbon atoms and one nitrogen atom. It has important pyrometer An instrument used in the biochemical derivatives, including chloro- chemical industry to measure high temper- phyll and heme. ature, e.g. in reactor vessels. pyruvate An intermediate in several pyrone (C H O ) A compound having a 5 4 2 metaoblic pathways, including glycolysis six-membered ring of five carbon atoms and gluconeogenesis, with the formula and one oxygen, with one of the carbon – atoms attached to a second oxygen in a CH3COCOO . Pyruvate is also the precur- carbonyl group. The pyrone ring system sor for the synthesis of the amino acids ala- has two forms depending on the position of nine, valine, and leucine. the carbonyl relative to the oxygen hetero atom. It occurs in many natural products. pyruvic acid A carboxylic acid, CH3- COCOOH. The systematic name is 2- pyrrhole ((CH)4NH) A heterocyclic liq- oxopropanoic acid.

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qualitative analysis Analysis carried quantum number An integer or half in- out with the purpose of identifying the teger that specifies the value of a quantized components of a sample. Classical meth- physical quantity (energy, angular momen- ods involved simple preliminary tests fol- tum, etc.). See atom. lowed by a carefully devised scheme of systematic tests and procedures. Modern quantum states States of an atom, elec- methods include the use of a range of spec- tron, particle, etc., specified by a unique set troscopic techniques. Compare quantita- of quantum numbers. For example, the hy- tive analysis. drogen atom in its ground state has an electron in the K shell specified by the four quanta See quantum. quantum numbers: n = 1, l = 0, m = 0, ms = ½. In the helium atom there are two elec- quantitative analysis Analysis carried trons: ½ out with the purpose of determining the n = 1, l = 0, m = 0, ms = ½ concentration of one or more components n = 1, l = 0, m = 0, ms = – of a sample. Classical wet methods include volumetric and gravimetric analysis. A quantum theory A mathematical theory originally introduced by the Ger- wide range of instrumental techniques are man physicist Max Planck (1848–1947) to also used, including polarography and var- explain the radiation emitted from hot ious types of chromatography and spec- bodies. Quantum theory is based on the troscopy. Compare qualitative analysis. idea that energy (or certain other physical quantities) can be changed only in certain quantized Describing a physical quan- discrete amounts for a given system. Other tity that can take only certain discrete val- early applications were the explanations of ues, and not a continuous range of values. the photoelectric effect and the Bohr Thus, in an atom or molecule the electrons theory of the atom. around the nucleus can have certain ener- Quantum mechanics is a system of me- gies, E1, E2, etc., and cannot have interme- chanics that developed from quantum diate values. Similarly, in atoms and theory and is used to explain the behavior molecules, the electrons have quantized of atoms, molecules, etc. In one form it is values of spin angular momentum and or- based on de Broglie’s idea that particles can bital angular momentum. have wavelike properties – this branch of quantum mechanics is called wave me- quantum (plural quanta) A definite chanics. See orbital. amount of energy released or absorbed in a process. Energy often behaves as if it were quantum yield The number of reactive ‘quantized’ in this way. The quantum of events per absorbed photon in a photo- electromagnetic radiation is the photon. chemical reaction.

quantum mechanics See quantum quaternary ammonium compound A theory. compound formed from an amine by addi-

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quaternary structure

tion of a proton to produce a positive ion. quinol See benzene-1,4-diol. Quaternary compounds are salts, the simplest example being ammonium com- quinoline (C9H7N) A colorless two-ring pounds formed from ammonia and an heterocyclic compound with an unpleasant acid, for example: odor, which acts as a base and forms salts → + – NH3 + HCl NH4 Cl with acids. First made from the alkaloid Other amines can also add protons to give quinine, it is found in bone oil and coal analogous compounds. For instance, tar and used for making drugs and dye- methylamine (CH3NH2) forms the com- stuffs. pound + – [CH3NH3] X quinone (cyclohexadiene-1,4-dione; ben- where X– is an acid radical. zoquinone; C6H4O2) A yellow crystalline The formation of quarternary com- organic compound with a pungent odor. pounds occurs because the lone pair on the Its molecules contain a nonaromatic six- nitrogen atom can form a coordinate bond carbon ring and it behaves as an unsatu- with a proton. This can also occur with rated diketone with conjugated double heterogeneous nitrogen compounds, such as adenine, cytosine, thymine, and gua- bonds. It is used in making dyestuffs. The nine. Such compounds are known as ni- systematic name is cyclohexadiene-1,4- trogenous bases. See also amine salt. dione. A platinum electrode in an equi- molar solution of quinone and hydro- quaternary structure See protein. quinone (benzene-1,4-diol; C6H4(OH)2) is used as a standard electrode in electro- quinhydrone See quinone. chemistry. The reaction is: ˆ C6H4(OH)2 C6H4O2 + + quinine A poisonous ALKALOID found in 2H + 2e the bark of the cinchona tree of South This type of electrode is called a quinhy- America. It was used in treating malaria. drone electrode.

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racemate See optical activity. three most important forms of radiation that occur. The process by which one nu- racemic mixture See optical activity. clide changes to another is radioactive decay. racemization The conversion of an optical isomer into an equal mixture of enan- radiocarbon dating See carbon dating. tiomers, which is not optically active. See optical activity. radiochemistry The chemistry of radioactive isotopes of elements. Radiochem- rad A unit of absorbed dose of ionizing istry involves such topics as the radiation, defined as being equivalent to an preparation of radioactive compounds, the –2 absorption of 10 joule of energy in one separation of isotopes by chemical reac- kilogram of material. tions, the use of radioactive LABELS in studies of mechanisms, and experiments on the radian Symbol: rad The SI unit of plane chemical reactions and compounds of π angle; 2 radian is one complete revolution transuranic elements. (360°). radiogenic Caused by radioactive radiation In general the emission of en- decay. ergy from a source, either as waves (light, sound, etc.) or as moving particles (beta radioisotope A radioactive isotope of rays or alpha rays). an element. Tritium, for instance, is a radioisotope of hydrogen. Radioisotopes are radical 1. See free radical. 2. A group of atoms in a molecule. See also extensively used in research as souces of ra- functional group. diation and as tracers in studies of chemical reactions. Thus, if an atom in a radioactive Describing an element or compound is replaced by a radioactive nu- nuclide that exhibits natural radioactivity. clide of the element (a label) it is possible to follow the course of the chemical reaction. radioactive dating (radiometric dating) Radioisotopes are also used in medicine for A technique for dating archaeological spec- diagnosis and treatment. imens, rocks, etc., by measuring the extent to which some radionuclide has decayed to radiolysis A chemical reaction pro- give a product. duced by high-energy radiation (x-rays, gamma rays, or particles). radioactive decay See radioactivity. radiometric dating See radioactive dat- radioactivity The disintegration of cer- ing. tain unstable nuclides with emission of radiation. The emission of alpha particles, radio waves A form of electromagnetic beta particles, and gamma rays are the radiation with wavelengths greater than a

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raffinate

few millimeters. See also electromagnetic mole fraction of the solvent in the solution radiation. (X) and that the proportionality constant is the vapor pressure of pure solvent, (p0), at raffinate The liquid remaining after the the given temperature: i.e. p = p0X. Solu- solvent extraction of a dissolved substance. tions that obey Raoult’s law are said to be See solvent extraction. ideal. There are some binary solutions for which Raoult’s law holds over all values of raffinose A SUGAR occurring in sugar X for either component. Such solutions are beet. It is a trisaccharide consisting of fruc- said to be perfect and this behavior occurs tose, galactose, and glucose units. when the intermolecular attraction between molecules within one component is r.a.m. See relative atomic mass. almost identical to the attraction of molecules of one component for molecules of Raman effect A change in the fre- the other (e.g. chlorobenzene and bro- quency of electromagnetic radiation, such mobenzene). Because of solvation forces as light, which occurs when a photon of ra- this behavior is rare and in general Raoult’s diation undergoes an inelastic collision law holds only for dilute solutions. with a molecule. This type of scattering is For solutions that are ideal but not per- called Raman scattering, in contrast to fect the solute behavior is similar in that normal (Rayleigh) scattering. The intensity the partial pressure of the solute, ps, is proof Raman scattering is much smaller portional to the mole fraction of the solute, (about 1/1000) than the intensity of Xs, but in this case the proportionality con- Rayleigh scattering. The Raman effect was stant, p′, is not the vapor pressure of the first observed by the Indian physicists Sir pure solute but must be determined exper- Chandrasekhara Venkata Raman (1888– imentally for each system. This solute 1970) and his colleague Sir Karia- equivalent of Raoult’s law has the form ′ manikkam Srinivasa Krishnan in 1928, ps = p Xs, and is called HENRY’S LAW. Be- having been predicted theoretically on the cause of intermolecular attractions p′ is basis of quantum mechanics by Hendrik usually less than p0. It is named for the Anton Kramers and Werner Heisenberg in French chemist François-Marie Raoult 1925. The Raman effect has been used ex- (1830–1901), who formulated it in 1882. tensively in Raman spectroscopy for the determination of molecular structure, par- Rashig process A method for the man- ticularly since then advent of the laser. In ufacture of chlorobenzene, and thence some cases it is possible to draw conclu- phenol, from benzene. Benzene vapor, hy- sions about molecular structure by observ- drogen chloride, and air are passed over a ing whether certain lines are present or copper(II) chloride catalyst (230°C): → absent. 2C6H6 + 2HCl + O2 2C6H5Cl + 2H2O Raney nickel A catalytic form of nickel The chlorobenzene is converted to phenol produced by treating a nickel–aluminum by reaction with water over a silicon cata- alloy with caustic soda. The aluminum dis- lyst (425°C): → solves (as aluminate) and Raney nickel is C6H5Cl + H2O HCl + C6H5OH left as a spongy mass, which is pyrophoric It is named for the German chemist Fritz when dry. It is used especially for catalyz- Rashig (1863–1928). ing hydrogenation reactions. It was discovered by the US chemist M. Raney in 1927. rate constant (velocity constant; specific reaction rate) Symbol: k The constant of Raoult’s law A relationship between proportionality in the rate expression for a the pressure exerted by the vapor of a solu- chemical reaction. For example, in a reaction and the presence of a solute. It states tion A + B → C, the rate may be propor- that the partial vapor pressure of a solvent tional to the concentration of A multiplied above a solution (p) is proportional to the by that of B; i.e.

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rectified spirit

rate = k[A][B] industry it may be simple (such as nitrogen where k is the rate constant for this partic- from air used to make ammonia) or com- ular reaction. The constant is independent plex (such as coal and petroleum, used to of the concentrations of the reactants but make a wide range of products). depends on temperature; consequently the temperature at which k is recorded must be rayon An artificial fiber formed from stated. The units of k vary depending on wood pulp (cellulose). There are two types. the number of terms in the rate expression, Viscose rayon is made by dissolving the cel- but are easily determined remembering lulose in carbon disulfide and sodium hy- that rate has the units s–1. droxide. The solution is forced through a fine nozzle into an acid bath, which regen- rate-determining step (limiting step) erates the fibers. Acetate rayon is made by The slowest step in a multistep reaction. dissolving cellulose acetate in an organic Many chemical reactions are made up of a solvent, and forcing the solution through a number of steps in which the one with the nozzle. The solvent is evaporated, and the lowest rate is the one that determines the cellulose acetate thus obtained as fibers. rate of the overall process. The overall rate of a reaction cannot ex- reactant A compound taking part in a ceed the rate of the slowest step. For exam- CHEMICAL REACTION. ple, the first step in the reaction between acidified potassium iodide solution and hy- reaction See chemical reaction. drogen peroxide is the rate-determining step: reactive dye A dye that sticks to the H O + I– → H O + OI– (slow) 2 2 2 fibers of a fabric by forming covalent H+ + OI– → HOI (fast) chemical bonds with the substance of the HOI + H+ + I– → I + H O (fast) 2 2 fabric. The dyes used to color the cellulose fibers in rayon are examples of reactive rate of reaction A measure of the dyes. amount of reactant consumed in a chemical reaction in unit time. It is thus a meas- A compound that reacts with ure of the number of effective collisions reagent between reactant molecules. The rate at another (the substrate). The term is often which a reaction proceeds can be measured also used for common laboratory chemi- by the rate the reactants disappear or by cals – sodium hydroxide, hydrochloric the rate at which the products are formed. acid, etc. – used for experiment and analy- The principal factors affecting the rate of sis. reaction are temperature, pressure, concentration of reactants, light, and the action of rearrangement A reaction in which the a catalyst. The units usually used to meas- groups of a compound rearrange them- ure the rate of a reaction are mol dm–3 s–1. selves to form a different compound. An See also mass action, law of. example is the BECKMANN REARRANGE- MENT. rationalized units A system of units in which the equations have a logical form re- reciprocal proportions, law of See lated to the shape of the system. SI units equivalent proportions, law of. form a rationalized system of units. In it formulae concerned with circular symme- recrystallization The repeated crystal- try contain a factor of 2π; those concerned lization of a compound to ensure purity of with radial symmetry contain a factor of the sample. 4π. rectified spirit A constant-boiling mix- raw material A substance from which ture of ethanol and water that contains other substances are made. In the chemical about 6% water; no more water can be re-

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redox

moved by further distillation. It is used as liquid can be maintained at its boiling an industrial solvent. point for long periods of time, without loss. The technique is a standard method of redox Relating to the process of oxida- carrying out reactions in organic chem- tion and reduction, which are intimately istry. connected in that during oxidation by chemical agents the oxidizing agent itself reforming The cyclization of straight- becomes reduced, and vice versa. Thus an chain hydrocarbons from crude oil by oxidation process is always accompanied heating under pressure with a catalyst, usu- by a reduction process. In electrochemical ally platinum on alumina. For example, the processes this is equally true, oxidation manufacture of methylbenzene from hep- taking place at the anode and reduction at tane: → the cathode. These systems are often called C7H16 C6H11CH3 redox systems, particularly when the inter- This first step is the production of methyl est centers on both compounds. cyclohexane, which then loses six hydro- Oxidizing and reducing power is in- gen atoms to give methylbenzene: → dicated quantitatively by the redox poten- C6H11CH3 C6H5CH3 + 3H2 tial or standard electrode potential, EŠ. See also steam reforming. Redox potentials are normally expressed as reduction potentials. They are obtained Regnault’s method A method used for by electrochemical measurements and the the determination of the density of gases. A + values are referred to the H /H2 couple for bulb of known volume is evacuated and which EŠ is set equal to zero. Thus in- weighed then the gas is admitted at a creasingly negative potentials indicate in- known pressure (from a vacuum line) and creasing ease of oxidation or difficulty of the bulb weighed again. The temperature is reduction. Thus in a redox reaction the half also noted and the data corrected to STP. reaction with the most positive value of EŠ The method is readily applicable to the de- is the reduction half and the half reaction termination of approximate relative mo- with the least value of EŠ (or most highly lecular masses of gaseous samples. It is negative) becomes the oxidation half. named for the French chemist Henri Victor Regnault (1810–78). reducing agent See reduction. relative atomic mass (r.a.m.) Symbol: Ar reduction The gain of electrons by The ratio of the average mass per atom of atoms, molecules, ions, etc. It often in- the naturally occurring element to 1/12 of volves the loss of oxygen from a com- the mass of an atom of nuclide 12C. It was pound, or addition of hydrogen. Reduction formerly called atomic weight can be effected chemically, i.e. by the use of reducing agents (electron donors), or elec- relative density Symbol: d The ratio of trically, in which case the reduction the density of a given substance to the den- process occurs at the cathode. See also sity of some reference substance. The rela- redox. tive densities of liquids are usually measured with reference to the density of refining The process of removing impu- water at 4°C. Relative densities are also rities from a substance or of extracting a specified for gases; usually with respect to substance from a mixture. The refining of air at STP. The temperature of the sub- PETROLEUM is a particularly important instance is stated or is understood to be dustrial process. 20°C. Relative density was formerly called specific gravity. refluxing The process of boiling a liquid in a vessel connected to a condenser, so relative molecular mass Symbol: Mr that the condensed liquid runs back into The ratio of the average mass per molecule the vessel. By using a reflux condenser, the of the naturally occurring form of an el-

