DOCSLIB.ORG

An Introduction to Functional Groups in Organic Chemistry What Are

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
An Introduction to Functional Groups in Organic Chemistry What Are What are Functional Groups An Introduction to Functional Functional groups are common bonding Groups in Organic Chemistry patterns found in organic molecules Examples O O C C H CH3 HO CH3 an aldehyde a carboxylic acid Reactivity of Functional Groups Reactivity of Functional Groups O O + Tollen's Reagent + silver mirror 2 Li + 2 H2O → 2 LiOH + H2 C C H CH3 HO CH3 2 Na + 2 H2O → 2 NaOH + H2 O O + silver mirror 2 K + 2 H2O → 2 KOH + H2 C + Tollen's Reagent C H CH2CH3 HO CH2CH3 1 Functional Groups Containing Reactivity of Functional Groups Carbon and Hydrogen Only O O 3 Alkanes contain only sp hybridized carbon C + Tollen's Reagent C + silver mirror atoms, and therefore possess only single H R HO R bonds. O A compound is only called an alkane if it contains + Tollen's Reagent no silver mirror none of the other functional groups. C produced HO R Alkenes contain at least one carbon-carbon double bond. O CH O 2 Alkynes contain at least one carbon-carbon C C + Tollen's Reagent ??? triple bond. H OH Cyclic Hydrocarbons An alkane which contains a ring made up of three CH CH or more atoms is called a cycloalkane. 2 3 CH3 H CC Likewise, a cycloalkene contains a carbon-carbon CH3 C CH3 H CCCH3 double bond somewhere in a ring of carbon atoms. H CH3 CH3 Cycloalkynes are relatively rare, for reasons we will see later (think bond angles). an alkane an alkene an alkyne We will not consider these as distinct functional groups We still classify a cycloalkene as an alkene a cycloalkane a cycloalkene 2 Aromatics Alkyl Halides The term aromatic has a rather detailed meaning which we will discuss next semester. Alkyl halides are compounds which have For now, when we describe a compound as containing an a halogen atom bonded to a carbon aromatic group, we mean to say that it contains a benzene ring. atom somewhere in the molecule. We are sometimes more specific, calling a compound an alkyl chloride, alkyl Three equivalent ways of representing the benzene molecule bromide, etc. Note that the benzene ring is not considered either an alkene or a cycloalkene; the special term aromatic makes the H H presence of the ring and double bonds understood. H C Cl H C Br As we shall see, aromatic compounds react in distinctly different ways than do alkenes, owing to delocalization of electrons within the ring. CH3 CH3 Alcohols Thiols and Amines Compounds which contain the hydroxyl Thiols are similar to alcohols in group -OH bonded to carbon (R–OH) structure, with a sulfur atom replacing make up the class of compounds called the oxygen atom: R–SH alcohols. 3 Amines have a nitrogen atom bonded to Technically, the carbon should be sp hybridized. at least one carbon atom; the H OH remainder of its bonds may be to carbon or hydrogen. H3CCCH3 OH 3 Thiols and Amines Ethers H OH alcohols H3CCCH3 The ether functional group is described as 3 OH an sp oxygen atom bonded to two carbon atoms, and may be symbolized as R-O-R'. H SH thiols R and R' may be the same groups or H CCCH 3 3 different. SH CH3 CH3 H N amines CH3 O C CH3 H3CCCH3 CH3 CH3CH2 O CH2CH3 O NH2 CH3 Carbonyl-Containing Functional Groups Ketones A carbonyl group is made up of a carbon atom The ketone functional group contains a carbonyl double bonded to an oxygen atom. The carbon group, with the carbon of that group bonded to atom is not bonded to any other heteroatom. two other carbon atoms. O R and R' may or may not be the same. C O the carbonyl group C R R' We will not consider the carbonyl group to be a functional group in-and-of itself; rather, it is the a ketone essential portion of two functional groups: aldehydes and ketones. 