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Home , Chemical bond

CHEMICAL BONDING  The TRANSFER or SHARING of .

In the formation of chemical compounds from elements, electrons are usually either: a. transferred from the outer shell of one to the outer shell of another atom and b. shared among the outer shells of the combining .

Ways to PRODUCE A CHEMICAL BOND: 1) ~ transfer of electrons from one atom to another.  The compound formed is called IONIC COMPOUND & the type of bond formed is an IONIC BOND.  Usually + non-metals produce ionic bonds. The compds they produce are vast lattices.

Ex. 11 Na + 17 Cl  NaCl

Electronic Configuration: Na 1s2 2s2 2p6 3s1 Cl 1s2 2s2 2p6 3s2 3p5 .. Dot Formula: Na   Cl : ..  Na atom losses 1 e, while Cl atom accepts to follow the .  Na atom becomes Na (cation) Cl atom becomes Cl ion (anion)

IONS~ are charged particles created when atoms either LOSE or GAIN ELECTRONS. CATIONS ~ are positively charged . Usually METALS. ANIONS ~ are negatively charged ions. Usually NON-METALS.

IONIZATION ~ the energy needed to remove an electron in an atom.

2) COVALENT BONDING~ sharing of electrons between two atoms, usually non-metals.  The type of bond formed is the & they produce Covalent Compounds.  Usually non-metals + non-metals produce covalent bonds.

Ex. 1H + 1H  H2

Electronic Configuration: H: 1s1 H: 1s1 Electron Dot Formula: H + H  H : H

MOLECULE~ a held together by Covalent Bonds.

SELF TEST: 1) Show the formation of a chemical bond of the following: a. Magnesium Bromide (MgBr2) d. N2 b. O2 e. NaCl c. Al2S3 f. H2 SO4

3) MULTIPLE BONDING~ a bond of electrons attained in several types of bonding such as: a) Single Covalent Bond – attained in a single pair.

Ex: H + H  H : H or H – H or H2

b) Double Covalent Bond – when 2 pairs of electrons are sharing between 2 nuclei represented by 2 dashes ( = ) Ex. CO2 c) Triple Covalent Bond – when 3 pairs of electrons are sharing & represented by 3 dashes ( = ) . Ex. (N2) ~ is found in the atmosphere as a diatomic . Each N atom has 5 valence electrons & need to share 3 electrons. The 3 shared pair electrons will form a by 3 dashes. Accounts for 80% of the of the gases in the atmosphere, stable; relatively UNREACTIVE. Because of being unreactive, Nitrogen is in the form of N2, useless to most forms of life. There is only 1 type of organism that utilize the atmospheric nitrogen—BACTERIA. They live on soil or in roots of plants such as peas & alfalfa. Roots—contains NODULES these nodules contain the Nitrogen-Fixing Bacteria. Using the nutrient provided through the roots, these bacteria CONVERT the Nitrogen in the air to form (NH3) or NITRATES (NO3). These compds w/c resulted upon conversion of the Nitrogen inside the roots to become compds that can be used by plants & animals then get N2 by eating the plant.

Nitrogen has 5 valence e-

CHEMICAL FORMULA A combination of elemental symbols and subscript numbers that is used to show the composition of a compound depending on the type of compound that the formula represents, the information that it provides will vary slightly whether it is a molecular compound or an ionic compound.

TYPES OF : 1. Molecular Formula – A chemical formula that denotes the constituent elements of a molecular substance & the # of atoms of each element composing 1 molecule.

Ex. C6 H6 (benzene) H2O2 ( peroxide)

2. Empirical Formula – A special type of chemical formula, that shows the composition of a molecule not as it actually exists, but in a simple whole number ratio.

Ex. CH HO Ionic compounds are composed of charged ions that are held together by electrostatic . A typical type of ionic compound, called a binary compound because it is made up of two elements, will be composed of metallic positive ions (cations) and nonmetal negative ions (anions). Another type of ionic compound, called a ternary compound as it contains three elements, is composed of monatomic ions and polyatomic ions. When dealing with ionic formulas it is very important to remember that the formula does not show how the compound actually exists in nature. It only shows the ratio by which the individual ions combine.

Ex. The ionic formula for calcium chloride is CaCl2. Since calcium chloride is an ionic compound, this formula does not mean that there are actually two chlorine atoms floating around attached to one calcium atom. Ionic compounds are actually continuous, lacking the discrete units that make up a sample of a molecular substance. Rather, the formula shows that a sample of calcium chloride contains twice as many chlorine atoms as calcium atoms. Remember that ionic compounds are not , so the formula CaCl2 is said to represent one formula unit of calcium chloride.