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respiration

ement or compound to 1/12 of the mass of resonance (mesomerism) The behavior an atom of nuclide 12C. This was formerly of many compounds cannot be adequately called molecular weight. It does not have to explained by a single structure using simple be used only for compounds that have dis- single and double bonds. The bonding elec- crete molecules; for ionic compounds (e.g. trons of the compound have a different dis- NaCl) and giant-molecular structures (e.g. tribution in the molecules. The actual BN) the formula unit is used. bonding in the molecule can be regarded as a hybrid of two or more conventional relaxation The process by which an ex- forms of the molecule, called resonance cited species loses energy and falls to a forms or canonical forms. The result is a lower energy level (such as the ground resonance hybrid. For example, the car- state). bonyl group in a ketone has negative charge on the oxygen atom. It can be de- rem (radiation equivalent man) A unit scribed as a resonance hybrid, somewhere for measuring the effects of radiation dose between =C=O, in which a pair of electrons on the human body. One rem is equivalent is shared between the C and the O, and + – to an average adult male absorbing one rad =C –O , in which the electrons are local- of radiation. The biological effects depend ized on the O atom. Note that the two on the type of radiation as well as the en- canonical forms do not contribute equally ergy deposited per kilogram. in the hybrid. The bonding of benzene can be represented by a resonance hybrid of resin A yellowish insoluble organic com- two Kekulé structures and, to a lesser ex- pound exuded by trees as a viscous liquid tent, three Dewar structures. It is conven- that gradually hardens on exposure to air tional to represent a resonance hybrid by two or more conventional structures joined to form a brittle amorphous solid. Syn- by double-headed arrows, as in: thetic resins are artificial polymers used in R C=O ↔ R C+O– making adhesives, insulators, and paints. 2 2 The double-headed arrow should not be See also rosin. confused with the equilibrium symbol (ˆ). The two forms are not in equilibrium resolution The separation of a racemate but represent different classical structures into the two optical isomers. This cannot that contribute to the actual structure. be done by normal methods, such as crystallization or distillation, because the iso- resonance ionization spectroscopy mers have identical physical properties. (RIS) A type of spectroscopy that detects The main methods are: specific types of atoms using lasers. The 1. Mechanical separation. Certain opti- laser ionizes the atoms of interest. The fre- cally active compounds form crystals with quency of the laser is chosen so that only distinct left- and right-handed shapes. The the atoms of interest in a sample are ex- crystals can be sorted by hand. cited by the laser. This method is very se- 2. Chemical separation. The mixture is re- lective because ionization only occurs for acted with an optical isomer. The products those atoms those whose energy levels fit in are then not optical isomers of each other, with the frequency of the laser light. This and can be separated by physical means. selectivity has led to many practical uses For instance, a mixture of D- and L-forms for this technique. of an acid, acting with a pure L-base, produces two salts that can be separated by resorcinol See benzene-1,3-diol. fractional crystallization and then reconverted into the acids. respiration The oxidation of organic 3. Biochemical separation. Certain organic molecules to provide energy in plants and compounds can be separated by using bac- animals. In animals, food molecules are teria that feed on one form only, leaving respired, but autotrophic plants respire the other. molecules that they have themselves syn-

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respiratory chain

thesized by photosynthesis. The energy retinene See retinal. from respiration is used to attach a high- energy phosphate group to ADP to form retinol See vitamin A. the short-term energy carrier ATP, which can then be used to power energy-requiring retort A piece of laboratory apparatus processes within the cell. The actual chem- consisting of a glass bulb with a long nar- ical reactions of respiration are known as row neck. In industrial chemistry, various internal (cell or tissue) respiration and they metallic vessels in which distillations or re- normally require oxygen from the environ- actions take place are called retorts. ment (aerobic respiration). Some organisms are able to respire, at least for a short retrosynthetic analysis A technique period, without the use of oxygen (an- for planning the synthesis of an organic aerobic respiration), although this process molecule. The structure of the molecule is produces far less energy than aerobic respi- considered and it is divided into imaginary ration, e.g. 38 molecules of ATP are gener- parts, which could combine by known re- ated for each molecule of glucose oxidized actions. These disconnections of the mol- in aerobic respiration, compared with only ecule suggest charged fragments, known as 2 in anerobic respiration. Respiration usu- synthons, which give a guide to possible reagents for the synthesis. ally involves an exchange of gases with the environment; this is known as external res- reversible change In thermodynamics, piration. In small animals and all plants ex- a change in the pressure, volume, or other change by diffusion is adequate, but larger properties of a system, in which the system animals generally have special respiratory remains at equilibrium throughout the organs with large moist and ventilated sur- change. Such processes could be reversed; faces (e.g. lungs, gills) and there is often a i.e. returned to the original starting posi- circulatory system to transport gases inter- tion through the same series of stages. They nally to and from the respiratory organs. are never realized in practice. An isother- See also electron-transport chain; glycoly- mal reversible compression of a gas, for ex- sis; Krebs cycle. ample, would have to be carried out infinitely slowly and involve no friction, respiratory chain The electron-trans- etc. Ideal energy transfer would have to port chain in aerobic respiration. take place between the gas and the surroundings to maintain a constant tempera- respiratory pigments Colored com- ture. pounds that can combine reversibly with In practice, all real processes are irre- oxygen. HEMOGLOBIN is the blood pigment versible changes in which there is not an in all vertebrates and a wide range of in- equilibrium throughout the change. In an vertebrates. Other blood pigments, such as irreversible change, the system can still be haemoerythrin (containing iron) and he- returned to its original state, but not mocyanin (containing copper), are found through the same series of stages. For a in lower animals, and in many cases are closed system, there is always an entropy dissolved in the plasma rather than present increase involved in an irreversible change. in cells. Their affinity for oxygen is compa- rable with hemoglobin, though oxygen ca- reversible reaction A chemical reaction pacity is generally lower. that can proceed in either direction. An example is the reaction of an acid with an al- retinal (retinene) An aldehyde derivative cohol to form an ester and water: of retinol (vitamin A). Retinal is a con- R1COOH + R2OH ˆ R1COOR2 + stituent of the light-sensitive conjugated H2O protein, rhodopsin, which occurs in the In general, there will be an equilibrium rod cells of the retina. See rhodopsin. mixture of reactants and products.

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rotary dryers

RF value The distance traveled by a volved in protein synthesis. It is a single given component divided by the distance polynucleotide chain similar in composi- travelled by the solvent front in chro- tion to a single strand of DNA except that matography. the sugar ribose replaces deoxyribose and the pyrimidine base uracil replaces rheology The study of the ways in thymine. RNA is synthesized on DNA in which matter can flow. This topic is of par- the nucleus and exists in three forms. In ticular interest in the study of polymers. certain viruses, RNA is the genetic material. riboflavin (vitamin B2) One of the water-soluble B-group of vitamins. It is Robinson, Sir Robert (1886–1975) found in cereal grains, peas, beans, liver, British organic chemist. Sir Robert Robin- kidney, and milk. Riboflavin is a con- son made many important contributions to stituent of several enzyme systems (flavo- experimental and theoretical organic proteins), acting as a coenzyme for chemistry. His experimental work con- hydrogen transfer in the reactions cat- cerned plant products, particularly the al- alyzed by these enzymes. Two forms of kaloids. Robinson won the 1947 Nobel phosphorylated riboflavin are known to Prize for chemistry for this work. His theo- exist in various enzyme systems: FMN retic work concerned the role of electrons (flavin mononucleotide) and FAD (flavin in organic reactions. He originated the rep- adenine dinucleotide). See also vitamin B resentation of electronic transfer by curly complex. arrows. In particular, he applied these ideas for benzene and its chemical reac- ribonucleic acid See RNA. tions using concepts such as electrophilic reagents and nucleophilic reagents. ribose (C5H10O5) A monosaccharide SUGAR. It rarely occurs naturally in the free roentgen Symbol: R A unit of radiation, state but ribose is an important component used for x-rays and gamma rays, defined in of RNA. The derivative compound, de- terms of the ionizing effect on air. One oxyribose, is a component of DNA. roentgen induces 2.58 × 10–4 coulomb of ring A closed loop of atoms in a mol- charge in one kilogram of dry air. The unit ecule, as in benzene or cyclohexane. A is named for the German physicist W. K. fused ring is one joined to another ring in Roentgen (1845–1923). such a way that they share two atoms. Naphthalene is an example of a fused-ring rosin A brittle yellow or brown resin compound. that remains after the distillation of turpentine. It is used as a flux in soldering and ring closure A reaction in which one in making paints and varnishes. Powdered part of an open chain in a molecule reacts rosin gives a ‘grip’ to violin bows and box- with another part, so that a ring of atoms ers’ shoes. See resin. is formed. For example, an amino group on one end of a long molecule can react with rotamer See conformation. a carboxyl group on the other end to form a cyclic amide containing the –NH.CO– rotary dryers Devices commonly used group (see lactam). Another example of in the chemical industry for the drying, ring closure is the conversion of the mixing, and sintering of solids. They con- straight-chain form of a SUGAR into the sist essentially of a rotating inclined cylin- cyclic form. There are many examples of der, which is longer in length than in ring-closure reactions in organic chemistry. diameter. Gases flow through the cylinder in either a countercurrent or cocurrent di- RNA (ribonucleic acid) A nucleic acid rection to regulate the flow of solids, which found mainly in the cytoplasm and in- are fed into the end of the cylinder. Rotary

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R-S convention

dryers can be applied to both batch and elastic material. Natural rubber is a poly- continuous processes. mer of methylbuta-1,3-diene (isoprene). Various synthetic rubbers are made by R-S convention See optical activity. polymerization; for example CHLOROPRENE rubber (from 2-chlorobuta-1,3-diene) and rubber A natural or synthetic polymeric silicone rubbers. See also vulcanization.

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Sabatier, Paul (1854–1941) French or- temperature to about 1500°C. Under these ganic chemist. Starting in 1897, Sabatier conditions, the rest of the methane is con- performed experiments in which he studied verted to ethyne and hydrogen: → the hydrogenation of ethene when it is 2CH4 C2H2 + 3H2 passed over nickel. When nickel is finely di- The process is important because it provided it catalyses this reaction. It also vides a source of ethyne from readily avail- catalyses other hydrogenation reactions able natural gas, thus avoiding the such as the conversion of benzene into cy- expensive carbide process. clohexane. Sabatier explained his results in terms of chemisorption of unstable com- salicylate (hydroxybenzoate) A salt or pounds onto the surface of a catalyst and ester of salicylic acid. produced evidence for this idea. He dis- cussed his findings in the book Catalysis in salicylic acid (2-hydroxybenzoic acid; Organic Chemistry (1912). Sabatier shared C6H4(OH)COOH) A crystalline aro- the 1912 Nobel Prize with François GRIG- matic carboxylic acid. It is used in medi- NARD for his work on catalysis. cines, as an antiseptic, and in the manufacture of azo dyes. Its ethanoyl Sabatier–Senderens process A method (acetyl) ester is aspirin. See also aspirin; of hydrogenating unsaturated vegetable methyl salicylate. oils to make margarine, using hydrogen and a nickel catalyst. It is named for the Sandmeyer reaction A method of pro- French chemists Paul Sabatier and Jean- ducing chloro- and bromo-substituted de- Baptiste Senderens (1856–1937). See also rivatives of aromatic compounds by using hardening. the DIAZONIUM SALT with a copper halide. The reaction starts with an amine, which is saccharide See sugar. diazotized at low temperature by using hydrochloric acid and sodium nitrite (to pro- saccharin (C7H5NO3S) A white crystalline organic compound used as an artifi- duce nitrous acid, HNO2). For example: → cial sweetener; it is about 550 times as C6H5NH2 + NaNO2 + HCl + – – + sweet as sugar (sucrose). It is nearly insolu- C6H5N2 + Cl + OH + Na + H2O ble in water and so generally used in the The copper halide (e.g. CuCl) acts as a cat- form of its sodium salt. Possible links with alyst to give the substituted benzene deriv- cancer in animals has restricted its use in ative: + – → some countries. C6H5N2 + Cl C6H5Cl + N2 This reaction was discovered by the Ger- saccharose See sucrose. man chemist Traugott Sandmeyer (1854– 1922) in 1884. A variation of the reaction, Sachse reaction A process for the man- in which the catalyst is freshly precipitated ufacture of ethyne (acetylene) from natural copper powder, was reported in 1890 by gas (methane). Part of the methane is the German chemist Ludwig Gatterman burned in two stages to raise the furnace (1860–1920). This is known as the Gatter-

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sandwich compound

man reaction (or Gatterman–Sandmeyer tions but not addition reactions since each reaction). atom in the structure will already have its maximum possible number of single sandwich compound A type of com- bonds. Compare unsaturated compound. plex formed between transition-metal ions and aromatic compounds, in which the saturated solution A solution that con- metal ion is ‘sandwiched’ between the tains the maximum equilibrium amount of rings. The bonding is between the d or- solute at a given temperature. A solution is bitals of the metal and the pi electrons on saturated if it is in equilibrium with its the ring. Four-, five-, six-, seven-, and solute. If a saturated solution of a solid is eight-membered rings are known to com- cooled slowly, the solid may stay tem- plex with a number of elements including porarily in solution; i.e. the solution may V, Cr, Mn, Co, Ni, and Fe. Ferrocene contain more than the equilibrium amount (Fe(C5H5)2) is the best-known example. of solute. Such solutions are said to be Compounds of this type are also known as supersaturated. metallocenes. saturated vapor A vapor that is in equi- Sanger, Frederick (1918– ) British librium with the solid or liquid. A satu- biochemist. Sanger is one of the very few rated vapor is at the maximum pressure people to win two Nobel Prizes. He was (the saturated vapor pressure) at a given awarded the 1958 Nobel Prize for chem- temperature. If the temperature of a satu- istry for determining the sequence of rated vapor is lowered, the vapor con- amino acids in the insulin of cows. This denses. Under certain circumstances, the work started in 1943. By 1945 he had dis- substance may stay temporarily in the covered a compound now known as vapor phase; i.e. the vapor contains more Sanger’s reagent which attaches itself to than the equilibrium concentration of the the amino acids, and breaks up the protein substance. The vapor is then said to be chain. After the discovery of the structure supersaturated. of DNA in 1953 Sanger and his colleagues started to investigate the sequence of the scavenger A compound or chemical nucleotides in DNA. Sanger and his col- species that removes a trace component leagues developed techniques for splitting from a system or that removes a reactive up the DNA. In 1977 they announced the intermediate from a reaction. complete sequence of more than 5000 nucleotides of the DNA of a bacterial virus. Schiff’s base A type of compound This resulted in Sanger sharing the 1980 formed by reacting an aldehyde or ketone Nobel Prize for chemistry with Paul Berg (e.g. RCOR) with an aryl amine (e.g. and Walter Gilbert. ArNH2). The product, an N-arylimide, which is usually crystalline, has the for- saponification The process of hy- mula R2C:NAr. Schiff’s bases were for- drolyzing an ester with a hydroxide. The merly used to identify aldehydes and carboxylic acid forms a salt. For instance, ketones (by forming the crystalline base with sodium hydroxide: and measuring its melting point). They are R1COOR2 + NaOH → NaOOCR1 + named after the German organic chemist R2OH Hugo Schiff (1834–1915). The saponification of fats, which are esters of propane-1,2,3-triol (glycerol) with long- Schiff’s reagent An aqueous solution of chain carboxylic acids, forms soaps. magenta dye decolorized by reduction with sulfur(IV) oxide (sulfur dioxide; SO2). It is saturated compound An organic com- a test for aldehydes and ketones. Aliphatic pound that does not contain any double or aldehydes restore the color quickly; triple bonds in its structure. A saturated aliphatic ketones and aromatic aldehydes compound will undergo substitution reac- slowly; aromatic ketones give no reaction.

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sequence rule

Schultze’s solution A solution of zinc seed A small crystal added to a gas or chloride, potassium iodide, and iodine liquid to assist solidification or precipita- used mainly for testing for cellulose and tion from a solution. The seed, usually a hemicellulose. Both materials stain a blue crystal of the substance to be formed, color with the reagent, that of hemicellu- enables particles to pack into predeter- lose being weaker. mined positions so that a larger crystal can form. scleroprotein One of a group of proteins obtained from the exoskeletal struc- self-assembly See supramolecular chem- tures of animals. They are insoluble in istry. water, salt solutions, dilute acids, and alkalis. This group exhibits a wide range of Seliwanoff’s test A test for ketose SUG- both physical and chemical properties. ARS in solution. The reagent used consists Typical examples of scleroproteins are ker- of benzene-1,3-diol (resorcinol) dissolved atin (hair), elastin (elastic tissue), and col- in hydrochloric acid. A few drops are lagen (connective tissue). added to the solution and a red precipitate indicates a ketonic sugar. The test is named SCP See single-cell protein. for the Russian chemist F. F. Seliwanoff.

scrubber A part of an industrial chemi- semicarbazide See semicarbazone. cal plant that removes impurities from a gas by passing it through a liquid. semicarbazone A type of organic compound containing the C:N.NH.CO.NH2 secondary alcohol See alcohol. grouping, formed by reaction of an ALDE- HYDE or KETONE with semicarbazide secondary amine See amine. (H2N.NH.CO.NH2). The compounds are crystalline solids with sharp melting secondary structure See protein. points, which were formerly used to characterize the original aldehyde or ketone. second-order reaction A reaction in which the rate of reaction is proportional semipermeable membrane A mem- to the product of the concentrations of two brane that, when separating a solution of the reactants or to the square of the con- from a pure solvent, permits the solvent centration of one of the reactants: molecules to pass through but does not rate = k[A][B] allow the transfer of solute molecules. Syn- or thetic semipermeable membranes are gen- rate =k[A]2 erally supported on a porous material, For example, the hydrolysis by dilute such as unglazed porcelain or fine wire alkali of an ester is a second-order reaction: screens, and are commonly formed of cel- rate = k[ester][alkali] lulose or related materials. They are used in The rate constant for a second-order re- osmotic studies, gas separations, and med- action has the units mol–1 dm3 s–1. Unlike a ical applications. first-order reaction, the time for a definite Equilibrium is reached at a semiperme- fraction of the reactants to be consumed is able membrane if the chemical potentials dependent on the original concentrations. on both sides become identical; migration of solvent molecules towards the solution sedimentation The settling of a suspen- is an attempt by the system to reach equi- sion, either under gravity or in a centrifuge. librium. The pressure required to halt this The speed of sedimentation can be used to migration is the OSMOTIC PRESSURE. estimate the average size of the particles. This technique is used with an ULTRA- semipolar bond A coordinate bond. CENTRIFUGE to find the relative molecular masses of macromolecules. sequence rule See CIP system.