4 Aldehydes Carboxylic Acids The aldehyde functional group contains a A carboxyl group looks similar to a carbonyl group, but at carbonyl group, with the carbon of that group least one of its sigma bonds is to a second oxygen atom. O bonded to at least one hydrogen atom. The O C C other bond is either to a carbon atom or another O OH hydrogen atom. the carboxyl group carboxylic acid R is an alkyl group or another hydrogen atom in The carboxylic acid functional group has a hydrogen atom the case of formaldehyde. bonded to this oxygen atom. It is generally acidic, for O reasons we discussed in Chapter One. The other bond need not be specified; it may be an alkyl C R H group, a hydrogen atom, or another –OH. an aldehyde Carboxylic Acid Derivatives Carboxylic Acid Derivatives There are several functional groups which are The anhydride functional group is a two-sided structurally similar to carboxylic acids. carboxyl group: O O The ester functional group has an alkyl group in place of the hydrogen bonded to the carboxylic C C oxygen atom. O R O R' C an anhydride R OR' an ester R and R' may or may not be the same. R and R' may or may not be the same. 5 Carboxylic Acid Derivatives Carboxylic Acid Derivatives 3 An amide has an sp hybridized nitrogen atom in The acid chloride (or acyl chloride) functional place of the carboxylic oxygen atom. group has a chlorine atom in place of the O carboxylic oxygen O C R N C R Cl an amide an acid chloride The nitrogen atom is bonded to 0-2 alkyl groups. The remaining bonds are to hydrogen. Do not confuse amides with amines! Practice Practice—Solutions Circle and label all the functional groups in the molecules below. ether ketone OH CH3 OH CH3 O alcohol O CH2 CH O C C CH2 CH O C C CH3 CH3 Cl CH3 ester Cl CH3 aldehyde O O O O alkyl halide H H alkene O CH3 O CH3 alkyne C CH C CH O N O N CH thiol CH SH 3 ether SH 3 H3C H3C alkyl halide amine F F 6.
Load more

Recommended publications
  • Nomenclature of Carboxylic Acid Derivatives Acid Halide Substituents
    Gentilucci, Carboxylic Acid Derivatives Nomenclature of Carboxylic Acid Derivatives Gentilucci, Carboxylic Acid Derivatives Acid halides 1. Alkane + the suffix -oyl followed by the halogen. 2. Select the longest continuous carbon chain, containing the acyl group. 3. Number the carbon chain, beginning at the end nearest to the acyl group. 4. Number the substituents and write the name, listing substituents alphabetically. Acid halide substituents attached to rings are named using the suffix - carbonyl. 1 Gentilucci, Carboxylic Acid Derivatives Anhydrides 1. Symmetrical: replace the ending "acid" with "anhydride ". 2. Asymmetrical: select the longest continuous carbon chain, containing the carboxylic acid group, and derive the parent name by replacing the -e ending with -oic anhydride . 3. Number the carbon chain, beginning at the end nearest to the acyl group. 4. Number the substituents and write the name, listing substituents alphabetically. Gentilucci, Carboxylic Acid Derivatives Amides are named by replacing the ending -oic acid with -amide . 1. Select the longest continuous carbon chain, containing the acyl group, and derive the parent name by replacing the -e ending with -amide . 2. Number the carbon chain, beginning at the end nearest to the acyl group. 3. Number the substituents and write the name, listing substituents alphabetically. 4. If the nitrogen atom is further substituted, the substituents are preceded by N- to indicate that they are attached to the nitrogen. Acid halide substituents attached to rings are named using the suffix - carboxamide. 2 Gentilucci, Carboxylic Acid Derivatives Carboxylate esters 1. Select the longest continuous carbon chain containing the acyl group, and derive the parent name by replacing the -e ending with –oate .