Molecular compounds are held together by covalent bonds, or shared pairs of electrons. Molecular formulas do show these molecules as they actually exist as discrete units in nature. Ex. When we say that the molecular formula of water is H2O, we can see that the molecules of water are made up of three atoms; two hydrogen atoms are covalently bonded to each atom.

Isomerism - is the phenomenon whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organisations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic . are molecules that have the same chemical formula but different structural formulas.

CH3CH2 CH2 CH2 CH2CH3 or C6H14 or

CH3 I

CH3 CH2 CH CH2 CH3 or Hexane

Structural Isomerism

Structural Isomers have different structural formulae because their atoms are linked together in different ways.

It arises owing to: • arrangement of skeleton e.g. The formula C4H10 represents two possible structural formulae, and methylpropane:

• position of e.g. propan-1-ol and propan-2-ol

• different Functional groups e.g. the molecular formula C2H60 represents both and methoxymethane.

ethanol Dimethyl ether ethyl alcohol, pure alcohol, grain alcohol IUPAC: methoxymethane or drinking alcohol

Cyclic are isomeric with , e.g. cyclopropane and propene

FUNCTIONAL GROUP Essentials

Many important organic chemistry molecules contain oxygen or nitrogen. It's a good idea to memorize the names and structures of these functional groups.

Benzyl acetate Has an ester functional group (in red), an acetyl moiety (circled with green) and a benzyl alcohol moiety (circled with orange). Other divisions can be made.

In organic chemistry, functional groups -- are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar (s) regardless of the size of the molecule it is a part of. However, its relative can be modified by nearby functional groups.

The word moiety is often used synonymously to "functional group," but, according to the IUPAC definition, a moiety -- is a part of a molecule that may include functional groups as substructures. For example: ester

A carboxylic acid ester. R and R' denote any alkyl( or aryl (functional)group Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, and most commonly from carboxylic acids and alcohols.

Is divided into an alcohol moiety and an acyl moiety, but has an ester functional group. Also, it may be divided into carboxylate and alkyl moieties.

Combining the names of functional groups with the names of the parent alkanes generates a powerful systematic nomenclature for naming organic compounds.

The atoms of functional groups are linked to each other and to the rest of the molecule by covalent bonds. When the group of atoms is associated with the rest of the molecule primarily by ionic forces, the group is referred to more properly as a polyatomic ion or complex ion. ( POLYATOMIC IONS~ a group of covalently bonded atoms, that as a group, carries an electrical charge, but since it is so stable, it can through most chem. rxns as a unit won’t come apart.)

And all of these are called radicals, by a meaning of the term that predates the free radical.

The first carbon atom after the carbon that attaches to the functional group is called the alpha carbon; the second, beta carbon, the third, gamma carbon, etc. If there is another functional group at a carbon, it may be named with the Greek letter, e.g. the gamma- amine in gamma-aminobutanoic acid is on the third carbon of the carbon chain attached to the carboxylic acid group. II The In Organic Chemistry it is the simplest consisting entirely of hydrogen and carbon.

Hydrocarbons from which one hydrogen atom has been removed are functional groups, called -- hydrocarbyls. (organic chemistry) Any univalent radical, derived from a , such as methyl or phenyl. Aromatic hydrocarbons (arenes), alkanes, alkenes, cycloalkanes and -based compounds are different types of hydrocarbons.

The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded, can catenate -- to form atoms of the same into a chain held together by chemical bonds.

A) Alkanes Organic Chemistry Nomenclature & Numbering A saturated hydrocarbon in which all of the carbon-carbon bonds are single bonds. Each carbon atom forms four bonds and each hydrogen forms a to a carbon. The bonding around each carbon atom is tetrahedral, so all bond angles are 109.5°. As a result, the carbon atoms in higher alkanes are arranged in zig-zag rather than linear patterns.

Straight Chain Alkanes Here is a table that gives the names of the straight chain alkanes. It's a good idea to commit this table to memory. The general formula for an is CnH2n+2 where n is the number of carbon atoms in the molecule. There are two ways of writing a condensed . For example, butane may be written as CH3CH2CH2CH3 or CH3(CH2)2CH3.