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serine

serine See amino acid. the German physicist Ernst Werner von Siemens (1816–92). serotonin (hydroxytryptamine) A substance that serves as a neurohormone that sievert Symbol: Sv The SI unit of dose acts on muscles and nerves, and a neuro- equivalent. It is the dose equivalent when transmitter found in both the central and the absorbed dose produced by ionizing ra- peripheral nervous systems. It controls di- diation multiplied by certain dimensionless lation and constriction of blood vessels and factors is 1 joule per kilogram (1 J kg–1). affects peristalsis and gastrointestinal tract The dimensionless factors are used to mod- motility. Within the brain it plays a role in ify the absorbed dose to take account of the mood behavior. Many hallucinogenic com- fact that different types of radiation cause pounds (e.g. LSD) antagonize the effects of different biological effects. The unit is serotonin in the brain. named for the Swedish physicist Rolf Siev- ert (1896–1966). sesquiterpene See terpene. sigma bond See orbital. shell A group of electrons that share the same principal quantum number. Early sigmatropic rearrangement See peri- work on x-ray emission studies used the cyclic reaction. terms K, L, M, and these are still sometimes used for the first three shells: n = 1, silica gel A gel made by coagulating K-shell; n = 2, L-shell; n = 3, M-shell. sodium silicate sol. The gel is dried by heating and used as a catalyst support and as a The main shikimic acid pathway drying agent. The silica gel used in desicca- metabolic pathway in the biosynthesis of tors and in packaging to remove moisture the aromatic amino acids tyrosine, pheny- is often colored with a cobalt salt to indi- lalanine, and tryptophan. The first step is cate whether it is still active (blue = dry; erythrose-4-phosphate, which comes from pink = moist). the pentose phosphate pathway, condensing with phosphenolpyruvate, from glycol- silicones Polymeric synthetic silicon ysis. This product cyclizes to shikimate, compounds containing chains of alternat- which is then converted by various pathways into the amino acids. ing silicon and oxygen atoms, with organic groups bound to the silicon atoms. Sili- short period See period. cones are used as lubricants and water re- pellants and in waxes and varnishes. SI See SI units. Silicone rubbers are superior to natural rubbers in their resistance to both high and side chain In an organic compound, an low temperatures and chemicals. aliphatic group or radical attached to a longer straight chain of atoms in an acyclic silver-mirror test A test for the alde- compound or to one of the atoms in the hyde group. A few drops of the sample are ring of a cyclic compound. See also chain. warmed with TOLLEN’S REAGENT. An aldehyde reduces Ag+ to silver metal, causing a side reaction A chemical reaction that brilliant silver mirror to coat the inside takes place to a limited extent at the same wall of the tube. time as a main reaction. Thus the main product of a reaction may contain small single bond A covalent bond between amounts of other compounds. two elements that involves one pair of electrons only. It is represented by a single line, siemens (mho) Symbol: S The SI unit of for example H–Br, and is usually a sigma electrical conductance, equal to a conduc- bond, although it can be a pi bond. Com- tance of one ohm–1. The unit is named for pare multiple bond. See also orbital.

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solubility product

single-cell protein (SCP) Protein pro- variety of sols are known; the colors often duced from microorganisms, such as bac- depend markedly on the particle size. The teria, yeasts, mycelial fungi, and unicellular term aerosol is used for solid or liquid algae, used as food for man and other ani- phases dispersed in a gaseous medium. mals. solid The state of matter in which the singlet state See carbene. particles occupy fixed positions, giving the substance a definite shape. The particles SI units (Système International d’Unités) are held in these positions by bonds. Three The internationally adopted system of kinds of attraction fix the positions of the units used for scientific purposes. It has particles: ionic, covalent, and intermolecu- seven base units and two dimensionless lar. Since these bonds act over short dis- units, formerly called supplementary units. tances the particles in solids are packed Derived units are formed by multiplication closely together. The strengths of these and/or division of base units. Standard pre- three types of bonds are different and so, fixes are used for multiples and submulti- therefore, are the mechanical properties of ples of SI units. The SI system is a coherent different solids. rationalized system of units. solid solution A solid composed of two SN1 reaction See nucleophilic substitu- or more substances mixed together at the tion. molecular level. Atoms, ions, or molecules of one component in the crystal are at lat- See nucleophilic substitu- SN2 reaction tice positions normally occupied by the tion. other component. SNG Substitute (or synthetic) natural solubility The amount of one substance gas. A mixture of hydrocarbons manufac- that could dissolve in another to form a tured from coal or the naphtha fraction of saturated solution under specified condi- petroleum for use as a fuel. tions of temperature and pressure. Solubil- ities are stated as moles of solute per 100 soap A substance consisting of sodium grams of solvent or as mass of solute per or potassium compounds of fatty acids used to improve the cleansing properties of unit volume of solvent. water. Soap is a SURFACTANT and was the solubility product Symbol: K If an earliest known DETERGENT. The basic s method of making soap involved treating ionic solid is in contact with its saturated animal fat (mainly beef tallow), which is a solution, there is a dynamic equilibrium triglyceride of octadecanoic acid (stearic between solid and solution: ˆ + – acid), with caustic soda (sodium hydrox- AB(s) A (aq) + B (aq) ide; NaOH) to produce sodium octade- The equilibrium constant for this is given canoate (sodium stearate). More refined by + – soaps are made from vegetable oils, such as [A ][B ]/[AB] palm oil, which contains hexadecanoic The concentration of undissolved solid acid (palmitic acid). Liquid soaps (soft [AB] is also constant, so + – soap) are made using potassium hydroxide Ks = [A ][B ] rather than sodium hydroxide. Other Ks is the solubility product of the salt (at a metal salts of long-chain carboxylic acids given temperature). For a salt A2B3, for in- are also known as ‘soaps’. stance: + 2 – 3 Ks = [A ] [B ] , etc. soft soap See soap. Solubility products are meaningful only for sparingly soluble salts. If the product of sol A COLLOID consisting of solid parti- ions exceeds the solubility product, precip- cles distributed in a liquid medium. A wide itation occurs.

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BASE AND DIMENSIONLESS SI UNITS

Physical quantity Name of SI unit Symbol for SI unit length meter m mass kilogram(me) kg time second s electric current ampere A thermodynamic temperature kelvin K luminous intensity candela cd amount of substance mole mol *plane angle radian rad *solid angle steradian sr *supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unit frequency hertz Hz energy joule J force newton N power watt W pressure pascal Pa electric charge coulomb C electric potential difference volt V electric resistance ohm Ω electric conductance siemens S electric capacitance farad F magnetic flux weber Wb inductance henry H magnetic flux density tesla T luminous flux lumen lm illuminance (illumination) lux lx absorbed dose gray Gy activity becquerel Bq dose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol 10–1 deci- d 101 deca- da 10–2 centi- c 102 hecto- h 10–3 milli- m 103 kilo- k 10–6 micro- µ 106 mega- M 10–9 nano- n 109 giga- G 10–12 pico- p 1012 tera- T 10–15 femto- f 1015 peta- P 10–18 atto- a 1018 exa- E 10–21 zepto- z 1021 zetta- Z 10–24 yocto- y 1024 yotta- Y

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specific reaction rate

solute A material that is dissolved in a moment and consequently will solvate solvent to form a solution. nonpolar species in preference to polar species. For example, benzene and tetra- solution A liquid system of two or more chloromethane are good solvents for io- species that are intimately dispersed within dine and paraffin wax, but do not dissolve each other at a molecular level. The system sodium chloride. is therefore totally homogeneous. The Amphiprotic. A solvent that undergoes major component is called the solvent self-ionization and can act both as a proton (generally liquid in the pure state) and the donator and as an acceptor. Water is a minor component is called the solute (gas, good example and ionizes according to: + – liquid, or solid). 2H2O = H3O + OH The process occurs because of a direct Aprotic. A solvent that can neither accept intermolecular interaction of the solvent nor yield protons. An aprotic solvent is with the ions or molecules of the solute. therefore the opposite to an amphiprotic This interaction is called SOLVATION. Part solvent. of the energy of this interaction appears as a change in temperature on dissolution. See solvent extraction (liquid–liquid extrac- also heat of solution; solid solution; solution) A method of removing a substance bility. from solution by shaking it with and dissolving it in another (better) solvent that is solvation The attraction of a solute not miscible with the original solvent. species (e.g. an ion) for molecules of solvent. In water, for example, a positive ion solvent naphtha See naphtha. will be surrounded by water molecules, which tend to associate around the ion be- solvolysis A reaction between a com- cause of attraction between the positive pound and the solvent in which it is discharge of the ion, and the negative part of solved. See also hydrolysis. the polar water molecule. The energy of this solvation (hydration in the case of s orbital See orbital. water) is the ‘force’ needed to overcome the attraction between positive and negative sorbitol (HOCH2(CHOH)4CH2OH) A ions when an ionic solid dissolves. The at- hexahydric alcohol that occurs in rose hips traction of the dissolved ion for solvent and rowan berries. It can be synthesized by molecules may extend for several layers. the reduction of glucose. Sorbitol is used to make vitamin C (ascorbic acid) and surfac- solvent A liquid capable of dissolving tants. It is also used in medicines and as a other materials (solids, liquids, or gases) to sweetener (particularly in foods for diabet- form a solution. The solvent is generally ics). It is an isomer of mannitol. the major component of the solution. Sol- vents can be divided into classes, the most sorption Absorption of gases by solids. important being: Polar. A solvent in which the molecules specific Denoting a physical quantity possess a moderate to high dipole moment per unit mass. For example, volume (V) per and in which polar and ionic compounds unit mass (m) is called specific volume: are easily soluble. Polar solvents are usu- V = Vm ally poor solvents for nonpolar com- In certain physical quantities the term pounds. For example, water is a good does not have this meaning: for example, solvent for many ionic species, such as specific gravity is more properly called rel- sodium chloride or potassium nitrate, and ative density. polar molecules, such as the sugars, but does not dissolve paraffin wax. specific gravity See relative density. Nonpolar. A solvent in which the molecules do not possess a permanent dipole specific reaction rate See rate constant.

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specific rotatory power

α specific rotatory power Symbol: m 2. Any of various other instruments for an- The rotation of plane-polarized light in de- alyzing the energies, masses, etc., of parti- grees produced by a 10 cm length of solu- cles. See mass spectrometer. tion containing 1 g of a given substance per milliliter of stated solvent. The specific ro- spectrophotometer A form of spec- tatory power is a measure of the optical ac- trometer able to measure the intensity of tivity of substances in solution. It is radiation at different wavelengths in a measured at 20°C using the D-line of spectrum, usually in the visible, infrared, sodium. or ultraviolet regions.

spectra See spectrum. spectroscope An instrument for examining the different wavelengths present in spectral line A particular wavelength of electromagnetic radiation. See also spec- light emitted or absorbed by an atom, ion, trometer. or molecule. See line spectrum. spectroscopy 1. The production and spectral series A group of related lines analysis of spectra. There are many spec- in the absorption or emission spectrum of troscopic techniques designed for investi- a substance. The lines in a spectral series gating the electromagnetic radiation occur when the transitions all occur be- emitted or absorbed by substances. Spec- tween one particular energy level and a set troscopy, in various forms, is used for of different levels. analysis of mixtures, for identifying and determining the structures of chemical spectrograph An instrument for pro- compounds, and for investigating energy levels in atoms, ions, and molecules. In the ducing a photographic record of a spec- visible and longer wavelength ultraviolet, trum. transitions correspond to electronic energy levels in atoms and molecules. The shorter spectrographic analysis A method of wavelength ultraviolet corresponds to analysis in which the sample is excited elec- transitions in ions. In the x-ray region, trically (by an arc or spark) and emits radi- transitions in the inner shells of atoms or ation characteristic of its component ions are involved. The infrared region cor- atoms. This radiation is passed through a responds to vibrational changes in mol- slit, dispersed by a prism or a grating, and ecules, with rotational changes at longer recorded as a spectrum, either photograph- wavelengths. ically or photoelectrically. The photo- 2. Any of various techniques for analysing graphic method was widely used for the energy spectra of beams of particles or qualitative and semiquantitative work but for determining mass spectra. photoelectric detection also allows wide quantitative application. spectrum (plural spectra) 1. A range of electromagnetic radiation emitted or ab- spectrometer 1. An instrument for ex- sorbed by a substance under particular cir- amining the different wavelengths present cumstances. In an emission spectrum, light in electromagnetic radiation. Typically, or other radiation emitted by the body is spectrometers have a source of radiation, analyzed to determine the particular wave- which is collimated by a system of lenses lengths produced. The emission of radia- and/or slits. The radiation is dispersed by a tion may be induced by a variety of prism or grating, and recorded photo- methods; for example, by high tempera- graphically or by a photocell. There are ture, bombardment by electrons, absorp- many types for producing and investigat- tion of higher-frequency radiation, etc. In ing spectra over the whole range of the an absorption spectrum a continuous flow electromagnetic spectrum. Often spec- of radiation is passed through the sample. trometers are called spectroscopes. The radiation is then analyzed to deter-

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states of matter

mine which wavelengths are absorbed. See tains a known weight of the reagent in a also band spectrum; continuous spectrum; definite volume of solution. A standard line spectrum. flask or volumetric flask is used for this 2. In general, any distribution of a prop- purpose. The solutions may be prepared by erty. For instance, a beam of particles may direct weighing for primary standards. If have a spectrum of energies. A beam of the reagent is not available in a pure form ions may have a mass spectrum (the distri- or is deliquescent the solution must be bution of masses of ions). See mass spec- standardized by titration against another trometer. known standard solution. See primary standard. spin A property of certain elementary particles whereby the particle acts as if it standard state The standard conditions were spinning on an axis; i.e. it has an in- used as a reference system in thermody- trinsic angular momentum. Such particles namics: pressure is 101 325 Pa; tempera- also have a magnetic moment. In a mag- ture is 25°C (298.15 K); concentration is 1 netic field the spins line up at an angle to mol. The substance under investigation the field direction and precess around this must also be pure and in its usual state, direction. Certain definite orientations to given the above conditions. π the field direction occur such that msh/2 is the component of angular momentum standard temperature An internation- along this direction. ms is the spin quantum ally agreed value for which many measure- number, and for an electron it has values ments are quoted. It is the melting +1/2 and –1/2. h is the Planck constant. temperature of water, 0°C (273.15 K). See also STP. spontaneous combustion The self- ignition of a substance that has a low igni- starch A polysaccharide that occurs ex- tion temperature. It occurs when slow oxi- clusively in plants. Starches are extracted dation of the substance (such as a heap of commercially from maize, wheat, barley, damp straw or oily rags) builds up suffi- rice, potatoes, and sorghum. The starches cient heat for ignition to take place. are storage reservoirs for plants; they can be broken down by enzymes to simple sug- stabilization energy The difference in ars and then metabolized to supply energy energy between the delocalized structure needs. Starch is a dietary component of an- and the conventional structure for a com- imals. pound. For example, the stabilization en- Starch is not a single molecule but a ergy of benzene is 150 kJ per mole, which mixture of two glucose polymers: amylose represents the difference in energy between (water-soluble, blue color with iodine) and a Kekulé structure and the delocalized amylopectin (not water-soluble, violet structure: the delocalized form being of color with iodine). The composition is lower energy is therefore more stable. The amylose 10–20%, amylopectin 80–90%. stabilization energy can be determined by comparing the experimental value for the states of matter The three physical con- heat of hydrogenation of benzene with that ditions in which substances occur: solid, calculated for Kekulé benzene from bond- liquid, and gas. The addition or removal of energy data. energy (usually in the form of heat) enables one state to be converted into another. stabilizer A substance added to prevent The major distinctions between the chemical change (i.e. a negative catalyst). states of matter depend on the kinetic energies of their particles and the distances be- staggered conformation See confor- tween them. In solids, the particles have mation. low kinetic energy and are closely packed; in gases they have high kinetic energy and standard solution A solution that con- are very loosely packed; kinetic energy and