    [Show full text]
  • Carbonyl Compounds
    Carbonyl Compounds What are Carbonyl Compounds? Carbonyl compounds are compounds that contain the C=O (carbonyl) group. Preparation of Aldehydes: 1. Preparation from Acid Chloride (Rosenmund Reduction): This reaction was named after Karl Wilhelm Rosenmund, who first reported it in 1918. The reaction is a hydrogenation process in which an acyl chloride is selectively reduced to an aldehyde. The reaction, a hydrogenolysis, is catalysed by palladium on barium sulfate, which is sometimes called the Rosenmund catalyst. 2. Preparation from Nitriles: This reaction involves the preparation of aldehydes (R-CHO) from nitriles (R- CN) using SnCl2 and HCl and quenching the resulting iminium salt ([R- + − CH=NH2] Cl ) with water (H2O). During the synthesis, ammonium chloride is also produced. The reaction is known as Stephen Aldehyde synthesis. Dr. Sumi Ganguly Page 1 3. Preparation from Grignard Reagent: When Grignard Reagent is reacted with HCN followed by hydrolysis aldehyde is produced. Preparation of Ketones: 1. Preparation from Acid Chloride (Friedel-Crafts Acylation): Acid chlorides when reacted with benzene in presence of anhydrous AlCl3, aromatic ketone are produced. However, only aromatic ketones can be prepared by following this method. In order to prepare both aromatic and aliphatic ketones acid chlorides is reacted with lithium dialkylcuprate (Gilman Reagnt). Dr. Sumi Ganguly Page 2 The lithium dialkyl cuprate is produced by the reaction of two equivalents of the organolithium reagent with copper (I) iodide. Example: 3. Preparation from Nitriles and Grignard Reagents: When Grignard Reagent is reacted with RCN followed by hydrolysis aldehyde is produced. Dr. Sumi Ganguly Page 3 Physical Characteristic of Carbonyl Compounds: 1) The boiling point of carbonyl compounds is higher than the alkanes with similar Mr.
    [Show full text]
  • Aldehydes and Ketones
    12 Aldehydes and Ketones Ethanol from alcoholic beverages is first metabolized to acetaldehyde before being broken down further in the body. The reactivity of the carbonyl group of acetaldehyde allows it to bind to proteins in the body, the products of which lead to tissue damage and organ disease. Inset: A model of acetaldehyde. (Novastock/ Stock Connection/Glow Images) KEY QUESTIONS 12.1 What Are Aldehydes and Ketones? 12.8 What Is Keto–Enol Tautomerism? 12.2 How Are Aldehydes and Ketones Named? 12.9 How Are Aldehydes and Ketones Oxidized? 12.3 What Are the Physical Properties of Aldehydes 12.10 How Are Aldehydes and Ketones Reduced? and Ketones? 12.4 What Is the Most Common Reaction Theme of HOW TO Aldehydes and Ketones? 12.1 How to Predict the Product of a Grignard Reaction 12.5 What Are Grignard Reagents, and How Do They 12.2 How to Determine the Reactants Used to React with Aldehydes and Ketones? Synthesize a Hemiacetal or Acetal 12.6 What Are Hemiacetals and Acetals? 12.7 How Do Aldehydes and Ketones React with CHEMICAL CONNECTIONS Ammonia and Amines? 12A A Green Synthesis of Adipic Acid IN THIS AND several of the following chapters, we study the physical and chemical properties of compounds containing the carbonyl group, C O. Because this group is the functional group of aldehydes, ketones, and carboxylic acids and their derivatives, it is one of the most important functional groups in organic chemistry and in the chemistry of biological systems. The chemical properties of the carbonyl group are straightforward, and an understanding of its characteristic reaction themes leads very quickly to an understanding of a wide variety of organic reactions.