# Carbon Name Molecular Structural Formula Formula

1 CH4 CH4

2 Ethane C2H6 CH3CH3

3 Propane C3H8 CH3CH2CH3

4 Butane C4H10 CH3CH2CH2CH3

5 C5H12 CH3CH2CH2CH2CH3

6 Hexane C6H14 CH3(CH2)4CH3

7 Heptane C7H16 CH3(CH2)5CH3

8 Octane C8H18 CH3(CH2)6CH3

9 Nonane C9H20 CH3(CH2)7CH3

10 Decane C10H22 CH3(CH2)8CH3

Rules for Naming Alkanes

• The parent name of the molecule is determined by the number of in the longest chain. • In the case where two chains have the same number of carbons, the parent is the chain with the most substituents. • The carbons in the chain are numbered starting from the end nearest the first substituent. • In the case where there are substituents having the same number of carbons from both ends, numbering starts from the end nearest the next substituent. • When more than one of a given substituent is present, a prefix is applied to indicate the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use the number assigned to the carbon to indicate the position of each substituent. Branched Alkanes • Branched substituents are numbered starting from the carbon of the substituent attached to the parent chain. From this carbon, count the number of carbons in the longest chain of the substituent. The substituent is named as an alkyl group based on the number of carbons in this chain. • Numbering of the substituent chain starts from the carbon attached to the parent chain. • The entire name of the branched substituent is placed in parentheses, preceded by a number indicating which parent-chain carbon it joins. • Substituents are listed in alphabetical order. To alphabetize, ignore numerical (di-, tri-, tetra-) prefixes (e.g., ethyl would come before dimethyl), but don't ignore don't ignore positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).

Conformations of Alkanes

Conformations are different forms of a molecule, related by simple rotation about a single bond. Such interconversion are usually very rapid, so that a sample of a given molecule may exist in many different conformations.

For ALKANES, various conformations can be represented by using dash-wedge notation in a structural "perspective" drawing. An alternate depiction is a Newman Projection, which views a molecule by looking down a C-C bond axis, showing the relative orientation of groups off the two carbon atoms.

Ethane

This conformation of ethane is the staggered form. The various hydrogen atoms on the two carbons form a dihedral angle of 60º.

This conformation of ethane is the eclipsed form. The hydrogen atoms have a dihedral angle of 0º.

The staggered conformation of ethane is more stable than the eclipsed form by 12.1 kJ/mol, so that as one rotates 360º relative to the other, the compound passes through three stable staggered conformers via three unstable eclipsed forms. This barrier is small enough that at 25ºC the compound changes conformation about 50 million times each second!

Butane

Butane has not only eclipsed and staggered conformations, but also forms that vary in the relative orientation of the methyl groups.

anti eclipsed gauche fully eclipsed

fully eclipsed

Relative energy: +15.9 kJ/mol +3.8 kJ/mol 0 kJ/mol anti eclipsed gauche

+18.8 kJ/mol

fully eclipsed

As with ethane, the eclipsed conformations are higher energy than the staggered. The staggered conformation where the two methyl groups are as far away from one another as possible (with a dihedral angle of 180º, seen in the Newman projection) is called the anti conformation, and is the lowest-energy arrangement. The two staggered forms with the methyl groups in closer proximity (60º) are the gauche conformations. Direct interconversion of one gauche form to the other requires passing through the highest- energy eclipsed form, where the two methyl groups are next to each other. A space- filling representation of this conformation shows that the methyl groups are physically touching, leading to the high energy.

Alkanes: Natural Sources The alkanes are isolated from natural gas and petroleum. Natural gas contains mainly methane, with smaller amounts of other low-molecular-weight alkanes. Petroleum, which is a complex mixture of many compounds, is the main source of all other alkanes. The lighter fractions are distilled from the mixture to produce the liquid alkanes, while the residue from the distillation produces the solid alkanes.

By far the most important economic use for alkanes is their use as fuels, an ingredient in kerosene. They are also used as lubricants and as non-polar . They are largely unreactive, but can react with such as bromine in the presence of sunlight to form halogenoalkanes, which are much more useful in organic synthesis.

Alkanes are used in hundreds of products such as plastics, paints, drugs, cosmetics, detergents, insecticides and many more. Environmental hazards and health risk of common liquid perfluoro-n-alkanes & a potent greenhouse gases. These liquid perfluoro-n-alkanes tend to be hydrophobic and partitioned into organic matter, and they have exceptionally low solubility in water and extremely high vaporization from the water bodies, suggesting that it will sink into the atmosphere if it is released into the environment.