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stationary phase

separation of particles in liquids are inter- steam reforming The conversion of a mediate. methane–steam mixture at 900°C with a Solids have fixed shapes and volumes, nickel catalyst into a mixture of carbon i.e. they do not flow, like liquids and gases, monoxide and hydrogen. The mixture of and they are difficult to compress. In solids gases (synthesis gas) provides a starting the atoms or molecules occupy fixed posi- material in a number of processes, e.g. the tions in space. In most cases there is a reg- manufacture of methanol. ular pattern of atoms – the solid is crystalline. stearate A salt or ester of stearic acid; an Liquids have fixed volumes (i.e. low octadecanoate. compressibility) but flow to take up the shape of the container. The atoms or mol- stearic acid See octadecanoic acid. ecules move about at random, but they are quite close to one another and the motion step An elementary stage in a chemical is hindered. reaction, in which energy may be trans- Gases have no fixed shape or volume. ferred from one molecule to another, They expand spontaneously to fill the con- bonds may be broken or formed, or elec- tainer and are easily compressed. The mol- trons may be transferred. ecules have almost free random motion. A plasma is sometimes considered to be steradian Symbol: sr The SI unit of solid a fourth state of matter. angle. The surface of a sphere, for example, subtends a solid angle of 4π at its center. stationary phase See chromatography. The solid angle of a cone is the area inter- cepted by the cone on the surface of a Staudinger, Hermann (1881–1965) sphere of unit radius. German organic chemist. Staudinger was a pioneer of polymer chemistry. It was stereochemistry The branch of chem- Staudinger who introduced the word istry concerned with the shapes of mol- ‘macromolecule’ into chemistry in 1922. ecules and the way these affect their He proposed that polymers are very long chemical properties. molecules held together by ordinary chemical bonds. This correct view was not uni- stereoisomerism See isomerism. versally accepted when it was first put forward but within a decade or so stereospecific Describing a chemical re- decisive evidence emerged that confirmed action that results in a particular arrange- Studinger’s view. In a book entitled Macro- ment of atoms in space. molecular Chemistry and Biology (1947) he anticipated some of the key features of stereospecific polymer See polymer- molecular biology. Staudinger won the ization. 1953 Nobel Prize for chemistry for his fundamental contributions to polymer chem- steric effect An effect in which the istry. shape of a molecule influences its reactions. A particular example occurs in mol- steam distillation A method of isolat- ecules containing large groups, which ing or purifying substances by exploiting hinder the approach of a reactant (steric Dalton’s law of partial pressures to lower hindrance). the boiling point of the mixture. When two immiscible liquids are distilled, the boiling steric hindrance See steric effect. point will be lower than that of the more volatile component and consequently will steroid Any member of a group of com- be below 100°C if one component is water. pounds having a complex basic ring struc- The method is particularly useful for re- ture. Examples are corticosteroid covering materials from tarry mixtures. hormones (produced by the adrenal gland),

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substitution reaction

sex hormones (progesterone, androgens, strong base A base that is completely or and estrogens), bile acids, and sterols (such almost completely dissociated into its com- as cholesterol). See also anabolic steroid; ponent ions in solution. See acid. sterol. structural formula See formula. sterol A steroid with long aliphatic side chains (8–10 carbons) and at least one hy- structural isomerism See isomerism. droxyl group. They are lipid-soluble and often occur in membranes (e.g. cholesterol styrene See phenylethene. and ergosterol). sublimate A solid formed by sublimation. still An apparatus for distillation. sublimation The conversion of a solid stoichiometric coefficient See chemi- into a vapor without the solid first melting. cal equation. For instance (at standard pressure) iodine, solid carbon dioxide, and ammonium chlo- The proportions in stoichiometry ride sublime. At certain conditions of ex- which elements form compounds. A stoi- ternal pressure and temperature an chiometric compound is one in which the equilibrium can be established between the atoms have combined in small whole num- solid phase and vapor phase. bers. subshell A subdivision of an electron STP (NTP) Standard temperature and shell. It is a division of the orbitals that pressure. Conditions used internationally make up a shell into sets of orbitals, which when measuring quantities that vary with are degenerate (i.e. have the same energy) both pressure and temperature (such as the in the free atom. For example, in the sec- density of a gas). The values are 101 325 ond shell (the L shell) there are the 2s and Pa (approximately 100 kPa) and 0°C 2p subshells. (273.15 K). substituent An atom or group substi- straight chain See chain. tuted for another in a compound. Often the term is used for groups that have replaced Strecker synthesis A method of synthe- hydrogen in organic compounds. For ex- sizing amino acids. Hydrogen cyanide ample, in chlorobenzene (C6H5Cl) chlorine forms an addition product (cyanohydrin) can be regarded as a substituent. with aldehydes: RCHO + HCN → RCH(CN)OH substitution reaction A reaction in which an atom or group of atoms in an or- With ammonia further substitution occurs: ganic molecule is replaced by another atom RCH(CN)OH + NH → 3 or group. The substitution of a hydrogen RCH(CN)(NH ) + H O 2 2 atom in an alkane by a chlorine atom is an Acid hydrolysis of the cyanide group then example. Substitution reactions fall into produces the amino acid three major classes depending upon the na- → RCH(CN)(NH2) ture of the attacking substituent. RCH(COOH)(NH2) Nucleophilic substitution: the attacking α The method is a general synthesis for - substituent is a nucleophile (i.e. a molecule amino acids (with the –NH2 and –COOH or ion that can donate electrons). Such re- groups on the same carbon atom). actions are very common with alcohols and halogen compounds, in which the elec- strong acid An ACID that is almost com- tron-deficient carbon atom attracts the nu- pletely dissociated into its component ions cleophile and the leaving group readily in solution. exists alone. Examples are the hydrolysis

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substrate H H CH2OH CH2OH O O HO H HO H H OH H H HO OH HO OH OH H H H α–D–glucose β–D–glucose Glucose — a monosaccharide sugar

CH2OH O O H H HOCH2 H H OH H O H HO HO CH2OH

OH OH H

Sucrose — a disaccharide sugar

of a haloalkane and the chlorination of an succinic acid See butanedioic acid. alcohol: – → – C2H5Cl + OH C2H5OH + Cl sucrose (cane sugar; saccharose; → C H O )A SUGAR that occurs in many C2H5OH + HCl C2H5Cl + H2O 12 22 11 Electrophilic substitution: the attacking plants. It is extracted commercially from substituent is an electrophile (i.e. a mol- sugar cane and sugar beet. Sucrose is a disaccharide formed from a glucose unit and ecule or ion that accepts electrons). Such a fructose unit. It is hydrolyzed to a mix- reactions are common in aromatic com- ture of fructose and glucose by the enzyme pounds, in which the electron-rich ring at- invertase. Since this mixture has a different tracts the electrophile. The nitration of optical rotation (levorotatory) than the + benzene in which the electrophile is NO2 original sucrose, the mixture is called in- is an example: vert sugar. + → + C6H6 + NO2 C6H5NO2 + H Free-radical substitution: a free radical is sugar (saccharide) One of a class of the attacking substituent. Such reactions sweet-tasting simple carbohydrates. Sugars can be used with compounds that are inert have molecules consisting of a chain of car- to either nucleophiles or electrophiles, for bon atoms with –OH groups attached, and instance the halogenation of an alkane: either an aldehyde or ketone group. They → can exist in a chain form or in a ring CH4 + Cl2 CH3Cl + HCl The term ‘substitution’ is very general formed by reaction of the ketone or aldehyde group with an OH group to form a and several reactions that can be consid- cyclic hemiacetal (see acetal). Monosac- ered as substitutions are more normally charides are simple sugars that cannot be given special names (e.g. esterification, hydrolyzed to sugars with fewer carbon hydrolysis, and nitration). See also elec- atoms. Two or more monosaccharide units trophilic substitution; nucleophilic substi- can be linked in disaccharides, trisaccha- tution. rides, etc., by a GLYCOSIDIC LINK. Monosaccharides are also classified ac- substrate A material that is acted on by cording to the number of carbon atoms: a a catalyst. pentose has five carbon atoms and a hex-

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sulfonic acid

CH2OH of the primary alcohol group (CH2OH) to O COOH yields uronic acid; oxidation of both the primary alcohol and carboxyl groups gives an aldaric acid. The uronic OH acids are biologically important, being HO OH components of many polysaccharides, for example glucuronic acid (from glucose) is a OH major component of gums and cell walls, while galacturonic acid (from galactose) glucose – a pyranose ring makes up pectin. Ascorbic acid or vitamin C is an important sugar acid found universally in plants, particularly in citrus fruits. O HOCH2 CH2OH sugar alcohol (alditol) An alcohol de- HO rived from a monosaccharide by reduction OH of its carbonyl group (CO) so that each carbon atom of the sugar has an alcohol OH group (OH). For example, glucose yields sorbitol, common in fruits, and mannose fructose – in a furanose ring form yields mannitol.

sulfa drug See sulfonamide. O HOCH2 sulfate A salt or ester of sulfuric(VI) acid.

OH sulfide See thioether.

OH OH sulfite A salt or ester of sulfurous acid.

ribose – a pyranose ring sulfonamide A type of organic compound with the general formula Sugars R.SO2.NH2. Sulfonamides, which are amides of sulfonic acids, are active against ose six. Monosaccharides with aldehyde bacteria, and some are used in pharmaceu- groups are aldoses; those with ketone ticals (‘sulfa drugs’). groups are ketoses. Thus, an aldohexose is a hexose with an aldehyde group; a ke- sulfonate A salt or ester of a SULFONIC topentose is a pentose with a ketone group, ACID. etc. The ring forms of monosaccharides are sulfonation A reaction introducing the derived by reaction of the aldehyde or ke- –SO2OH (sulfonic acid) group into an or- tone group with one of the carbons at the ganic compound. Sulfonation of aromatic other end of the chain. It is possible to have compounds is usually accomplished by re- a six-membered (pyranose) ring or a five- fluxing with concentrated sulfuric acid for membered (furanose) ring. See also several hours. The attacking species is SO3 anomer; carbohydrate; polysaccharide. See (sulfur trioxide; sulfur(VI) oxide) and the illustration overleaf. reaction is an example of electrophilic substitution. sugar acid An acid formed from a monosaccharide by oxidation. Oxidation sulfonic acid A type of organic com- of the aldehyde group (CHO) of the aldose pound containing the –SO2.OH group. monosaccharides to a carboxyl group The simplest example is benzenesulfonic (COOH) gives an aldonic acid; oxidation acid (C6H5SO2OH). Sulfonic acids are 205 iranchembook.ir/edu

sulfonium compound

Glucose (an aldohexose) 6 6 CH OH CH OH 1 2 2 H C O 5 5 C O C O 2 H C OH H H H H H 1 3 C C C HO C O 4 OH H 4 OH H 4 HO OH HO H C OH C C C C 3 2 3 2 5 H C OH H OH H OH

6 α β CH2OH -glucose -glucose straight-chain form ring forms

Fructose (a ketohexose)

1 H C OH

2 C O 6 1 HOCH2 O CH2OH HO 3 C H 5 2 C H OH C 4 H C OH HO OH C C 5 4 3 H C OH 6 OH H CH2OH

straight-chain form ring form

Sugar: straight-chain and ring forms

strong acids and ionize in water to form the organic radicals. An example is dimethyl – sulfonate ion (–SO2O ). Electrophilic sub- sulfoxide, (CH3)2SO, commonly used as a stitution can introduce other groups onto solvent. the benzene ring; the –SO2.OH group directs substituents into the 3-position. sulfur A low-melting nonmetallic solid, sulfonium compound An organic yellow colored in its common forms; the second member of group 16 (formerly compound of general formula R3SX, where R is an organic radical and X is an VIA) of the periodic table. It has the elec- electronegative radical or element; it con- tronic configuration [Ne]3s23p4. + tains the ion R3S . An example is diethyl- Sulfur occurs in the elemental form in methylsulfonium chloride, (C2H5)2- Sicily and some southern states of the USA, CH S+Cl–, made by reacting diethyl sulfide 3 and in large quantities in combined forms with chloromethane. such as sulfide ores (FeS2) and sulfate rocks sulfoxide An organic compound of gen- (CaSO4). It forms about 0.5% of the eral formula R1SOR2, where R1 and R2 are Earth’s crust.

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supersaturated solution

Sulfur forms a wide range of organic sulfurous acid (sulfuric(IV) acid; sulfur compounds, most of which have the H2SO3) A weak acid found only in solu- typically revolting smell of H2S. tion, made by passing sulfur(IV) oxide into Symbol: S; m.p. 112.8°C; b.p. 444.6°C; water. The solution is unstable and smells r.d. 2.07; p.n. 16; r.a.m. 32.066. of sulfur(IV) oxide. It is a reducing agent, converting iron(III) ions to iron(II) ions, sulfur dichloride dioxide (sulfuryl chlo- chlorine to chloride ions, and orange ride; SO2Cl2) A colorless fuming liquid dichromate(VI) ions to green chrom- formed by the reaction of chlorine with sul- ium(III) ions. fur(IV) oxide in sunlight. It is used as a chlorinating agent. sulfur(IV) oxide (sulfur dioxide; SO2) A colorless choking gas prepared by burning sulfur dichloride oxide (thionyl chlo- sulfur or heating metal sulfides in air, or by treating a sulfite with an acid. It is a pow- ride; SOCl2) A colorless fuming liquid formed by passing sulfur(IV) oxide over erful reducing agent, used as a bleach. It phosphorus(V) chloride and distilling the dissolves in water to form sulfurous acid mixture obtained. Sulfur dichloride oxide (sulfuric(IV) acid) and combines with oxy- is used in organic chemistry to introduce gen, in the presence of a catalyst, to form chlorine atoms (for example, into ethanol sulfur(VI) oxide. This latter reaction is im- to form monochloroethane), the organic portant in the manufacture of sulfuric(VI) product being easily isolated as the other acid. products are gases. sulfur(VI) oxide (sulfur trioxide; SO3) A fuming volatile white solid prepared by sulfur dioxide See sulfur(IV) oxide. passing sulfur(IV) oxide and oxygen over hot vanadium(V) oxide (acting as a cata- sulfuretted hydrogen See hydrogen lyst) and cooling the product in ice. Sul- sulfide. fur(VI) oxide reacts vigorously with water to form sulfuric acid. See also contact sulfuric(IV) acid See sulfurous acid. process. sulfuric(VI) acid (oil of vitriol; sulfur trioxide See sulfur(VI) oxide. H2SO4) A colorless oily liquid manufactured by the contact process. The concen- sulfuryl chloride See sulfur dichloride trated acid is diluted by adding it slowly to dioxide. water, with careful stirring. Concentrated sulfuric acid acts as an oxidizing agent, giv- superacid An acid with a high proton- ing sulfur(IV) oxide as the main product, donating ability. They are substances such and also as a dehydrating agent. The di- as HF–SbF3 and HSO3–SbF5. Sometimes luted acid acts as a strong dibasic acid, neu- known as magic acids, they are able to pro- tralizing bases and reacting with active duce carbenium ions from some saturated metals and carbonates to form sulfates. hydrocarbons. Sulfuric acid is used in the laboratory to dry gases (except ammonia), to prepare ni- superheating The raising of a liquid’s tric acid and ethene, and to absorb alkenes. temperature above its boiling temperature, In industry, it is used to manufacture fertil- by increasing the pressure. izers (e.g. ammonium sulfate), rayon, and detergents, to clean metals, and in vehicle supernatant Denoting a clear liquid batteries. See contact process. that lies above a sediment or a precipitate.

sulfur monochloride See disulfur di- supersaturated solution See saturated chloride. solution.

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supersaturated vapor

supersaturated vapor See saturated surfactant A substance that lowers sur- vapor. face tension and has properties of wetting, foaming, detergency, dispersion, and emul- supplementary units See dimensionless sification. SOAPS and other DETERGENTS units. have surfactant properties.

supramolecular chemistry A branch suspension A system in which small of chemistry concerned with the synthesis particles of a solid or liquid are dispersed in and study of large structures consisting of a liquid or gas. molecules assembled together in a definite pattern. In a supramolecule the molecular synclinal conformation See conforma- units are joined by intermolecular bonds – tion. i.e. by hydrogen bonds or by ionic attractions. A particular interest in supramolecu- syndiotactic polymer See polymeriza- lar research is the idea of self-assembly – tion. i.e. that the molecules form well-defined structures spontaneously as a result of their Synge, Richard Laurence Millington geometry and chemical properties. In this (1914–94) British biochemist. Synge is best way, supramolecular chemistry is ‘chem- known for his work with Archer MARTIN istry beyond molecules’. on paper chromatography which led to There are various different examples of them sharing the 1952 Nobel Prize for supramolecular structures. For instance, chemistry. Synge used this technique to de- single layers of carboxylic acid molecules termine the exact structure of a simple anti- held by hydrogen bonds may form two-di- biotic peptide gramicidin-S. mensional crystalline structures with novel electrical properties. Large organic poly- synperiplanar conformation See con- meric structures may be formed in which a formation. number of individual chains radiate from a central point or region. These polymers are synthesis The preparation of chemical called dendritic polymers or dendrimers compounds from simpler compounds. and they have a number of possible applications. Another type of supramolecule is a synthesis gas A mixture of carbon helicate, which has a double helix made of monoxide and hydrogen produced by two chains of bipyridyl units held by cop- steam reforming of natural gas. → per ions along the axis. The structure is CH4 + H2O CO + 3H2 analogous to the double helix of DNA. See Synthesis gas is a useful starting ma- also host–guest chemistry. terial for the manufacture of a number of organic compounds. supramolecule See supramolecular chemistry. synthon See retrosynthetic analysis.