    [Show full text]
  • Aldehydes and Ketones Are Simple Organic Compounds Containing a Carbonyl Group
    Aldehydes and Ketones are simple organic compounds containing a carbonyl group. Carbonyl group contains carbon- oxygen double bond. These organic compounds are simple because the carbon atom presents in the carbonyl group lack reactive groups such as OH or Cl. By Dr. Sayed Hasan Mehdi Assistant Professor Department of Chemistry Shia P.G. College, Lucknow Dr. S.Hasan Mehdi 6/13/2020 This is to bring to kind notice that the matter of this e- content is for the B.Sc. IV semester students. It has been taken from the following sources. The students are advised to follow these books as well. •A TEXTBOOK OF ORGANIC CHEMISTRY by Arun Bahl & B.S. Bahl, S. Chand & Company Ltd. Publication. •Graduate Organic Chemistry by M. K. Jain and S.C. Sharma, Vishal Publishing Co. •Pradeep’s Organic Chemistry Vol II by R. N. Dhawan, Pradeep Publication, Jalandhar. Dr. S.Hasan Mehdi 6/13/2020 An aldehyde is one of the classes of carbonyl group- containing alkyl group on one end and hydrogen on the other end. The R and Ar denote alkyl or aryl member respectively. In the condensed form, the aldehyde is written as –CHO. Dr. S.Hasan Mehdi 6/13/2020 Dr. S.Hasan Mehdi 6/13/2020 1. From Alcohols: a. By oxidation of Alcohols: Aldehydes and ketones can be prepared by the controlled oxidation of primary and secondary alcohols using an acidified solution of potassium dichromate or permanganate. Primary alcohol produces aldehydesRef. Last slide. O K Cr O RCH2OH + [O] 2 2 7 + R C H H 10 Alcohol Aldehyde O CH3CH2OH+ [O] K2Cr2O7 + CH3 C H H Ethyl Alcohol Acetaldehyde The aldehydes formed in the above reaction are very easily oxidised to carboxylc acids if allowed to remain in the reaction mixture.
    [Show full text]
  • The Relative Rates of Thiol–Thioester Exchange and Hydrolysis for Alkyl and Aryl Thioalkanoates in Water
    Orig Life Evol Biosph (2011) 41:399–412 DOI 10.1007/s11084-011-9243-4 PREBIOTIC CHEMISTRY The Relative Rates of Thiol–Thioester Exchange and Hydrolysis for Alkyl and Aryl Thioalkanoates in Water Paul J. Bracher & Phillip W. Snyder & Brooks R. Bohall & George M. Whitesides Received: 14 April 2011 /Accepted: 16 June 2011 / Published online: 5 July 2011 # Springer Science+Business Media B.V. 2011 Abstract This article reports rate constants for thiol–thioester exchange (kex), and for acid- mediated (ka), base-mediated (kb), and pH-independent (kw) hydrolysis of S-methyl thioacetate and S-phenyl 5-dimethylamino-5-oxo-thiopentanoate—model alkyl and aryl thioalkanoates, respectively—in water. Reactions such as thiol–thioester exchange or aminolysis could have generated molecular complexity on early Earth, but for thioesters to have played important roles in the origin of life, constructive reactions would have needed to compete effectively with hydrolysis under prebiotic conditions. Knowledge of the kinetics of competition between exchange and hydrolysis is also useful in the optimization of systems where exchange is used in applications such as self-assembly or reversible binding. For the alkyl thioester S-methyl thioacetate, which has been synthesized in −5 −1 −1 −1 −1 −1 simulated prebiotic hydrothermal vents, ka = 1.5×10 M s , kb = 1.6×10 M s , and −8 −1 kw = 3.6×10 s . At pH 7 and 23°C, the half-life for hydrolysis is 155 days. The second- order rate constant for thiol–thioester exchange between S-methyl thioacetate and 2- −1 −1 sulfonatoethanethiolate is kex = 1.7 M s .