PHYSICAL PROPERTIES:  Alkanes are non-polar  Intermolecular forces are van der Waals forces  Insoluble in water  Less dense than water

CHEMICAL PROPERTIES: Combustion of Alkanes All Alkanes burn in air to give Carbon Dioxide & Water

Alkanes: Preparations

Alkanes are rarely prepared from other types of compounds because of economic reasons. However, ignoring financial considerations, alkanes can be prepared from the following compounds:

1. Unsaturated compounds via catalytic reduction

2. Alkyl halides via coupling (Wurtz reaction) 3. Alkyl halides via Grignard reagent

4. Alkyl halides via reduction

Methyl Chloride Methane Although organic refer to the above diagrams as “equations,” they are not balanced. In addition, not every product formed is shown. These diagrams are really reaction schemes.

5. Combustion of methane

CH4 + 2O2 CO2 + 2H2O 6. Alkanes: Halogenation The reaction of a with an alkane in the presence of ultraviolet (UV) light or heat leads to the formation of a haloalkane (alkyl halide). An example is the chlorination of methane.

7. Alkanes: Oxidation (Combustion) Alkanes can be oxidized to carbon dioxide and water via a free-radical mechanism. The energy released when an alkane is completely oxidized is called the heat of combustion. For example, when propane is oxidized, the heat of combustion is 688 kilocalories per mole.

CH3CH2CH3 + 5O2  3CO2 + 4H2O + energy

CYCLOALKANE

Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect.

The reason has to do with the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are about 109.5°. In cyclopropane, they are 60°. With the electron pairs this close together, there is a lot of repulsion between the bonding pairs joining the carbon atoms. That makes the bonds easier to break.

Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect.

Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest Cycloalkane is cyclopropane. Of cyclohexane is formed known as the "boat" form. In this arrangement, both of these atoms are either pointing up or down at the same time.

If you count the carbons and , you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms.

You are unlikely to ever need it, but the general formula for a cycloalkane is CnH2n. Don't imagine that these are all flat molecules. All the cycloalkanes from cyclopentane upwards exist as "puckered rings".

Cyclohexane, for example, has a ring structure which looks like this:

This is known as the "chair" form of cyclohexane - from its shape which vaguely resembles a chair.

III B) Alkenes The second class of simple hydrocarbons that consists of molecules that contain at least one double-bonded carbon pair. Alkenes follow the same naming convention used for alkanes. A prefix (to describe the number of carbon atoms) is combined with the ending "ene" to denote an . The simplest alkene which has the International Union of Pure and Applied Chemistry (IUPAC) is properly named Ethene, is the two- carbon molecule that contains one but is almost universally known by its common name, ethylene( C 2H4). The chemical formula for the simple alkenes follows the expression CnH2n. Because one of the carbon pairs is double bonded, simple alkenes have two fewer hydrogen atoms than alkanes.

Ethene

Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry). Aromatic compounds are often drawn as cyclic alkenes, BUT their structure and properties are different and they are not considered to be alkenes.

 Carbon-carbon double bonds  Names end in -ene H2C=CH2 ethene (ethylene) H2C=CH-CH3 propene (propylene) cyclohexene Each carbon atom in ETHYLENE is attached not to four other atoms, as is the carbon in methane or ethane, but to three. It seems we will need a different bonding rationale with which to describe Nature now. Our strategy will be to develop a bonding scheme for the simplest trivalent compound of carbon, methyl (CH3 ) , and then extend it to ethylene , in which each carbon is attached not to three hydrogens as in methyl, but to two hydrogens and the other methylene (CH2) group.

H H H

C – H C –– C

H H H Replacement of one hydrogen in methyl (CH3), with a methylene (CH2) group leads to the framework of ethylene (H2CCH2). Note that each carbon so far has only three bonds. In this drawing the full bonding scheme for ethylene is not yet in place.

IUPAC Naming of Alkenes The rules for naming alkenes are basically the same as those of alkanes but with two differences. (1)The parent chain must include the double bond even if it makes it shorter than the others. (2)And the parent alkene chain must be numbered from whichever end gives the first carbon of the double bond the lower of two possible numbers. Also, the location number should be given as to where the double bond is (except ethene or propene, where the location will always be 1). For example:

CH3 CH3 | | CH3CH2CH=CH2 CH3CH=CHCH3 CH3CH2CHCH2CH=CCH3 4 3 2 1 1 2 3 4 7 6 5 4 3 2 1 1-butene 2-butene 2,5-dimethyl-2-heptene

Alkanes which have two double bonds are dienes, those with three are trienes, and so forth. Each double bond has to be located by a number.

CH2=CHCH=CHCH3 CH2=CHCH2CH=CH2 CH2=CHCH=CHCH=CH2 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 1,3-pentadiene 1,4-pentadiene 1,3,5-hexatriene * also follow similar naming conventions *

Geometric Isomerism among the Alkenes There is no free rotation of a double bond. Doing so would break it. Therefore, many alkenes exhibit geometric isomerism. For example, cis-2-butene and trans-2-butene are geometric isomers. Cis means "on the same side," while trans means "on opposite sides."