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tactic polymer See polymerization. Teflon (Trademark) The synthetic polymer polytetrafluoroethane. tannic acid See tannin. temperature scale A practical scale for tannin Any of several yellow organic measuring temperature. A temperature compounds found in vegetable sources scale is determined by fixed temperatures such as bark of trees, oak galls, and tea. (fixed points), which are reproducible sys- They are used in tanning animal skins to tems assigned an agreed temperature. On make leather and as mordants in dyeing. the Celsius scale the two fixed points are Tannic acid (a type of tannin) is a white the temperature of pure melting ice (the ice solid heterocyclic organic acid extracted temperature) and the temperature of pure from oak galls and used for making dyes boiling water (the steam temperature). The and inks. difference between the fixed points is the fundamental interval of the scale, which is tar (bitumen) A dark oily viscous liquid subdivided into temperature units. The In- obtained by the destructive distillation of ternational Temperature Scale has 11 fixed coal or the fractionation of petroleum. points that cover the range 13.81 kelvin to 1337.58 kelvin. Tars are mixtures of mainly high-molecular weight hydrocarbons and phenols. tera- Symbol: T A prefix denoting 1012. For example, 1 terawatt (TW) = 1012 watts tartaric acid A crystalline hydroxy car- (W). boxylic acid with the formula: HOOC(CHOH) COOH 2 terephthalic acid See benzene-1,4- Its systematic name is 2,3-dihydroxy- dicarboxylic acid. butanedioic acid. It is used as an additive in foodstuffs. See also optical activity. ternary compound A chemical compound formed from three elements; e.g. tartrate A salt or ester of tartaric acid. Na2SO4 or LiAlH4.

tautomerism Isomerism in which each terpene Any of a class of natural unsat- isomer can convert into the other, so that urated hydrocarbons with formulae the two isomers are in equilibrium. The (C5H8)n, found in plants. Terpenes consist isomers are called tautomers. Tautomerism of isoprene units, often results from the migration of a hy- CH2=C(CH3)CH=CH2. drogen atom. See keto–enol tautomerism. Monoterpenes have two units (C10H16), diterpenes four units (C20H32), triterpenes TCA cycle (tricarboxylic acid cycle) See six units (C30H48), etc. Sesquiterpenes have Krebs cycle. three isoprene units (C15H24).

TCCD See dioxin. tertiary alcohol See alcohol.

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tertiary amine

tertiary amine See amine. theobromine An alkaloid found in the cacao bean. Its action is similar to caffeine. tertiary structure See protein. The systematic name is 3,7-dimethylxanthine. Terylene (Trademark) A polymer made by condensing benzene-1,4-dicarboxylic theophylline An alkaloid similar in ac- acid (terephthalic acid) and ethane-1,2-diol tion to caffeine. Its systematic name is 1,3- (ethylene glycol), used for making fibers dimethylxanthine. for textiles. thermal dissociation The decomposi- tesla Symbol: T The SI unit of magnetic tion of a chemical compound into compo- flux density, equal to a flux density of one nent atoms or molecules by the action of heat. Often it is temporary and reversible. weber of magnetic flux per square meter. 1 T = 1 Wb m–2. The unit is named for the thermochemistry The branch of chem- Croatian–US electrical engineer Nikola istry concerned with heats of reaction, sol- Tesla (1870–1943). vation, etc.

tetrachloroethene (ethylene tetrachlo- thermodynamics The study of heat and ride; tetrachloroethylene; CCl2:CCl2)A other forms of energy and the various re- colorless poisonous liquid organic com- lated changes in physical quantities such as pound (a haloalkene) used as a solvent in temperature, pressure, density, etc. dry cleaning and as a de-greasing agent. The first law of thermodynamics states that the total energy in a closed system is tetrachloroethylene See tetrachloro- conserved (constant). In all processes en- ethene. ergy is simply converted from one form to another, or transferred from one system to tetrachloromethane (carbon tetrachlo- another. ride; CCl4) A colorless nonflammable A mathematical statement of the first liquid made by the chlorination of law is: methane. Its main use is as a solvent al- δQ = δU + δW though it is being replaced by other com- Here, δQ is the heat transferred to the δ pounds for safety reasons. system, U the change in internal energy (resulting in a rise or fall of temperature), δ tetraethyl lead See lead tetraethyl. and W is the external work done by the system. The second law of thermodynamics can tetrafluoroethene (CF2:CF2) A gaseous organic compound (a fluorocarbon and a be stated in a number of ways, all of which are equivalent. One is that heat cannot pass haloalkene) used to make the plastic poly- from a cooler to a hotter body without tetrafluoroethene (PTFE). See polytetraflu- some other process occurring. Another is oroethene. the statement that heat cannot be totally converted into mechanical work, i.e. a heat tetrahydrofuran (THF; C4H8O) A col- engine cannot be 100% efficient. orless liquid widely used as a solvent and The third law of thermodynamics states for making polymers. that the entropy of a substance tends to zero as its thermodynamic temperature ap- tetrapyrrole A structure of four pyrrole proaches zero. molecules linked together, found in heme, Often a zeroth law of thermodynamics chlorophyll, and other compounds. Usu- is given: that if two bodies are each in ther- ally a metal ion is coordinated to the four mal equilibrium with a third body, then nitrogen atoms on the pyrrole rings. See they are in thermal equilibrium with each porphyrin. other. This is considered to be more funda-

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thiol

mental than the other laws because they as- tum mechanics. Thiele also worked exten- sume it. sively on organic compounds of nitrogen.

thermodynamic temperature Symbol: thin-layer chromatography A type of T A temperature measured in kelvins. CHROMATOGRAPHY widely used for the See also absolute temperature. analysis of mixtures. Thin-layer chromatography employs a solid stationary thermoplastic polymer See polymer. phase, such as alumina or silica gel, spread evenly as a thin layer on a glass plate. A thermosetting polymer See polymer. base line is carefully scratched near the bottom of the plate, and a small sample of the THF See tetrahydrofuran. mixture is spotted onto the base line. The plate is then stood upright in solvent,

thiamine (vitamin B1) One of the water- which rises up to the base line and beyond soluble B-group of vitamins. Good sources by capillary action. The components of the of thiamine are unrefined cereal grains, spot of the sample will dissolve in the sol- liver, heart, and kidney. Thiamine defi- vent and tend to be carried up the plate. ciency predominantly affects the peripheral However, some of the components will nervous system, the gastrointestinal tract, cling more readily to the solid phase than and the cardiovascular system. Thiamine others and will not move up the plate so has been shown to be of value in the treat- rapidly. In this way, different fractions of ment of beriberi. Thiamine, in the form of the mixture eventually become separated. thiamine diphosphate, is the coenzyme for When the solvent has almost reached the the decarboxylation of acids such as pyru- top, the plate is removed and quickly dried. The plate is developed to locate the posi- vic acid. See also vitamin B complex. tions of colorless fractions by spraying with a suitable chemical or by exposure to thiazine (C H NS) Any of a group of 4 4 ultraviolet radiation. The components are heterocyclic organic compounds that have identified by comparing the distance they a six-membered ring containing four car- have moved up the plate with standard so- bon atoms, one nitrogen atom, and one lutions that have been run simultaneously, sulfur atom. or by computing an RF VALUE.

thiazole (C3H3NS) A colorless volatile thio alcohol See thiol. liquid, a beterocyclic compound with a five-membered ring containing three car- thiocarbamide (thiourea; NH2CSNH2) bon atoms, one nitrogen atom, and one A colorless crystalline organic compound sulfur atom. It resembles PYRIDINE in its re- (the sulfur analog of urea). It is converted actions and is used in making dyes. to the inorganic compound ammonium thiocyanate on heating. It is used as a sen- Thiele, Friedrich Karl Johannes sitizer in photography and in medicine. (1865–1918) French organic chemist. Thiele is best known for his concept of par- thioether (sulfide) A compound of the tial valence which he put forward in 1899. type RSR′. They are the sulfur analogs of In order to explain the lack of reactivity of ethers and are generally more reactive than the benzene molecule Thiele postulated the corresponding oxygen compound. that when single and double bonds alter- With halogen compounds they form sulfonate then the overall bonding is less con- nium compounds: → + – ducive to reactivity than purely double CH3SCH3 + CH3Cl (CH3)3 S Cl bonds. The concept of partial valence was They can also be oxidized to sulfoxides: → not properly understood until the chemical (CH3)2S + O (CH3)2S=O bonding and valence of molecules such as benzene were explained in terms of quan- thiol (thio alcohol; mercaptan) A com-

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thionyl chloride

H3C NH

N O HOCH2 O

Thymidine

pound of the type RSH, similar to an alco- tincture A solution in which alcohol hol with the oxygen atom replaced by sul- (ethanol) is the solvent. fur. Typically they have a strong unpleasant odor. They are more reactive Tiselius, Arne Wilhelm Kaurin than the corresponding alcohols and can (1902–71) Swedish chemist. Tiselius is best form salts of the type RS–M+. known for developing electrophoresis as a technique for studying proteins. He was thionyl chloride See sulfur dichloride able to separate the proteins in horse serum oxide. using this technique and to confirm that there are four types of proteins: albumins thiophene (C4H4S) A colorless liquid and alpha, beta and gamma globulins. that smells like benzene, a heterocyclic Tiselius also used other techniques such as compound with a five-membered ring con- chromatography and partition and gel fil- taining four carbon atoms and one sulfur tration to separate proteins. Tiselius won atom. It occurs as an impurity in commer- the 1948 Nobel Prize for chemistry for his cial benzene and is used as a solvent and in work using electrophoresis and other tech- organic syntheses. niques in the analysis of proteins.

thiourea ((NH2)2CS) See thiocarbamide. titrant See titration.

threonine See amino acid. titration A procedure in volumetric analysis in which a solution of known con- thymidine The NUCLEOSIDE formed centration (called the titrant) is added to a when thymine is linked to D-ribose by a solution of unknown concentration from a β-glycosidic bond. burette until the equivalence point or end point of the titration is reached. See volu- thymine A nitrogenous base found in metric analysis. DNA. It has a PYRIMIDINE ring structure. TNT See trinitrotoluene.

tocopherol See vitamin E. O

H3C Todd, Alexander Robertus, Lord NH (1907–97) British organic chemist. In the early part of his career, Todd was largely N O H concerned with the vitamins B1, B12 and E. In the late 1940s and 1950s he synthesized the purine and pyrimidine bases that Thymine occur in nucleic acids such as DNA and

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triazine

RNA and established their structures. He trans- Designating an isomer with also synthesized a number of other im- groups that are on opposite sides of a bond portant compounds such as adenosine or structure. See isomerism. triphosphate (ATP) and adenosine diphosphate (ADP), which are of crucial impor- trans fat See hardening. tance in energy transfer in biological systems. Todd won the 1957 Nobel Prize transient species A short-lived interme- for chemistry for his work on biologically diate in a chemical reaction. important molecules. He published an autobiography entitled A Time to Remember transition state (activated complex) Sym- in 1983. bol: ‡ A short-lived high-energy molecule, radical, or ion formed during a Tollen’s reagent A solution of the com- reaction between molecules possessing the + necessary activation energy. The transition plex ion Ag(NH3)2 produced by precipitation of silver oxide from silver nitrate with state decomposes at a definite rate to yield a few drops of sodium hydroxide solution, either the reactants again or the final prod- and subsequent dissolution of the silver ucts. The transition state can be considered oxide in aqueous ammonia. Tollen’s to be at the top of the energy profile. For the reaction, reagent is used in the ‘silver-mirror test’ for X + YZ = X…Y…Z‡ → XY+ Z aldehydes, where the Ag+ ion is reduced to the sequence of events is as follows. X ap- silver metal. It is also a test for alkynes with proaches YZ and when it is close enough a triple bond in the 1-position. A yellow the electrons are rearranged producing a precipitate of silver carbide is formed in weakening of the bond between Y and Z. this case. A partial bond is now formed between X RCCH + Ag+ → RCC–Ag+ + H+ and Y producing the transition state. De- It is named for Bernhard Tollens (1841– pending on the experimental conditions, 1918). See also silver-mirror test. the transition state either breaks down to form the products or reverts back to the re- toluene See methylbenzene. actants. See also activation energy.

toluidine (methylaniline; aminotoluene; transition temperature A temperature CH3C6H4NH2) An aromatic amine used at which some definite physical change oc- in making dyestuffs and drugs. There are curs in a substance. Examples of such tran- three isomers; the 1,2- (ortho-amino- sitions are change of state, change of toluene) and 1,3- (meta-) forms are liquids, crystal structure, and change of magnetic the 1,4- (para-) isomer is a solid. behavior.

tonne Symbol: t A unit of mass equal to triacylglycerol See triglyceride. 103 kilograms (i.e. one megagram). tricarboxylic acid cycle (TCA cycle) torr A unit of pressure equal to a pres- See Krebs cycle. sure of 101 325/760 pascals (133.322 Pa). One torr is equal to one mmHg. The unit is triaminotriazine See melamine. named for the Italian physicist Evangelista Torricelli (1609–47). triatomic molecule A molecule consisting of three atoms, such as O3 or H2O. torsion angle See conformation. triazine (C3H3N3) A heterocyclic or- tracer An isotope of an element used to ganic compound with a six-membered ring investigate chemical reactions or physical containing three carbon atoms and three processes (e.g. diffusion). See isotope. nitrogen atoms. There are three isomers,

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triazole

– – → used as dyestuffs and herbicides. See also CH3CHO + 3I + 4OH CHI3 + – melamine. HCOO + 3H2O The reaction also occurs with all ketones of triazole (C2H3N3) A heterocyclic or- general formula CH3COR (R is an alkyl ganic compound with a five-membered group) and with secondary alcohols ring containing two carbon atoms and CH3CH(OH)R. Iodoform is used as a test three nitrogen atoms. There are two iso- for such reactions (the iodoform reaction). mers. See also haloform.

tribasic acid An acid with three replace- trimer A molecule (or compound) able hydrogen atoms (such as phos- formed by addition of three identical mol- phoric(V) acid, H PO ). See acid. 3 4 ecules. See ethanal; methanal. tribromomethane (bromoform; CHBr ) 3 trimethylaluminum (aluminum tri- A colorless liquid compound. See halo- methyl; (CH ) Al) A colorless liquid pro- form. 3 3 duced by the reduction of dimethyl trichloroacetic acid See chloroethanoic aluminum chloride using sodium. It ignites acid. spontaneously on contact with air and reacts violently with water, acids, halogens, alcohols, and amines. Aluminum alkyls are trichloroethanal (chloral; CCl3CHO) A colorless liquid aldehyde made by chlori- used in the ZIEGLER PROCESS for the manu- nating ethanal. It was used to make the in- facture of high-density polyethene. secticide DDT. It can be hydrolyzed to give 2,2,2-trichloroethanediol (chloral hydrate, trimolecular Describing a reaction or CCl3CH(OH)2). Most compounds with step that involves three molecules interact- two –OH groups on the same carbon atom ing simultaneously with the formation of are unstable. However, in this case the ef- a product. For example, the final step in fect of the three chlorine atoms stabilizes reaction between hydrogen peroxide and the compound. It is used as a sedative. acidified potassium iodide is trimolecular: trichloroethanoic acid See chloro- + – → HOI + H + I I2 + H2O ethanoic acid. It is uncommon for reactions to take place involving trimolecular steps. trichloromethane (chloroform; CHCl3) A colorless volatile liquid formerly used as trinitroglycerine See nitroglycerine. an anesthetic. Now its main use is as a solvent and raw material for making other trinitrophenol See picric acid. chlorinated compounds. Trichloromethane is made by reacting trinitrotoluene (TNT; CH C H (NO ) ) ethanal, ethanol, or propanone with chlo- 3 6 2 2 3 A yellow crystalline solid. It is a highly un- rinated lime. See also haloform. stable substance, used as an explosive. The compound is made by nitrating methylben- triglyceride A GLYCERIDE in which esters are formed with all three –OH groups of zene and the nitro groups are in the 2, 4, glycerol. and 6 positions.

trihydric alcohol See triol. triol (trihydric alcohol) An alcohol that has three hydroxyl groups (–OH) per mol-

triiodomethane (iodoform; CHI3)A ecule of compound. yellow crystalline compound made by warming ethanal with an alkaline solution triose A SUGAR that contains three car- of an iodide: bon atoms.