    [Show full text]
  • Chapter 7, Haloalkanes, Properties and Substitution Reactions of Haloalkanes Table of Contents 1
    Chapter 7, Haloalkanes, Properties and Substitution Reactions of Haloalkanes Table of Contents 1. Alkyl Halides (Haloalkane) 2. Nucleophilic Substitution Reactions (SNX, X=1 or 2) 3. Nucleophiles (Acid-Base Chemistry, pka) 4. Leaving Groups (Acid-Base Chemistry, pka) 5. Kinetics of a Nucleophilic Substitution Reaction: An SN2 Reaction 6. A Mechanism for the SN2 Reaction 7. The Stereochemistry of SN2 Reactions 8. A Mechanism for the SN1 Reaction 9. Carbocations, SN1, E1 10. Stereochemistry of SN1 Reactions 11. Factors Affecting the Rates of SN1 and SN2 Reactions 12. --Eliminations, E1 and E2 13. E2 and E1 mechanisms and product predictions In this chapter we will consider: What groups can be replaced (i.e., substituted) or eliminated The various mechanisms by which such processes occur The conditions that can promote such reactions Alkyl Halides (Haloalkane) An alkyl halide has a halogen atom bonded to an sp3-hybridized (tetrahedral) carbon atom The carbon–chlorine and carbon– bromine bonds are polarized because the halogen is more electronegative than carbon The carbon-iodine bond do not have a per- manent dipole, the bond is easily polarizable Iodine is a good leaving group due to its polarizability, i.e. its ability to stabilize a charge due to its large atomic size Generally, a carbon-halogen bond is polar with a partial positive () charge on the carbon and partial negative () charge on the halogen C X X = Cl, Br, I Different Types of Organic Halides Alkyl halides (haloalkanes) sp3-hybridized Attached to Attached to Attached
    [Show full text]
  • Esters Introduction Structurally, an Ester Is a Compound That Has an Alkoxy (OR) Group Attached to the Carbonyl Group
    Esters Introduction Structurally, an ester is a compound that has an alkoxy (OR) group attached to the carbonyl group. O R C O R' R may be H, alkyl or aryl, while R’ may be alkyl or aryl only. Esters are widespread in nature. Many of the fragrances of flowers and fruits are due to the esters present. Ethyl butanoate is the chief component that accounts for the pineapple-like aroma and flavour of pineapples. 1:18 PM 1 Nomenclature of Esters Names of esters consist of two words that reflect the composite structure of the ester. The first word is derived from the alkyl group of the alcohol component, and the second word from the carboxylate group of the carboxylic acid component of the ester. The name of the carboxylate portion is derived by substituting the -ic acid suffix of the parent carboxylic acid with the –ate suffix. The alkyl group is cited first followed by the carboxylate group separated by a space. An ester is thus named as an alkyl 1:18 PM alkanoate. 2 IUPAC Nomenclature of Esters Examples 1:18 PM 3 Synthesis of Esters Preparative Strategies Highlighted below are some of the most common strategies by which esters are prepared. The esters are commonly prepared from the reaction of carboxylic acids, acid chlorides and acid anhydrides with alcohols. 1:18 PM 4 Synthesis of Esters Acid-Catalysed Esterification of a Carboxylic Acid and an Alcohol The acid-catalysed reaction of carboxylic acids and alcohols provides esters. Typically, a catalytic amount of a strong inorganic (mineral) acid such as H2SO4, HCl and H3PO4 is used.
    [Show full text]
  • Indium Promoted-Convenient Method for Acylation of Alc이iols with Acyl Chlorides
    Communications to the Editor Bull. Korean Chem. Soc. 2003, Vol. 24, No. 2 155 Indium Promoted-Convenient Method for Acylation of Alc이iols with Acyl Chlorides Dae Hyan Cho, Joong Gon Kim,f and Doo Ok Jang* Department of Chemistry, Yonsei University, Wonju 220-710, Korea ‘Biotechnology Division, Hanwha Chemical R & D Center, Daejeon 305-345, Korea Received November 9, 2002 Key Words : Indium, Alcohol, Acylation, Acyl chloride Even though various reagents for coupling of alcohols tions for the acylation of alcohols with acyl chlorides in the with carboxylic acids and transesterification of esters have presence of indium. The results are summarized in Table 1. been developed,1 there is still a great demand for a process Reaction of 2 (1 equiv) with 1 (1 equiv) in the presence of by using acyl chlorides for the acylation of alcohols in the indium (1 equiv) in CH3CN at room temperature produced case of substrates having steric hindrance or low reactivity. the corresponding ester in only 21% yield and the starting The acylation of alcohols with acyl chlorides is commonly acyl chloride and alcohol were recovered. The optimal yield carried out in the presence of tertiary amines such as 4- of the ester was attained with 3 equiv of 1 or 2 in the presence (methylamino) pyridine or 4 -pyrrolidinopyridine.2 Many of 3 equiv of indium. The solvent effect of acylation of 2 methods for the acylation of alcohols with acyl chlorides with 1 in the presence of indium was studied. The reaction have been developed using a variety of reagents.3 Most proceeded efficiently in common organic solvents such as recently, benzoylation of alcohols with lithium perchlorate DMF, Et2。，THF or CHzCh whereas non-polar solvents hase been reported.4 However, these methods have their own such as n-hexane or benzene gave poor yields of the ester.