CH3 CH3 CH3 H \ / \ / C = C C = C / \ / \ H H H CH3 cis-2-butene trans-2-butene Generally, the differences in physical properties are measurable, but their chemical properties are very similar.

Different Reactions of Alkenes:

Alkenes are relatively stable compounds, but are more reactive than alkanes due to the presence of a carbon-carbon pi-bond. The majority of the reactions of alkenes involve the rupture of this , forming new single bonds.

Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions.

1) Addition Reactions of Alkenes

Alkenes are Lewis bases ( donors) because the bond of the carbon- carbon double bond is projected outward where electron-seeking reactants are able to get it. They will react very readily with Lewis acids (electron pair acceptors) and strong Brønsted acids ( donors). The addition reactions of alkenes make pieces of a reactant become separately attached to the carbons at the ends of a double bond. Ethene readily reacts with hydrogen chloride to make 1-chloroethane:

H H H H + H H \ / | | | | C = C + H-Cl ==> H-C-C-H + Cl- ==> H-C-C-H / \ | | | | H H H H Cl + - CH2=CH2 + H-Cl ==> CH3-CH2 + Cl ==> CH3-CH2-Cl ethyl carbocation (exists for short duration)

2) Addition of Sulfuric Acid Reaction Another reaction involves ethene and water, giving ethanol (ethyl alcohol), while sulfuric acid acts like a catalyst called the.

+ H H H H H H

\ / | | | | - C = C + H2SO4 + H2O ==> H-C-C-H + HSO4 + H2O ==> H-C-C-H / \ | | | | H H H H OH ethene water

- + + HSO4 + H ==> C2H5OH + H2SO4 ethanol

3) Hydrogenation--Addition of Hydrogen

Hydrogenation of alkenes produces the corresponding alkanes. When an alkene is hydrogenated, it becomes and alkane. The reaction is carried out under pressure in the presence of a metallic catalyst. It requires a catalyst--powdered platinum, for example-- Common industrial catalysts are based on platinum, nickel or palladium and often high heat and pressure. This is the hydrogenation of 2-butene:

Pt

CH3CH=CHCH3 + H-H ==> CH3CH-CHCH3 or CH3CH2CH2CH3 2-butene | | H H butane

Another is for laboratory syntheses, Raney nickel (an alloy of nickel and ) is often employed. The simplest example of this reaction is the catalytic hydrogenation of ethylene to yield ethane:

CH2=CH2 + H2 → CH3-CH3 ethylene ethane

4) Halogenation In electrophilic halogenation, the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:

CH2=CH2 + Br2 → BrCH2-CH2Br Ethene Bromine 1,2 - dibromoethene

It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture. The reaction works because the high at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced . This makes the Br closest to the double bond slightly positive and therefore an electrophile.

5) Oxidation Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen, alkenes burn with a bright flame to produce carbon dioxide and water. Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides. Reaction with ozone in ozonolysis leads to the breaking of the double bond, yielding two aldehydes or ketones. Reaction with Concentrated, Hot KMnO4 (or other oxidizing salts) in an acidic solution will yield ketones or carboxylic acids.

R1-CH=CH-R2 + O3 → R1-CHO + R2-CHO + H2O

This reaction can be used to determine the position of a double bond in an unknown alkene.

6) Hydrohalogenation - is the addition of hydrohalic acids such as HCl or HBr to alkenes to yield the corresponding haloalkanes.

CH3-CH=CH2 + HBr → CH3-CHBr-CH2-H

1- propene Hydrgen 2- Bromo-propane Bromide Markovnikov's rule

This rule states that in the ionic addition of an unsymmetrical reagent to an unsymmetrical double bond, the electrophilic agent (usually, though not necessarily, a proton) will attach itself to the doubly bonded carbon containing the smaller # of alkyl groups – that is, to the one containing the larger # of hydrogens. Thus the Markonikov rule predict the addition of HBr to 1- ethene will give primarily 2-Bromoethane. This is, therefore, often referred to as Markonikov addition. If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents.

1-ethene 2-Bromoethane

7) Polymerization A process by which an organic compound reacts with itself to form a high-molecular- weight compound composed of repeating units of the original compound. The polymerization of ethene by an ionic, or free-radical, reagent A−B is an example. An example of alkene polymerization, in which each Styrene monomer unit's double bond reforms as a single bond with another styrene monomer and forms .