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tyrosine

trioxygen See ozone. Tritium decays with emission of low- energy beta radiation to give 3He. The half- tripeptide See peptide. life is 12.3 years. It is useful as a tracer in studies of chemical reactions. Compounds triple bond A covalent bond formed be- in which 3H atoms replace the usual 1H tween two atoms in which three pairs of atoms are said to be tritiated. A positive tri- electrons contribute to the bond. One pair tium ion, T+, is a triton. forms a sigma bond (equivalent to a single bond) and two pairs give rise to two pi triton See tritium. bonds. It is conventionally represented as three lines, thus H–C≡C–H. The bond oc- + tropylium ion The positive ion C7H7 , curs in ALKYNES. See multiple bond. having a symmetrical seven-membered ring of carbon atoms. The tropylium ion ring triple point The only point at which the shows nonbenzenoid aromatic properties. gas, solid, and liquid phases of a substance See aromatic compound. can coexist in equilibrium. The triple point of water (273.16 K at 101 325 Pa) is used to define the kelvin. tryptophan See amino acid.

triplet state See carbene. turpentine (pine-cone oil) A yellow viscous RESIN obtained from coniferous trees. triterpene See terpene. It can be distilled to produce turpentine oil (also known simply as turpentine), used in tritiated compound A compound in medicine and as a solvent in paints, pol- which one or more 1H atoms have been re- ishes, and varnishes. placed by tritium (3H) atoms. twist-boat conformation See cyclo- tritium Symbol: T, 3H A radioactive iso- hexane. tope of hydrogen of mass number 3. The nucleus contains 1 proton and 2 neutrons. tyrosine See amino acid.

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→ ubiquinone An electron-transporting N2O4 2NO2 → coenzyme that is a component in the PCl5 PCl3 + Cl2 → electron-transport chain. It was formerly CH3CH2Cl C2H4 + HCl called coenzyme Q. unit A reference value of a quantity used ultracentrifuge A high-speed centrifuge to express other values of the same quan- used for separating out very small parti- tity. See also SI units. cles. The sedimentation rate depends on the particle size, and the ultracentrifuge unit cell The smallest group of atoms, can be used to measure the mass of col- ions, or molecules that, when repeated at loidal particles and large molecules (e.g. regular intervals in three dimensions, will proteins). produce the lattice of a crystal system. There are seven basic types of unit cells, ultraviolet (UV) A form of electromag- which result in seven CRYSTAL SYSTEMS. netic radiation, shorter in wavelength than visible light. Ultraviolet wavelengths range unit processes (chemical conver- between about 1 nm and 400 nm. Ordi- sions) The recognized steps used in nary glass is not transparent to these chemical processes, e.g. alkylation, distilla- waves; quartz is a much more effective mation, hydrogenation, pyrolysis, nitration, terial for making lenses and prisms for use etc. Industrial processing and the econom- with ultraviolet. Like light, ultraviolet radi- ics, design, and use of the equipment are ation is produced by electronic transitions based on these unit processes rather than between the outer energy levels of atoms. consideration of each reaction separately. However, having a higher frequency, ultraviolet photons carry more energy than univalent See monovalent. those of light and can induce photolysis of compounds and photoionization. See also universal gas constant See gas con- electromagnetic radiation. stant.

unimolecular Describing a reaction (or universal indicator (multiple-range indi- step) in which only one molecule is in- cator) A mixture of indicator dyestuffs volved. For example, radioactive decay is a that shows a gradual change in color over unimolecular reaction: a wide pH range. A typical formulation Ra → Rn + α contains methyl orange, methyl red, bro- Only one atom is involved in each disinte- mothymol blue, and phenolphthalein and gration. changes through a red, orange, yellow, In a unimolecular chemical reaction, green, blue, and violet sequence between the molecule acquires the necessary energy pH 3 and pH 10. Several commercial to become activated and then decomposes. preparations are available as both solu- The majority of reactions involve only uni- tions and test papers. or bimolecular steps. The following reactions are all unimolecular: unsaturated compound An organic

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compound that contains at least one dou- urethane (ethyl carbamate; CO(NH2)- ble or triple bond between two of its car- OC2H5) A poisonous flammable organic bon atoms. The ALKENES and ALKYNES are compound, used in medicine, as a solvent, examples of unsaturated compounds. Un- and as an intermediate in the manufacture saturated compounds typically undergo of POLYURETHANE resins. addition reactions to form single bonds. O Compare saturated compound. H N HN unsaturated solution See saturated so- O lution. O N N H H unsaturated vapor See saturated vapor. Uric acid

UPVC Unplasticized PVC; a hard- uric acid A nitrogen compound pro- wearing form of PVC used in building duced from purines. In certain animals work (e.g. for window frames). (uricotetic animals), it is the main excretory product resulting from breakdown of amino acids. In humans, uric acid crystals O in the joints are the cause of gout.

O N O H HN Uracil O N uracil A nitrogenous base that is found in RNA, replacing the thymine of DNA. It HOCH2 O has a pyrimidine ring structure.

urea (carbamide; CO(NH2)2) A white crystalline compound made from ammonia OH OH and carbon dioxide. It is used in the manufacture of urea–formaldehyde (methanal) resins. Urea is the end product of metabo- Uridine lism in many animals and is present in urine. uridine The nucleoside formed when uracil is linked to D-ribose by a β-glycosidic urea cycle See ornithine cycle. bond.

urea–formaldehyde resin A synthetic uronic acid See sugar acid. POLYMER made by copolymerizing urea with formaldehyde (methanal, HCHO). UV See ultraviolet.

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vacuum distillation The distillation of valence electron An outer electron in liquids under a reduced pressure, so that an atom that can participate in forming the boiling point is lowered. Vacuum dis- chemical bonds. tillation is a common laboratory technique for purifying or separating compounds valency See valence. that would decompose at their ‘normal’ boiling point. valeric acid See pentanoic acid.

vacuum flask See Dewar flask. valine See amino acid.

valence (valency) The combining power van der Waals equation An equation of an element or radical, equal to the num- of state for real gases. For n moles of gas ber of hydrogen atoms that will combine the equation is 2 2 with or displace one atom of the element. (p + n a/V )(V – nb) = nRT For simple covalent molecules the valence where p is the pressure, V the volume, and is obtained directly, for example C in CH T the thermodynamic temperature. a and b 4 are constants for a given substance and R is is tetravalent; N in NH3 is trivalent. For ions the valence is regarded as equivalent the gas constant. The equation gives a bet- to the magnitude of the charge; for exam- ter description of the behavior of real gases than the perfect gas equation (pV = nRT). ple Ca2+ is divalent, CO 2– is a divalent 3 The equation contains two corrections: radical. The rare gases are zero-valent be- b is a correction for the nonnegligible size cause they do not form compounds under of the molecules; a/V2 corrects for the fact normal conditions. As the valence for that there are attractive forces between the many elements is constant, the valence of molecules, thus slightly reducing the pres- some elements can be deduced without ref- sure from that of an ideal gas. It is named erence to compounds formed with hydro- for the Dutch physicist Johannes Diderik gen. Thus, as the valence of chlorine in HCl van der Waals (1837–1923). See also gas is 1, the valence of aluminum in AlCl3 is 3; laws. as oxygen is divalent (H2O) silicon in SiO2 is tetravalent. The product of the valence van der Waals force An intermolecular and the number of atoms of each element force of attraction, considerably weaker in a neutral compound must be equal. For than chemical bonds and arising from example, in Al2O3 for the two aluminum weak electrostatic interactions between atoms (valence 3) the product is 6 and for molecules (the energies are often less than the three oxygen atoms (valence 2) the 1 J mol–1). product is also 6. The van der Waals interaction contains The valence of an element is generally contributions from three effects; perma- equal to either the number of valence elec- nent dipole–dipole interactions found for trons or eight minus the number of valence any polar molecule; dipole–induced dipole electrons. Transition metal ions display interactions, where one dipole causes a variable valence. slight charge separation in bonds that have

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vicinal positions

a high polarizability; and dispersion forces, near the surface of a liquid acquire suffi- which result from temporary polarity aris- cient energy in collisions with other parti- ing from an asymmetrical distribution of cles to escape from the liquid and enter the electrons around the nucleus. Even atoms vapor; some particles in the vapor lose en- of the rare gases exhibit dispersion forces. ergy in collisions and re-enter the liquid. At a given temperature an equilibrium is es- Van’t Hoff, Jacobus Henricus (1852– tablished, which determines the vapor 1911) Dutch theoretical chemist. Van’t pressure of the liquid at that temperature. Hoff made important contributions to stereochemistry, thermodynamics, the ki- vapor density The ratio of the mass of a netics of chemical reactions and the theory certain volume of a vapor to the mass of an of chemical solutions. In 1874 Van’t Hoff equal volume of hydrogen (measured at the initiated the subject of stereochemistry same temperature and pressure). Determi- when he postulated that the four chemical nation of vapor densities is one method of bonds which a carbon atom can form are finding the relative molecular mass of a directed toward the corners of a regular compound (equal to twice the vapor den- tetrahedron. This enabled the phenomenon sity). VICTOR MEYER’S METHOD, DUMAS’ of optical activity to be understood in METHOD, or HOFMANN’S METHOD can be terms of the structures of optical isomers. used. Van’t Hoff introduced this idea independently of Joseph le Bel. Many of the contri- vaporization The process by which a butions of Van’t Hoff to thermodynamics, liquid or solid is converted into a gas or kinetics and solutions were expounded in vapor by heat. Unlike boiling, which oc- his book Studies of Chemical Dynamics curs at a fixed temperature, vaporization (1884). This included the application of can occur at any temperature. Its rate in- thermodynamics to chemical equilibrium. creases as the temperature rises. He was awarded the first Nobel Prize for chemistry in 1901. vapor pressure The pressure exerted by a vapor. The saturated vapor pressure is van’t Hoff factor Symbol: i The ratio the pressure of a vapor in equilibrium with of the number of particles present in a so- its liquid or solid. It depends on the nature lution to the number of undissociated mol- of the liquid or solid and the temperature. ecules added. It is used in studies of colligative properties, which depend on the vat dyes A class of insoluble dyes ap- number of entities present. For example, if plied by first reducing them to derivatives n moles of a compound are dissolved and that are soluble in dilute alkali. In this con- dissociation into ions occurs, then the dition they have a great attraction for cer- number of particles present will be in. Os- tain fibers, such as cotton. The solution is motic pressure (π), for instance, will be applied to the material and the insoluble given by the equation dye is regenerated in the fibers by atmos- πV = inRT pheric oxidation. Indigo and indanthrene It is named for the Dutch theoretical are examples of vat dyes. chemist Jacobus Henricus van’t Hoff. velocity constant See rate constant. van’t Hoff isochore The equation: ∆ 2 d(logeK)/dT = H/RT vesicant A substance that causes blister- showing how the equilibrium constant, K, ing of the skin. Mustard gas is an example. of a reaction varies with thermodynamic temperature, T. ∆H is the enthalpy of reac- vicinal positions Positions in a mol- tion and R is the gas constant. ecule at adjacent atoms. For example, in 1,2-dichloroethane the chlorine atoms are vapor A gas formed by the VAPORIZA- in vicinal positions, and this compound TION of a solid or liquid. Some particles can thus be named vic-dichloroethane.

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Viktor Meyer’s method

Viktor Meyer’s method A method for vitamin One of a number of organic determining VAPOR DENSITIES in which a compounds that are essential in small given weight of sample is vaporized and the quantities for metabolism. The vitamins volume of air displaced by it is measured. have no energy value; most of them seem to In practice, a bulb in a heating bath is con- act as catalysts for essential chemical nected via a fairly long tube to a water- changes in the body, each one influencing a bath gas-collection arrangement. The number of vital processes. Vitamins A, D, system is brought to equilibrium and the E, and K are the fat-soluble vitamins, oc- sample is then added (without opening the curring mainly in animal fats and oils. apparatus to the atmosphere). As the air Vitamins B and C are the water-soluble displaced by gas is collected over water a vitamins. If a diet lacks vitamins, this re- correction for the vapor pressure of water sults in the breakdown of normal bodily is necessary and the method may fail if the activities and produces disease symptoms. vapor is soluble in water. It is named for Such deficiency diseases can usually be the German chemist Viktor Meyer (1848– remedied by including the necessary vita- 97). mins in the diet. Plants can synthesize vitamins from simple substances, but animals violaxanthin A xanthophyll pigment generally require them in their diet, though found in the brown algae. See photosyn- there are exceptions to this. These include thetic pigments. vitamins synthesized by bacteria in the gut, and some that can be manufactured by the vinegar A dilute solution (about 4% by animal itself. A precursor of vitamin D2 volume) of ethanoic acid (acetic acid), (ergosterol), for example, can be converted often with added coloring and flavoring in the skin by ultraviolet radiation. such as caramel. Natural vinegar is produced by the bacterial fermentation of vitamin A (vitamin A1; retinol) A fat- cider or wine; it can also be made synthet- soluble vitamin (a derivative of the yellow ically. See ethanoic acid. pigment, carotene) occurring in milk, butter, cheese, liver, and cod-liver oil. It can vinylation A catalytic reaction in which also be formed in the body by oxidation of a compound adds across the triple bond of carotene, which is present in fresh green ethyne (acetylene) to form an ethenyl vegetables and carrots. Deficiency in vita- (vinyl) compound. For example, an alcohol min A can result in a reduced resistance to can add as follows: disease and in night blindness. ROH + HC≡CH → RHC=CH(OH) vitamin B complex A group of ten or vinyl benzene See phenylethene. more water-soluble vitamins, which tend to occur together. They can be obtained vinyl chloride See chloroethene. from whole grains of cereals and from meat and liver. Since the B vitamins are

vinyl group The group CH2:CH–. present in most unprocessed food, deficiency diseases only occur in populations viscose rayon See rayon. living on restricted diets. Many of the B vitamins act as coenzymes involved in the viscosity Symbol: η The resistance to normal oxidation of carbohydrates during flow of a fluid. respiration. The vitamins of the B complex include visible radiation See light. thiamine (vitamin B1), riboflavin (vitamin B2), nicotinic acid (niacin), pantothenic visual purple See rhodopsin. acid (vitamin B5), pyridoxine (vitamin B6), cyanocobalamin (vitamin B12), biotin, vitalism See Wöhler’s synthesis. lipoic acid, and folic acid.

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vulcanization

vitamin C (ascorbic acid) A water-solu- ciency may be observed, but this is readily ble vitamin, which is widely required in rectified by a small injection of the vitamin. metabolism. The major sources of vitamin C are fresh fruit and vegetables and severe VLDL (very low-density lipoprotein) See deficiency results in scurvy. lipoprotein.

vitamin D A fat-soluble vitamin found volatile Easily converted into a vapor. in fish-liver oil, butter, milk, cheese, egg yolk, and liver. Its principal action is to in- volt Symbol: V The SI unit of electrical crease the absorption of calcium and phos- potential, potential difference, and e.m.f., phorus from the intestine. The vitamin also defined as the potential difference between has a direct effect on the calcification two points in a circuit between which a process in bone. Deficiency results in inad- constant current of one ampere flows when equate deposition of calcium in the bones, the power dissipated is one watt. 1 V = 1 J –1 causing rickets in young children and C . The unit is named for the Italian physi- osteomalacia in adults. cist Alessandro Volta (1745–1827). The term vitamin D refers, in fact, to a group of compounds, all sterols, of very volumetric analysis One of the classi- similar properties. The most important are cal wet methods of quantitative analysis. It involves measuring the volume of a solu- vitamin D (calciferol) and vitamin D 2 3 tion of accurately known concentration (cholecalciferol). Precursors of these are that is required to react with a solution of converted to the vitamins in the body by the substance being determined. The solu- the action of ultraviolet radiation. tion of known concentration (the standard solution) is added in small portions from a vitamin E (tocopherol) A fat-soluble burette. The process is called a titration vitamin found in wheat germ, dairy prod- and the equivalence point is called the end ucts, and in meat. Severe deficiency in in- point. End points are observed with the aid fants may lead to high rates of red-blood of indicators or by instrumental methods, cell destruction and hence to anemia. such as conduction or light absorption. However, there are very few deficiency Volumetric analysis can also be applied to effects apparent in adults. gases. The gas is typically held over mercury in a graduated tube, and volume vitamin K (phylloquinone; menaquin- changes are measured on reaction or after one) A fat-soluble vitamin that is re- absorption of components of a mixture. quired to catalyze the synthesis of prothrombin, a blood-clotting factor, in vulcanite (ebonite) A hard black insula- the liver. Intestinal microorganisms are cator made by vulcanizing rubber with a pable of synthesizing considerable large amount of sulfur. amounts of vitamin K in the intestine and this, together with dietary supply, insures vulcanization A process of improving that deficiency is unlikely to occur in any the quality of rubber (hardness and resis- but the newborn. A newborn child may be tance to temperature changes) by heating it deficient because the intestine is sterile at with sulfur (about 150°C). Accelerators birth and the level supplied by the mother are used to speed up the reaction. Certain during gestation is limited. Thus during the sulfur compounds can also be used for vul- first few days of life blood-clotting defi- canization.