    [Show full text]
  • Fermentation and Ester Taints
    Fermentation and Ester Taints Anita Oberholster Introduction: Aroma Compounds • Grape‐derived –provide varietal distinction • Yeast and fermentation‐derived – Esters – Higher alcohols – Carbonyls – Volatile acids – Volatile phenols – Sulfur compounds What is and Esters? • Volatile molecule • Characteristic fruity and floral aromas • Esters are formed when an alcohol and acid react with each other • Few esters formed in grapes • Esters in wine ‐ two origins: – Enzymatic esterification during fermentation – Chemical esterification during long‐term storage Ester Formation • Esters can by formed enzymatically by both the plant and microbes • Microbes – Yeast (Non‐Saccharomyces and Saccharomyces yeast) – Lactic acid bacteria – Acetic acid bacteria • But mainly produced by yeast (through lipid and acetyl‐CoA metabolism) Ester Formation Alcohol function Keto acid‐Coenzyme A Ester Ester Classes • Two main groups – Ethyl esters – Acetate esters • Ethyl esters = EtOH + acid • Acetate esters = acetate (derivative of acetic acid) + EtOH or complex alcohol from amino acid metabolism Ester Classes • Acetate esters – Ethyl acetate (solvent‐like aroma) – Isoamyl acetate (banana aroma) – Isobutyl acetate (fruit aroma) – Phenyl ethyl acetate (roses, honey) • Ethyl esters – Ethyl hexanoate (aniseed, apple‐like) – Ethyl octanoate (sour apple aroma) Acetate Ester Formation • 2 Main factors influence acetate ester formation – Concentration of two substrates acetyl‐CoA and fusel alcohol – Activity of enzyme responsible for formation and break down reactions • Enzyme activity influenced by fermentation variables – Yeast – Composition of fermentation medium – Fermentation conditions Acetate/Ethyl Ester Formation – Fermentation composition and conditions • Total sugar content and optimal N2 amount pos. influence • Amount of unsaturated fatty acids and O2 neg. influence • Ethyl ester formation – 1 Main factor • Conc. of precursors – Enzyme activity smaller role • Higher fermentation temp formation • C and N increase small effect Saerens et al.
    [Show full text]
  • Chapter 14 – Aldehydes and Ketones
    Chapter 14 – Aldehydes and Ketones 14.1 Structures and Physical Properties of Aldehydes and Ketones Ketones and aldehydes are related in that they each possess a C=O (carbonyl) group. They differ in that the carbonyl carbon in ketones is bound to two carbon atoms (RCOR’), while that in aldehydes is bound to at least one hydrogen (H2CO and RCHO). Thus aldehydes always place the carbonyl group on a terminal (end) carbon, while the carbonyl group in ketones is always internal. Some common examples include (common name in parentheses): O O H HH methanal (formaldehyde) trans-3-phenyl-2-propenal (cinnamaldehyde) preservative oil of cinnamon O O propanone (acetone) 3-methylcyclopentadecanone (muscone) nail polish remover a component of one type of musk oil Simple aldehydes (e.g. formaldehyde) typically have an unpleasant, irritating odor. Aldehydes adjacent to a string of double bonds (e.g. 3-phenyl-2-propenal) frequently have pleasant odors. Other examples include the primary flavoring agents in oil of bitter almond (Ph- CHO) and vanilla (C6H3(OH)(OCH3)(CHO)). As your book says, simple ketones have distinctive odors (similar to acetone) that are typically not unpleasant in low doses. Like aldehydes, placing a collection of double bonds adjacent to a ketone carbonyl generally makes the substance more fragrant. The primary flavoring agent in oil of caraway is just a such a ketone. 2 O oil of carraway Because the C=O group is polar, small aldehydes and ketones enjoy significant water solubility. They are also quite soluble in typical organic solvents. 14.2 Naming Aldehydes and Ketones Aldehydes The IUPAC names for aldehydes are obtained by using rules similar to those we’ve seen for other functional groups (e.g.