8) Synthesis Industrial methods: The most common industrial synthesis of alkenes is based on of petroleum. Large alkanes are broken apart at high temperatures, often in the presence of a zeolite catalyst, to give alkenes and smaller alkanes, and the mixture of products is then separated by fractional distillation. This is mainly used for the manufacture of small alkenes (up to six carbons). Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbentsNatural zeolites form where volcanic rocks and ash layers react with alkaline groundwater. Zeolites also crystallize in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins. Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, , or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential.

Related to this is catalytic dehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene. This is the reverse of the catalytic hydrogenation of alkenes.

9) HYDRATION REACTION In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group (OH-) and a hydrogen cation (an acidic proton) are added to the two carbon atoms bonded together in thecarbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous solution. Hydration differs from hydrolysis in that hydrolysis cleaves the non-water component in two. Hydration leaves the non-water component intact. The general chemical equation of the reaction is the following:

RRC=CH2 in H2O/acid → RRC(-OH)-CH3 In the first step, the acidic proton bonds to the less substituted carbon of the double

bond following Markovnikov's rule. In the second step an H2O molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge. Another water molecule comes along and takes up the extra proton. When carried out in the laboratory, this reaction tends to yield many undesirable side products and in its simple form described here is not considered very useful for the production of alcohol. Conceptually similar reactions include:

MECHANISM This is an example reaction mechanism of the hydration of 1- methylcyclohexene to 1-methylcyclohexanol. Alkenes are very useful compounds. Major uses are, • Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol. • For the manufacture of a wide variety of , e.g., polyethene, polyvinylchloride (PVC) and teflon etc. • As a raw material for the manufacture of industrial Chemicals such as alcohols, aldehydes, etc. • For producing lamp black.

ALKYNES

The alkynes are the third homologous series of organic compounds of hydrogen and carbon, where there is at least one triple-bond between the atoms in the molecules. H-C≡C

The alkenes are said to be unsaturated because of the existence of a multiple bond in the molecule. The general structure of the alkene series of hydrocarbons is CnH2n-2. The first member of the ethene series is ethyne (previously called ). The names of all alkynes end in "-yne". Rules for the systematic naming of alkynes are similar to those for alkenes. In the case of higher members of the alkene series, the triple bond may be between the terminal carbon atoms of the chain, or may be between internal carbon atoms in the chain.

Ethyne (Acetylene) HC≡CH Propyne HC≡C— CH3 1-Butyne HC≡CCH2CH3 1-Pentyne HC≡C(CH2)2CH3 1-Hexyne HC≡C(CH2)3CH3 1-Heptyne HC≡C(CH2)4CH3 1-Octyne HC≡C(CH2)5CH3 1-Nonyne HC≡C(CH2)6CH3 1-Decyne HC≡C(CH2)7CH3

2-Butyne CH3CCCH3

2-Pentyne CH3CCCH2CH3

The bond formed between the hydrogen atom and the unsaturated carbon atom, and first bond between the unsaturated carbon atoms in the ethynes are s bonds ( bonds) and these bonds are formed by the end-on overlap of sp hybrid orbitals of the carbon atoms and the bonds are arranged as far apart in space as possible (i.e. at 180 degree) to form a linear molecule. The second and third bonds that makes up the triple bond of the unsaturated carbon atoms in alkenes are p-bonds (pi-bonds), formed by the side-on overlap of the two p-orbitals on each of the carbon atoms. The p-bonds (pi- bonds) are much more reactive than the s bonds (sigma bonds), and react readily in addition reactions.

Acetylene is a linear molecule, all four atoms lying along a straight line. This linear structure can only be explained by the existence of sp hybridisation of the orbitals of the carbon atoms of ethyne.

The carbon-carbon triple bond is thus made up of one strong bond and two weaker (bonds; it has a total strength 123 kcal. It is stronger than the carbon-carbon double bond of ethylene 100 kcal or the single carbon-carbon bond of ethane 83 kcal, and therefore is shorter than either.

The C-C distance is 1.2 A, as compared with 1.34 Å in ethylene and 1.54 Å in ethane and is a more electronegative grouping than that formed by carbon atoms joined by either a double or a single bond.

The hydrogen attached to the carbon-carbon triple bond in ethyne or in any alkyne where the carbon-carbon triple bond is situated at the end of a carbon chain is able to separate from the rest of the molecule as a hydrogen ion; the electronegative carbon is able to retain both electrons from the broken covalent bond.

A significant result of this bonding is that ethyne can unite with metals and so be distinguished from alkenes by chemical means.