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Wacker process An industrial process water consists of single H2O molecules. for making ethanal (and other carbonyl Due to the presence of two lone pairs the compounds). To produce ethanal, ethene atoms do not lie in a straight line, the angle and air are bubbled through an acid solu- between the central oxygen atom and the tion of palladium(II) chloride and cop- two hydrogen atoms being 105°; the dis- per(II) chloride (20–60°C and moderate tance between each hydrogen atom and the pressure): oxygen atom is 0.099 nm. When ice forms, 2+ → C2H4 + Pd + O2 CH3CHO + hydrogen bonds some 0.177 nm long de- Pd + 2H+ velop between the hydrogen atom and oxy- The reaction involves an intermediate gen atoms in adjacent molecules, giving ice complex between palladium(II) ions and its tetrahedral crystalline structure with a ethane in which the palladium bonds to the density of 916.8 kg m–3 at STP. Different pi electrons. The purpose of the copper(II) ice structures develop under higher pres- chloride is to oxidize the palladium back to sures. When ice melts to form liquid water, Pd2+ ions: the tetrahedral structure breaks down, but Pd + 2Cu2+ → Pd2+ + 2Cu+ some hydrogen bonds continue to exist; The copper(I) ions spontaneously oxidize liquid water consists of groups of associ- to copper(II) ions in air. The process pro- ated water molecules, (H2O)n, mixed with vides a cheap source of ethanal (and, by some monomers and some dimers. This oxidation, ethanoic acid) from the readily mixture of molecular species has a higher available ethene. It is named for Alexander density than the open-structured crystals. von Wacker (1846–1922). The maximum density of water, 999.97 kg m–3, occurs at 3.98°C. This accounts for Walden inversion A reaction in which the ability of ice to float on water and for an optically active compound reacts to give the fact that water pipes burst as ice ex- an optically active product in which the pands on freezing. configuration has been inverted. This hap- Although water is predominantly a co- pens in the SN2 mechanism. See nucle- valent compound, a very small amount of ˆ + ophilic substitution. It is named for the ionic dissociation occurs (H2O H + German chemist Paul Walden (1863– OH–). In every liter of water at STP there is 1957), who discovered it in 1896. approximately 10–7 mole of each ionic species. It is for this reason that, on the pH Walker, John E. (1941– ) British bio- scale, a neutral solution has a value of 7. chemist who worked on the enzyme mech- As a polar liquid, water is the most anism underlying the synthesis of ATP. He powerful solvent known. This is partly a was awarded the 1997 Nobel Prize for result of its high dielectric constant and chemistry jointly with P. D. Boyer. The partly its ability to hydrate ions. This latter prize was shared with J. C. Skou. property also accounts for the incorpora- tion of water molecules into some ionic water (H2O) A colorless liquid that crystals as water of crystallization. freezes at 0°C and, at atmospheric pres- Water is decomposed by reactive metals sure, boils at 100°C. In the gaseous state (e.g. sodium) when cold and by less active

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weber

metals (e.g. iron) when steam is passed wave. Wavelength is related to the speed over the hot metal. It is also decomposed (c) and frequency (v) thus: by electrolysis. c = vλ

water gas A mixture of carbon monox- wave mechanics A formulation of ide and hydrogen produced when steam is quantum mechanics put forward by the passed over red-hot coke or made to com- German physicist Erwin Schrödinger bine with hydrocarbons, e.g. (1887-1961) in 1926, following the sug- → C(s) + H2O(g) CO(g) + H2(g) gestion of the French physicist Louis de The amount of hydrogen can be increased broglie (1892-1987) that particles such as by combining it with the water gas shift re- electrons might also have wavelike proper- action: ties. The basic equation of wave mechanics ˆ CO + H2O CO2 + H2 is the Schrödinger equation, which is a Water gas was once an important source of wave equation describing the system. Solu- hydrogen for the production of ammonia. tions of the equation give rise to WAVE Most hydrogen is now obtained from FUNCTIONS. See also quantum theory; methane by steam REFORMING. orbital. → CH4(g) + H2O(g) CO(g) + 3H2(g) The production of water gas using wave number Symbol: σ The reciprocal methane is an important step in the prepa- of the wavelength of a wave. It is the num- ration of hydrogen for ammonia synthesis. ber of wave cycles in unit distance, and is Compare producer gas. often used in spectroscopy. The unit is the meter–1 (m–1). The circular wave number Watson, James Dewey (1928– ) (symbol: k) is given by: American molecular biologist. James Wat- k = 2πσ son is famous for his work in 1953 with Francis CRICK on the structure of DNA. wax One of a group of water-insoluble Watson gave a controversial account of substances with a very high molecular this work in his book The Double Helix weight; they are esters of long-chain alco- (1968). He also wrote several other autobi- hols with fatty acids. Waxes form protec- ographical and popular books as well as tive coverings to leaves, stems, fruits, seeds, co-authoring textbooks on molecular biol- animal fur, and the cuticles of insects, serv- ogy. He shared the 1962 Nobel Prize for ing principally as waterproofing. For ex- medicine with Crick and Maurice WILKINS ample, waxy deposits on some plant for their discovery of the structure of DNA. organs add to the efficiency of the cuticle in reducing transpiration, as well as cutting watt Symbol: W The SI unit of power, down airflow over the surface and forming defined as a power of one joule per second. a highly reflective surface, thus reducing 1 W = 1 J s–1. The unit is named for the energy available for evaporation. They British inventor James Watt (1739–1819). may also occur in plant cell walls, e.g. leaf mesophyll. They are used in varnishes, pol- wave function Symbol: Ψ A function ishes, and candles. that describes the quantum state of a system in WAVE MECHANICS. The physical sig- weak acid An ACID that is not fully dis- nificance of the wave function for a particle sociated in solution. is that the square of its absolute value at a point is proportional to the probability of weak base A base that is not fully disso- finding the particle in a small element of ciated in solution. See acid. volume, dxdydz, at that point. See also orbital. weber Symbol: Wb The SI unit of magnetic flux, equal to the magnetic flux that, wavelength Symbol: λ The distance be- linking a circuit of one turn, produces an tween the ends of one complete cycle of a e.m.f. of one volt when reduced to zero at

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white spirit

uniform rate in one second. 1 Wb = 1 V s. This compound is known as Wilkinson’s The unit is named for the German physicist catalyst. It is used as a catalyst in the hy- Wilhelm Weber (1804–91). drogenation of alkenes. Wilkinson shared the 1973 Nobel Prize for chemistry with white spirit A liquid hydrocarbon re- Ernst Otto Fischer for their work on sand- sembling kerosene obtained from petro- wich compounds. leum, used as a solvent and in the manufacture of paints and varnishes. Williamson’s synthesis 1. A method of preparing simple ethers by dehydration Wieland, Heinrich Otto (1877–1957) of alcohols with concentrated sulfuric acid. German organic chemist. Much of The reaction is carried out at 140°C under Wieland’s early work was devoted to the reflux with an excess of the alcohol: → organic compounds of nitrogen. He started 2ROH ROR + H2O to study the bile acids in 1912. He found The concentrated sulfuric acid both cat- that three of these acids: cholic acid, de- alyzes the reaction and displaces the equi- oxycholic acid, and lithocholic acid, are all librium to the right. Also the ether may be steroids and are all related to cholesterol. distilled off during the reaction (in which This led him to propose a structure of cho- case it is called Wilkinson’s continuous lesterol. He won the 1927 Nobel Prize for process). The product, ether, is termed chemistry for his work on steroids. How- ‘simple’, because the R groups are identi- ever, subsequent work by Wieland and his cal. There are two possible mechanisms for colleagues in 1932 showed that the struc- the process, depending on the nature of the ture he proposed was incorrect, leading to alcohol. In the case of primary alcohols, the now generally accepted modified struc- there is a hydrogensulfate formed. For ex- ture being proposed by Wieland et al. ample, with ethanol: ˆ C2H5OH + H2SO4 C2H5O.SO2.OH Wilkins, Maurice Hugh Freder- + H2O ick (1916– ) New Zealand-born With another alcohol molecule, an oxo- British molecular biologist. Maurice nium ion is formed: → Wilkins was one of the key figures in the C2H5OH + C2H5O.SO2.OH + – determination of the structure of DNA. He (C2H5)2OH + HSO4 was originally a physicist but turned to bio- The oxonium ion loses a proton to give the physics after the end of World War II. He ether: + – → began to study DNA by x-ray diffraction. (C2H5)2OH + HSO4 C2H5O + Some of the x-ray diffraction pictures pro- H2SO4 duced by his colleague Rosalind FRANKLIN In the case of tertiary alcohols the first provided essential clues to Francis CRICK step is production of a carbocation. For ex- and James WATSON in their search for the ample, with isobutanol (2-methylpronan- structure of DNA. Wilkins shared the 1962 2-ol; (CH3)3COH)): + → + Nobel Prize for Medicine with Crick and (CH3)3COH + H H2O + (CH3)3C Watson. Wilkins also determined the struc- In such cases the ion is stabilized by the ture of ribonucleic acid (RNA) using x-ray alkyl groups. The ion is attached by the diffraction. In 2003 Wilkins published his lone pair on the oxygen of another alcohol autobiography. molecule to form an oxonium ion: + → (CH3)3C + (CH3)3COH + Wilkinson, Sir Geoffrey (1921–96) ((CH3)3C)2OH British chemist. Wilkinson did a lot of no- As in the above mechanism, this loses a table work on ‘sandwich compounds’, i.e. proton to give the ether: + → molecules such as ferrocene in which an ((CH3)3C)2OH (CH3)3C.O.C(CH3)3 iron atom is sandwiched between two car- + H+ bon rings which each have five sides. 2. A method for the preparation of mixed Wilkinson studied many organo-metallic ethers by nucleophilic substitution. A compounds including RhCl(P(C6H5)3). haloalkane is refluxed with an alcoholic 224 iranchembook.ir/edu

Woodward–Hoffmann rules

solution of sodium alkoxide (from sodium Wöhler’s synthesis A synthesis of urea dissolved in alcohol): by evaporating the inorganic compound 1 2 – + 1 2 + – R Cl + R O Na → R OR + NaCl ammonium cyanate (NH4 NCO ), per- The product ether is termed ‘mixed’ if the formed in 1828 by Friedrich WÖHLER. At alkyl groups R1 and R2 are different. This the time it was believed that there was a synthesis can be used to produce both sim- distinction between organic and inorganic ple and mixed ethers. compounds in that organic compounds Both reactions are named for the British could be made only be living organisms (an chemist Alexander Williamson (1824– idea known as vitalism). Urea is present in 1904). the urine of mammals and was regarded as definitely ‘organic’ in this sense. Ammo- will-o’-the-wisp See ignis fatuus. nium cyanate is a definite inorganic compound. The discovery is sometimes said to Willstätter, Richard (1872–1942) Ger- mark the death of vitalism, although, in man organic chemist. Willstätter’s early fact, it was many years before the idea was work was concerned with the structure of finally abandoned. alkaloids such as atropine and cocaine. In 1905 he started the work on plant pig- wood alcohol See methanol. ments such as chlorophyll for which he is best known. Using the technique of chro- Woodward, Robert Burns (1917–79) matography he was able to establish that American organic chemist. Robert Wood- ward and his colleagues synthesized many chlorophyll is not a single compound. He complicated molecules. In 1944 together was able to work out the chemical formu- with William von Eggers Doering, Wood- lae of these compounds. He won the 1915 ward succeeded in synthesizing quinine. In Nobel Prize for chemistry for his work on the late 1940s he determined the structures plant pigments. of penicilin and strychnine. Among the molecules he synthesized were cholesterol Windaus, Adolf Otto Reinhold (1876– and cortisone, lysergic acid (1954), chloro- 1959) German chemist. He was awarded phyll (1960), and vitamin B (1971). His the Nobel Prize for chemistry in 1928 for 12 work on vitamin B led Woodward and his research into the constitution of the 12 Roald HOFFMANN to put forward the sterols and their connection with the vita- Woodward–Hoffmann rules governing the mins. conservation of orbital symmetry in the mid 1960s. Woodward and Hoffmann Wöhler, Friedrich (1800–82) German gave an account of this work in the book chemist. Wöhler is most famous for his dis- Conservation of Orbital Symmetry (1970). covery in 1828 that urea can be made by Woodward won the 1965 Nobel Prize for heating ammonium thiocyanate. The sig- chemistry for his work in synthesizing nificance of this discovery was that it was complex organic molecules. He would the first time an organic substance had have shared another Nobel Prize with been synthesized in the laboratory. Previ- Hoffmann if he had not died in 1979. ously there was a widespread view, called vitalism, that organic substances could be Woodward–Hoffmann rules A set of synthesized only by living organisms. To- rules used in analyzing the progress of PER- gether with Justus von LIEBIG, he was re- ICYCLIC REACTIONS. They are based on a sponsible for a number of important fundamental analysis of the symmetry of discoveries in organic chemistry such as the p orbitals in the reactants and how it isomerism. Wöhler also made significant correlates with the symmetry of the prod- discoveries in inorganic chemistry. This in- ucts. It is an alternative to the related FRON- cluded the isolation of aluminium (1827) TIER ORBITAL theory. This approach was and beryllium (1828). put forward in the 1960s by Woodward

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Wurtz–Fittig reaction

and the Polish–American theoretical compounds. For example, to obtain chemist Roald HOFFMANN. methylbenzene: → C6H5Cl + CH3Cl + 2Na C6H5CH3 + Wurtz–Fittig reaction See Wurtz reac- 2NaCl tion. In this mixed reaction phenylbenzene (C6H5C6H5) and ethane (CH3CH3) are Wurtz reaction A reaction for prepar- also produced by side reactions. ing alkanes by refluxing a haloalkane (RX) The Wurtz reaction is named for the with sodium metal in dry ether: French chemist Charles Adolphe Wurtz 2RX + 2Na → RR + 2NaX (1817–84), who developed the method in The reaction involves the coupling of two 1855. The Fittig reaction is named for alkyl radicals. The Fittig reaction is a simi- Rudolph Fittig (1835–1910), a German lar process for preparing alkyl-benzene hy- chemist who worked with Wurtz. It is drocarbons by using a mixture of halogen often called the Wurtz–Fittig reaction.

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mally produced by absorbing high-energy xanthate A salt or ester of XANTHIC ACID electrons in matter. The radiation can pass containing the ion –SCS(OR) or the group through matter to some extent (hence its –SCS(OR) (where R is an organic group). use in medicine and industry for investigat- Cellulose xanthate is used to make RAYON. ing internal structures). It can be detected with photographic emulsions and devices xanthene (CH2(C6H4)2O) A yellow like the Geiger-Müller tube. crystalline organic compound, used in X-ray photons result from electronic making dyestuffs and fungicides. transitions between the inner energy levels of atoms. When high-energy electrons are xanthic acid (HSCS(OR)) Any of sev- absorbed by matter, an x-ray line spectrum eral unstable organic acids (where R is an results. The structure depends on the sub- organic group). The esters and salts of xan- stance and is thus used in x-ray spec- thic acid have various industrial applica- troscopy. The line spectrum is always tions. See xanthate. formed in conjunction with a continuous background spectrum. The minimum (cut- λ xanthine (2,6-dioxypurine; C5H4N2O2) off) wavelength 0 corresponds to the max- A poisonous colorless crystalline organic imum x-ray energy, Wmax. This equals the compound that occurs in blood, coffee maximum energy of electrons in the beam beans, potatoes, and urine. It is used as a producing the x-rays. Wavelengths in the λ chemical intermediate. continuous spectrum above 0 are caused by more gradual energy loss by the elec- xanthone (dibenzo-4-pyrone; CO- trons, in the process called bremsstrahlung (C6H4)2O) A colorless crystalline organic (braking radiation). compound found as a pigment in gentians and other flowers. It is used as an insecti- x-ray crystallography The study of the cide and in making dyestuffs. internal structure of crystals using the technique of x-ray diffraction. xanthophyll One of a class of yellow to orange pigments derived from carotene, x-ray diffraction A technique used to the commonest being lutein. See carot- determine crystal structure by directing enoids; photosynthetic pigments. x-rays at the crystals and examining the diffraction patterns produced. At certain xanthoproteic test A standard test for angles of incidence a series of spots are pro- proteins. Concentrated nitric acid is added duced on a photographic plate; these spots to the test solution. A yellow precipitate are caused by interaction between the produced either immediately or on gentle x-rays and the planes of the atoms, ions, or heating indicates a positive result. molecules in the crystal lattice.

x-radiation An energetic form of elec- x-rays See x-radiation. tromagnetic radiation. The wavelength range is 10–11 m to 10–8 m. X-rays are nor- xylene See dimethylbenzene.