    [Show full text]
  • An Indicator of Triplet State Baird-Aromaticity
    inorganics Article The Silacyclobutene Ring: An Indicator of Triplet State Baird-Aromaticity Rabia Ayub 1,2, Kjell Jorner 1,2 ID and Henrik Ottosson 1,2,* 1 Department of Chemistry—BMC, Uppsala University, Box 576, SE-751 23 Uppsala, Sweden; [email protected] (R.A.); [email protected] (K.J.) 2 Department of Chemistry-Ångström Laboratory Uppsala University, Box 523, SE-751 20 Uppsala, Sweden * Correspondence: [email protected]; Tel.: +46-18-4717476 Received: 23 October 2017; Accepted: 11 December 2017; Published: 15 December 2017 Abstract: Baird’s rule tells that the electron counts for aromaticity and antiaromaticity in the first ππ* triplet and singlet excited states (T1 and S1) are opposite to those in the ground state (S0). Our hypothesis is that a silacyclobutene (SCB) ring fused with a [4n]annulene will remain closed in the T1 state so as to retain T1 aromaticity of the annulene while it will ring-open when fused to a [4n + 2]annulene in order to alleviate T1 antiaromaticity. This feature should allow the SCB ring to function as an indicator for triplet state aromaticity. Quantum chemical calculations of energy and (anti)aromaticity changes along the reaction paths in the T1 state support our hypothesis. The SCB ring should indicate T1 aromaticity of [4n]annulenes by being photoinert except when fused to cyclobutadiene, where it ring-opens due to ring-strain relief. Keywords: Baird’s rule; computational chemistry; excited state aromaticity; Photostability 1. Introduction Baird showed in 1972 that the rules for aromaticity and antiaromaticity of annulenes are reversed in the lowest ππ* triplet state (T1) when compared to Hückel’s rule for the electronic ground state (S0)[1–3].
    [Show full text]
  • 8.6 Acidity of Alcohols and Thiols 355
    08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 355 8.6 ACIDITY OF ALCOHOLS AND THIOLS 355 ural barrier to the passage of ions. However, the hydrocarbon surface of nonactin allows it to enter readily into, and pass through, membranes. Because nonactin binds and thus transports ions, the ion balance crucial to proper cell function is upset, and the cell dies. Ion Channels Ion channels, or “ion gates,” provide passageways for ions into and out of cells. (Recall that ions are not soluble in membrane phospholipids.) The flow of ions is essen- tial for the transmission of nerve impulses and for other biological processes. A typical chan- nel is a large protein molecule imbedded in a cell membrane. Through various mechanisms, ion channels can be opened or closed to regulate the concentration of ions in the interior of the cell. Ions do not diffuse passively through an open channel; rather, an open channel contains regions that bind a specific ion. Such an ion is bound specifically within the channel at one side of the membrane and is somehow expelled from the channel on the other side. Remark- ably, the structures of the ion-binding regions of these channels have much in common with the structures of ionophores such as nonactin. The first X-ray crystal structure of a potassium- ion channel was determined in 1998 by a team of scientists at Rockefeller University led by Prof. Roderick MacKinnon (b. 1956), who shared the 2003 Nobel Prize in Chemistry for this work. The interior of the channel contains binding sites for two potassium ions; these sites are oxygen-rich, much like the interior of nonactin.
    [Show full text]