The linear structure does not permit geometric isomerism of ethyne.

ALKYNES CHEMICAL PROPERTIES

1) Combustion of Alkynes Alkynes are characteristically more unsaturated than alkenes. Thus they add two equivalents of bromine whereas an alkene adds only one equivalent. Other reactions are listed below. Alkynes are usually more reactive than alkenes. They show greater tendency to polymerize or oligomerize than alkenes do. The resulting polymers, called polyacetylenes (which do not contain alkyne units) are conjugated and can exhibit semiconducting properties.

Ethyne burn in air with a luminous, smoky flame, (forming carbon dioxide and water).

2 HC≡CH + 5 02 ==> 4 CO2 + 2 H2O

The ethynes are highly dangerously when mixed with air or oxygen.

2) Oxidation of Alkynes Ethyne is oxidised by a dilute aqueous solution of potassium permanganate to form oxalic acid. Thus, if ethyne is bubbled through a solution of potassium permanganate the solution is decolourised. This is Baeyer's test for unsaturated organic compounds.

KMnO4 HC≡CH ==> HOO = C – C = OOH Ethyne or

O = COH O = COH

Oxalic Acid

3) Addition Reactions of Alkynes Because of the unsaturated nature of ethyne addition reactions can occur across the triple bond. 3.1 Addition of Hydrogen When acetylene and hydrogen are passed over a nickel catalyst at 150 deg C, (or over platinum black catalyst at room temperature) ethene is first formed and then this is further reduced to ethane. Ni HC≡CH + H2 ===> C2H6 150 °C Ethyne Ethane

Ni H2C=CH2 + H2 ===> C2H6 150 °C Ethene Ethane

3.2 Addition of Halogens Ethyne reacts explosively with chlorine at room temperature, forming hydrogen chloride and carbon. To control the reaction, acetylene and chlorine (also bromine) are added in retorts filled with kieselguhr (hydrated silica) and iron filings.

HC≡CH + Cl2 ==> ClHC=CHCl + Cl2 ==> Cl2HCCHCl2 Ethyne Acethyne Chloride

3.3 Addition of Hydrogen Halides. Ethyne reacts with the halogen acids. Hydrogen iodide adding on the most readily, at room temperature. A similar reaction occur with hydrogen bromide at 100 °C. Reaction with hydrogen chloride occurs very slowly.

HC≡CH + HCl ==> H2C=CHCl + HCl ==> CH3CHCl2 Ethyne

3.4) Addition of Water (Hydration) Hydration of ethyne occurs when the gas is passed into dilute sulphuric acid at 60°C. Mercuric sulphate is used as a catalyst for the reaction, and the product formed is ethanal (i.e. acetaldehyde).

HgSO4

HC≡CH + H2O ==> CH3CHO Ethyne 60 °C ethanal

4) Nitrile Formation When a slight excess of ethyne and ammonia are passed over an alumina catalyst at 573 degK, ethanonitrile (i.e. acetonitrile) is produced.

573 °C HC≡CH + NH3 ==> CH3CN + H2 Ethyne Ethanonitrile

5) Polymerization of Alkynes due to Triple Bond The products obtained by polymerising ethyne depend on the conditions used.

When ethyne is passed through a tube at 4000°C a little benzene is formed. This is not a suitable way to make benzene in quantity but it is an example of direct conversion from an open chain to an aromatic compound, (i.e. one with a closed-ring benzenoid structure)

400°C 3HC≡CH ==> C6H6 Ethyne Benzene

Two molecules of ethyne can be combined to produce vinyl ethyne, HCCCH=CH2, by passing the ethyne into a saturated solution of cuprous chloride in ammonium chloride continuously in such a way that low conversions of starting material occur.

Cu2Cl2 NH4Cl 2HC≡CH ==> HCCCH = CH2 Ethyne Vinyl Ethyne

This linear polymerisation can be extended by altering the conditions of reaction. For example,

HCCCH=CH2 + HC≡CH ==> CH2=CHCCCH=CH2 Vinyl Ethyne Ethyne DiVinyl Ethyne

6) Substitution Reaction of Alkynes The reactions of ethynes indicate acidic properties for the hydrogens which are attached to the carbon atoms involved in the triple bond. Ethynes readily form compounds with .

When ethyne is passed through a solution of sodium in liquid ammonia then sodium acetylide is formed and hydrogen is liberated.

liq.NH3 HC≡CH + 2Na ==> 2HCCNa + H2 Ethyne Sodium Acetylide

The other hydrogen atom in ethyne can be similarly replaced. When ethyne is passed into a solution of cuprous chloride in ammonia, cuprous acetylide is produced.