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ylid (ylide) A type of ZWITTERION in 10–24. For example, 1 yoctometer (ym) = which the two charges are on adjacent 10–24 meter (m). atoms. yotta- Symbol: Y A prefix denoting ylide See ylid. 1024. For example, 1 yottameter (Ym) = yocto- Symbol: y A prefix denoting 1024 meter (m).

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Zeisel reaction The reaction of an ether zero point energy The energy pos- with excess concentrated hydroiodic acid. sessed by the atoms and molecules of a sub- On refluxing, a mixture of iodoalkanes is stance at absolute zero (0 K). formed: ′ → ′ zetta- Symbol: Z A prefix denoting 1021. ROR + 2HI H2O + RI + R I 21 Analysis to identify the iodoalkanes For example, 1 zettameter (Zm) = 10 gives information about the composition meter (m). of the original ether. It was developed by S. (1898–1973) German or- Zeisel in 1886. Ziegler, Karl ganic chemist. Karl Ziegler is best known for his research into polymers, particularly zeolite A member of a group of hydrated Ziegler–Natta catalysts. In 1953 Ziegler aluminosilicate minerals, which occur in found that catalysts consisting of nature and are also manufactured for their organometallic compounds mixed with ion-exchange and selective-absorption metals such as titanium polymerize ethene properties. They are used for water soften- into a long-chain polymer with useful ing and for sugar refining. The zeolites properties such as a high melting point. have an open crystal structure and can be This method of producing polymers does used as molecular sieves. not need high temperatures or pressures. See also ion exchange; molecular sieve. This type of catalysis was developed further by Giulio NATTA. Ziegler and Natta zepto- Symbol: z A prefix denoting shared the 1963 Nobel Prize for chemistry 10–21. For example, 1 zeptometer (zm) = for their work on Ziegler–Natta catalysts. 10–21 meter (m). Ziegler process A method for the man- zero order Describing a chemical reac- ufacture of high-density polyethene using a tion in which the rate of reaction is inde- catalyst of titanium(IV]] chloride and tri- pendent of the concentration of a reactant; ethyl aluminum (Al(C2H5)3) under slight i.e. pressure. The mechanism involves forma- rate = k[X]0 tion of titanium alkyls TiCl (C H ) The concentration of the reactant re- 3 2 5 which coordinate the sigma orbitals of tita- mains constant for a period of time al- nium with the π bond of ethene. The chain- though other reactants are being length, and henfce the density, of the consumed. The hydrolysis of 2-bromo-2- polymer can be controlled. It is named for methylpropane using aqueous alkali has a the German chemist Karl Ziegler rate expression, (1896–1973), who introduced it in 1953. rate = k[2-bromo-2-methylpropane] See also Natta process. i.e. the reaction is zero order with respect to the concentration of the alkali. The zwitterion (ampholyte ion) An ion that rate constant for a zero reaction has the has both a positive and negative charge on units mol dm–3 s–1. the same species. Zwitterions occur when a

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zwitterion

molecule contains both a basic group and can be formed by transfer of a proton from an acidic group; formation of the ion the carboxyl group to the amine group. At can be regarded as an internal acid-base low pH (acidic conditions) the ion +H N.- reaction. For example, amino-ethanoic 3 CH2.COOH is formed; at high pH (basic acid (glycine) has the formula H2N.CH2.- – COOH. Under neutral conditions it exists conditions) H2N.CH2.COO is formed. – as the zwitterion H3N.CH2.COO , which See also amino acid.

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APPENDIXES iranchembook.ir/edu iranchembook.ir/edu

Appendix I Carboxylic Acids

In the examples below, the systematic name is given first, followed by the trivial (common) name.

Simple saturated monocarboxylic acids:

methanoic formic HCOOH ethanoic acetic CH3COOH propanoic proprionic C2H5COOH butanoic butyric C3H7COOH pentanoic valeric C4H9COOH hexanoic caproic C5H11COOH heptanoic enathic C6H13COOH octanoic caprylic C7H15COOH nonanoic pelargonic C8H17COOH decanoic capric C9H19COOH

Other simple saturated acids are found in naturally occurring glycerides. They all contain even numbers of carbon atoms:

dodecanoic lauric C11H23COOH tetradecanoic myristic C13H27COOH hexadecanoic palmitic C15H31COOH octadecanoic stearic C17H35COOH eicosanoic arachidic C19H39COOH docosanoic behenic C21H43COOH tetracosanoic lignoceric C23H47COOH hexacosanoic cerotic C25H51COOH

Certain important unsaturated monocarboxylic acids occur naturally:

octadec-9-enoic oleic C8H17CH=CHC7H14COOH octadeca-9,12- dienoic linoleic C5H11CH=CHCH2CH=CHC7H1 4COOH octadeca-9,12,15- trienoic linolenic C2H5CH=CHCH2CH=CHCH2CH=CH- C 7H14COOH

There are a number of common saturated dicarboxylic acids:

ethanedioic oxalic HOOCCOOH propanedioic malonic HOOCCH2COOH butanedioic succinic HOOCC2H4COOH pentanedioic glutaric HOOCC3H6COOH hexanedioic adipic HOOCC4H8COOH

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Appendix I

Examples of unsaturated dicarboxylic acids are:

cis-butenedioic maleic HOOCCH=CHCOOH trans-butenedioic fumaric HOOCCH=CHCOOH

Certain hydroxy acids occur naturally:

hydroxyethanoic glycolic CH2(OH)COOH 2-hydroxypropanoic lactic CH3CH(OH)COOH hydroxybutanedioic malic CH(OH)CH2(COOH)2 2-hydroxy-propane-1,2,3-tricarboxylic citric (CH2)2C(OH)(COOH)3

Naturally occurring amino carboxylic acids are shown in Appendix II.

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Appendix II Amino Acids

NH CH 3 H COOH C 2NH H2 H N C 2 NH H C 2 C H COOH C H2 NH alanine arginine 2

H H2 2 O C H COOH O C H COOH C C

NH OH 2 NH2 NH2

asparagine aspartine

HOOC H H2 2 C C H COOH C H COOH C C SH H2

NH2 NH2

cysteine glutamic acid

H COOH NH 2 CH 2 C 2 C H COOH C H2 NH2 NH glutamine 2 glycine

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Appendix II Amino Acids

O CH O H2 3 C H COOH N C COOH C 3CH H C H C CH H CH NH 2 N 2 NH H 2

histidine isoleucine

H 2NH 2 H C H2 2 C CH C COOH C 3 H H H C H COOH C C 2 C H2 CH NH 3 2 NH2

leucine lysine

H H 2 C C S H 2 CH H COOH 3CH C C C H COOH H C 2 CH CH NH C 2 NH2 H

methionine phenylalanine

H 2 H C H COOH 2 C C H COOH 2CH OH C NH C NH2 H2

proline serine

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Appendix II Amino Acids

H H OH C C H2 C H COOH C CH C H H COOH 3CH C C H CH NH2 NH N 2 H threonine tryptophan

H CH H 2 3 C C CH H COOH C COOH C H H 3CH C CH NH2 OH C NH2 H

tyrosine valine

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Appendix III Sugars

Some simple monosaccharides. The β-D-form is shown in each case

H H O OH HOCH2 O OH H H OH HO HO H CH2OH OH H OH β arabinose fructose

HOCH2 HOCH2 HO O OH O OH

OH OH HO OH OH

galactose glucose

O OH HOCH2 O OH OH HO

OH OH OH

ribose xylose

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Appendix IV

Nitrogenous Bases and Nucleosides

NH2 NH2 N 6 N N N1 7 9 3 N N N N H HOCH2 O adenine

OH OH

adenosine

O O

6 N N HN175 HN 8 2 4 9 3 N N H2N N H2N N H HOCH2 O guanine

OHOH

guanosine

H3C O NH

H3C NH N O HOCH2 O N O H

OH thymine

thymidine

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Appendix IV Nitrogenous Bases and Nucleosides

NH2 NH2 N 3 N 1 N O N O H HOCH2 O cytosine

OH OH

cytidine

O O

HN HN

O N O N H HOCH O uracil 2

OH OH

uridine

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Appendix V

The Chemical Elements (* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

actinium Ac 89 227* europium Eu 63 151.965 aluminum Al 13 26.982 fermium Fm 100 257* americium Am 95 243* fluorine F 9 18.9984 antimony Sb 51 112.76 francium Fr 87 223* argon Ar 18 39.948 gadolinium Gd 64 157.25 arsenic As 33 74.92 gallium Ga 31 69.723 astatine At 85 210 germanium Ge 32 72.61 barium Ba 56 137.327 gold Au 79 196.967 berkelium Bk 97 247* hafnium Hf 72 178.49 beryllium Be 4 9.012 hassium Hs 108 265* bismuth Bi 83 208.98 helium He 2 4.0026 bohrium Bh 107 262* holmium Ho 67 164.93 boron B 5 10.811 hydrogen H 1 1.008 bromine Br 35 79.904 indium In 49 114.82 cadmium Cd 48 112.411 iodine I 53 126.904 calcium Ca 20 40.078 iridium Ir 77 192.217 californium Cf 98 251* iron Fe 26 55.845 carbon C 6 12.011 krypton Kr 36 83.80 cerium Ce 58 140.115 lanthanum La 57 138.91 cesium Cs 55 132.905 lawrencium Lr 103 262* chlorine Cl 17 35.453 lead Pb 82 207.19 chromium Cr 24 51.996 lithium Li 3 6.941 cobalt Co 27 58.933 lutetium Lu 71 174.967 copper Cu 29 63.546 magnesium Mg 12 24.305 curium Cm 96 247* manganese Mn 25 54.938 darmstadtium Ds 110 269* meitnerium Mt 109 266* dubnium Db 105 262* mendelevium Md 101 258* dysprosium Dy 66 162.50 mercury Hg 80 200.59 einsteinium Es 99 252* molybdenum Mo 42 95.94 erbium Er 68 167.26 neodymium Nd 60 144.24

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Appendix V The Chemical Elements Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

neon Ne 10 20.179 scandium Sc 21 44.956 neptunium Np 93 237.048 seaborgium Sg 106 263* nickel Ni 28 58.69 selenium Se 34 78.96 niobium Nb 41 92.91 silicon Si 14 28.086 nitrogen N 7 14.0067 silver Ag 47 107.868 nobelium No 102 259* sodium Na 11 22.9898 osmium Os 76 190.23 strontium Sr 38 87.62 oxygen O 8 15.9994 sulfur S 16 32.066 palladium Pd 46 106.42 tantalum Ta 73 180.948 phosphorus P 15 30.9738 technetium Tc 43 99* platinum Pt 78 195.08 tellurium Te 52 127.60 plutonium Pu 94 244* terbium Tb 65 158.925 polonium Po 84 209* thallium Tl 81 204.38 potassium K 19 39.098 thorium Th 90 232.038 praseodymium Pr 59 140.91 thulium Tm 69 168.934 promethium Pm 61 145* tin Sn 50 118.71 protactinium Pa 91 231.036 titanium Ti 22 47.867 radium Ra 88 226.025 tungsten W 74 183.84 radon Rn 86 222* uranium U 92 238.03 rhenium Re 75 186.21 vanadium V 23 50.94 rhodium Rh 45 102.91 xenon Xe 54 131.29 rubidium Rb 37 85.47 ytterbium Yb 70 173.04 ruthenium Ru 44 101.07 yttrium Y 39 88.906 rutherfordium Rf 104 261* zinc Zn 30 65.39 samarium Sm 62 150.36 zirconium Zr 40 91.22

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Appendix VI 18 Kr Ar Xe He Ne Rn 2 36 18 86 10 54 18 I 0 (or VIIIB) VIIIA (or 0) F Cl At Br 17 35 17 85 9 53 symbol Lr Lu 17 71 103 VIIB VIIA S O Se 16 Po Te Uuh 34 116 16 84 8 52 Yb No 16 VIB VIA 70 102 P N Bi Sb 15 As 33 115 15 83 7 51 Tm Md 15 VB VA 69 101 C Si Sn 14 Pb Ge Uuq 32 114 14 82 6 50 Er Fm 14 IVB IVA 68 100 B In Al Tl 13 Ga 31 113 13 81 5 49 Es Ho 13 IIIB IIIA 67 99 12 Cd Zn Hg Uub 30 112 80 48 Cf Dy 12 IIB IIB 66 98 11 Ag Cu Au Uuu 29 111 79 47 Bk Tb IB IB 11 65 97 Pt 10 Ni Pd Ds 28 110 78 46 Gd Cm 10 64 96 9 Ir Rh Co Mt 27 109 77 45 Eu 9 Am 63 95 8 Fe Hs Os Ru 26 108 76 44 VIII (or VIIIB) VIII (or VIIIA) - giving group, atomic number, and chemical Pu 8 Sm 62 94 7 Tc Re Bh Mn 25 107 75 43 7 Np Pm 61 93 VIIB VIIA 6 W Sg Cr Mo 24 106 74 42 U 6 Nd VIB VIA 60 92 5 V Ta Db Nb 23 105 73 41 groups. Older group designations are shown below. Pr Pa 5 VB VA 59 91 4 Ti Rf Zr Hf 22 104 72 40 The above is the modern recommended form of table using 18 Ce Th 4 IVB IVA 58 90 - Lu 3 Y Sc 21 39 Ac - Lr 57-71 La 89-103 La Ac 3 IIIB IIIA 57 89 2 Sr Be Ba Ca Ra Mg 20 88 12 56 4 38 2 IIA IIA 1 K H Li Fr Cs Rb Na Periodic Table of the Elements Actinides 1 19 87 11 55 3 37 Lanthanides

1 IA IA 6 7 3 6 2 5 1 4 Period 7 European convention Modern form N. American convention

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Appendix VII

The Greek Alphabet

A α alpha N ν nu B β beta Ξξxi Γγgamma O ο omikron ∆δdelta Ππ pi E ε epsilon P ρ rho Z ζ zeta Σσsigma H η eta T τ tau Θθ theta Υυupsilon I ι iota Φφ phi K κ kappa X χ chi Λλlambda Ψψpsi M µ mu Ωωomega

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Appendix VIII

Fundamental Constants

speed of light c 2.997 924 58 × 108 m s–1 µ π × –7 permeability of free space o 4 10 = 1.256 637 0614 × 10–6 H m–1 ε µ –1 –2 × –12 –1 permittivity of free space 0= 0 c 8.854 187 817 10 F m charge of electron or proton e ±1.602 177 33 × 10–19 C × –31 rest mass of electron me 9.109 39 10 kg × –27 rest mass of proton mp 1.672 62 10 kg × –27 rest mass of neutron mn 1.674 92 10 kg electron charge-to-mass ratio e/m 1.758 820 × 1011 C kg–1 × –15 electron radius re 2.817 939 10 m Planck constant h 6.626 075 × 10–34 J s Boltzmann constant k 1.380 658 × 10–23 J K–1 Faraday constant F 9.648 531 × 104 C mol–1

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Appendix IX

Webpages

Chemical society webpages include:

American Chemical Society www.chemistry.org/

Royal Society of Chemistry www.rsc.org/

The International Union of Pure and www.iupac.org Applied Chemistry

Information on nomenclature is available at:

Queen Mary College, London www.chem.qmul.ac.uk/iupac/

Advanced Chemistry Development, Inc www.acdlabs.com/iupac/nomenclature/

An extensive set of organic chemistry links can be found at:

WWW Virtual Library, Chemistry Section www.liv.ac.uk/Chemistry/Links/

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Bibliography

There are a number of comprehensive texts covering organic chemistry. These include:

Carey, Francis & Sundberg, Richard J. Advanced Organic Chemistry: Structure and Mechanism. 4th ed. New York: Plenum, 2000

Carey, Francis & Sundberg, Richard J. Advanced Organic Chemistry: Reactions. 4th ed. New York: Plenum, 2000

Clayden, Jonathon; Greeves, Nick; Warren, Stuart; & Wothers, Peter Organic Chemistry. Oxford, U.K.: Oxford University Press, 2000

McMurray, John Organic Chemistry. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2004

March, Jerry Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 4th ed. New York: Wiley, 1992

Volhardt, Peter K. & Schore, Neil E. Organic Chemistry: Structure and Function. 4th ed. New York: W. H. Freeman, 2003

More specialized books are:

Eliel, Ernest L.; Wilen, Samuel H.; & Doyle, Michael P. Basic Organic Stereochemistry. New York: Wiley, 2001

Gilchrist, Thomas L. Heterocyclic Chemistry. 3rd ed. Harlow, U.K.: Longman, 1997

Isaacs, Neil S. Physical Organic Chemistry. Harlow, U.K.: Longman, 1987

Sykes, Peter Guidebook to Mechanism in Organic Chemistry. 6th ed. Harlow, U.K.: Longman, 1996

An advanced text on biochemistry is:

Nelson, David L. & Cox, Michael M. Lehninger Principles of Biochemistry. 3rd ed. New York: Worth Publishers, 2000

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