HC≡CH + Cu2Cl2 + NH4OH ==> CuCCCu

Copper Acetylide

Silver acetylide is formed when ethyne is passed into an ammoniacal solution of silver nitrate.

AgNO3 NH4OH HC≡CH ==> AgCCAg + 2 HNO3 Ethyne Silver Acetylide

These substitution reactions which ethynes undergo to form compounds with metals does not occur with the alkenes. These reactions can be used as tests to distinguish between acetylene and ethylene. When acetylene is passed through an ammonical solution of silver nitrate or cuprous chloride, at room temperature, precipitates of silver acetylide (white) or cuprous acetylide (red) are formed.

In addition to distinguishing ethyne from ethene by chemical means, these reactions provide a useful method for the preparation of higher alkynes:

+ HC≡C-Na + CH3I ==> HC≡CCH3 + NaI Propyne

Warning: Methyl acetylides are when dry so great care should be taken in their preparation. The metal acetylides can be destroyed when they are still wet by warming with dilute acid which will regenerate the parent ethyne.

HC≡C-Na+ + HNO3 ==> HC≡CH + NaNO3

ALKYNES PHYSICAL PROPERTIES

Alkynes are compounds which have low polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.

1. They are insoluble in water. 2. They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.). 3. They are less dense than water. 4. Their boiling points show the usual increase with increasing carbon number. 5. They are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons. Alkynes Preparation:

The carbon-carbon triple bond of the alkynes is formed in the same way as a double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons.

W X W X HC - CH ==> HC = CH ==> HC≡CH X X Alkane Alkene Alkyne

The groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.

Alkynes Reactivity:

The unsaturated nature of alkynes means that most of their reactions will be similar to those of alkenes (i.e. electrophilic addition), because of the availability of the loosely held pi-electrons. The carbon to carbon triple bond is less reactive than the carbon to carbon double bond towards electrophilic reagents. As well as the addition reactions, alkynes undergo reactions that are due to the acidity of a hydrogen atom attached to the triple bonded carbon.

The carbon-carbon triple bond in ethyne is thus made up of one strong sigma-bond and two weaker pi-bonds. It has a total strength 123 kcal/mole. This is stronger than the carbon-carbon double bond of ethylene which has a total strength of 100 kcal/mole or the single carbon-carbon bond of ethane which has a total strength of 83 kcal/mole.

The carbon-carbon bond lengths, which depend on the strengths of the bonds are: HC≡CH 1.20 Angstrom Units Ethyne

H2C=CH2 1.34 Angstrom Units Ethylene

H3C — CH3 1.54 Angstrom Units Ethane

The ethynyl radical, CHC*, is a more electronegative group than that formed by carbon atoms joined by either a double or a single bond. Thus, the hydrogen attached to the carbon-carbon triple bond in ethyne, or in any alkyne where the carbon-carbon triple bond is situated at the end of a carbon chain, is able to separate from the rest of the molecule as a hydrogen ion, so that the alkyne shows acidic properties. The electronegative carbon is able to retain both electrons from the broken covalent bond. A significant result of this bonding is that ethyne can form compounds with metals and so be distinguished from alkenes by chemical means.

USES OF ALKYNES: Alkynes are generally used as the starting materials for the manufacture of a large number of organic compounds of industrial importance such as, chloroprene, vinyl chloride etc. CHLOROPRENE~ has the formula CH2=CCl-CH=CH2. This colorless liquid is the monomer for the production of the polychloroprene, a type of synthetic rubber. Polychloroprene is better known to the public as Neoprene, the trade name given by DuPont. VINYL CHLORIDE~ the formula CH2:CHCl. It is also called vinyl chloride monomer, or VCM. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC). At ambient pressure and temperature, vinyl chloride is a gas with a sickly sweet odor. It is highly toxic, flammable and carcinogenic.

CYCLOALKYNE

Cyclooctyne: the smallest isolable cycloalkyne

In organic chemistry, a cycloalkyne is the cyclic analog of an alkyne. A cycloalkyne consists of a closed ring of carbon atoms containing one or more triple bonds. Because of the linear nature of the C-CΞC-C alkyne unit, cycloalkynes are usually highly strained and can only exist when the number of carbon atoms in the ring is great enough to provide the flexibility necessary to accommodate this geometry. Consequently, cyclooctyne (C8H12) is the smallest cycloalkyne capable of being isolated and stored as a stable compound. Despite this, smaller cycloalkynes can be produced and trapped by a suitable reagent.