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Alkenes : Aliphatic Unsaturated Hydrocarbons

Those compounds are called Aliphatic Unsaturated Hydrocarbons :-

1 – Aliphatic are those in which all the chains of atoms present are open.

2 – Those that are unsaturated i.e. in which at least one carbon-carbon double bond or carbon-carbon triple bond is present.

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3- In which only carbon and hydrogen elements are present. These aliphatic unsaturated hydrocarbons are called alkenes in which only one carbon-carbon double bond is present.

Those aliphatic unsaturated hydrocarbons are called alkynes in which only one carbon-carbon triple bond is present.

What is Alkenes?

Those aliphatic unsaturated hydrocarbons are called alkenes in which only one carbon-carbon double bond is present. The general formula of alkenes is CnH2n and all alkenes belong to the same homologous series. Substituting the value of n as 1 in the general formula CnH2n, the formula obtained is CH2. In this formula all four valencies of carbon are not satisfied.

Hence it does not show any compound. It exhibits a bivalent free radical named Ethylene (C2H4) as the simplest member of the series. This compound reacts with chlorine to form an oil-like substance called Ethylene dichloride. Hence the members of this category are also called Olefins. The double bonds present in olefines are also called Olefinic bonds.

Nomenclature of Alkenes :

In the IUPAC method, the nomenclature of alkenes is done by removing ene from the end of the name of the alkane and adding ene. Following are the molecular formula, structure formula, common name IUPAC name of some major alkenes –

FormulaStructure FormulaNameIUPAC Name
C₂H₄H₂C=CH₂EthyleneEthene
C3H6CH3CH=CH2PropylenePropene
C4H8CH₃CH₂CH=CH₂1-Butylene1-Butene
C4H8CH3-HC=CH-CH3β-Butylene2-Butene
C4H8(CH3)2C=CH2Isobutylene2-methylpropene

Isomerism in Alkenes :

The first two members of the alkene series (ethylene and propene) do not show isomerism. The other members of this category represent Place Isomerism, Series Isomerism and Geometric Isomerism.

example :

The molecular formula C4H8 shows four isomerism of alkenes –

Butene Isomers structure

Of these four isomerisms, I and II are series isomerism, II and III are series isomerism, II and IV are series isomerism, I and III are space isomerism, I and IV are space isomerism and III and IV are geometric isomerism.

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Structure of Alkenes

The structure of alkenes can be explained by the example of ethylene. In ethylene (C2H4) each carbon atom is sp2 hybridized. So each carbon atom has three sp2 hybridized orbitals and one pure p orbital. Three sp2 hybridized orbitals are used to form three sigma bonds. The remaining one p orbital on both carbon atoms form a pi bond by lateral overlap.

ethylene structure

Thus ethylene molecule has one carbon-carbon double bond and four carbon-hydrogen single bond. In other words ethylene molecule has 5 sigma bond and 1 pi bond. Since the pi bond is formed by lateral overlap and is weaker than the sigma bond formed by linear overlap, the pi bond is easily broken in the reactions of ethylene.

Like ethylene in other alkenes also 1 pi bond is present. Due to the presence of pi bond, these compounds are highly reactive and show additive reactions. For this reason these compounds are called unsaturated compounds.

The bond angle in sp2 hybridization is 120°. Therefore, the value of each H–C–H and H–C–C angle in ethylene molecule is 120° and ethylene molecule is planer. In alkenes, the C–C bond length is 1.34 A and the C–H bond length is 1.60 A. The C = C binding energy is 145 k cal per mole.

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General Methods of Preparation of Alkenes

By Dehydration of Alcohols : Alkene is obtained by heating alcohols with concentrated sulfuric acid or concentrated phosphoric acid.

Example-

Chemistry formula

In this reaction one molecule of water is removed from one molecule of alcohol. Such reactions are called dehydration.

Dehydration of alcohols can also be done by passing its vapors over heated alumina (Al2O3) or anhydrous aluminum sulfate.

In this reaction one molecule of water is removed from one molecule of alcohol. Such reactions are called dehydration.

Dehydration of alcohols can also be done by passing its vapors over heated alumina (Al2O3) or anhydrous aluminum sulfate.

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CH3 – CH2OH CH2 = CH2 + H2O

The order of ease of dehydration in different alcohols is as follows –

Tertiary Alcohol > Secondary Alcohol > Primary Alcohol

If the dehydration of an alcohol can yield more than one alkene, then dehydration occurs according to Saytzeff’s rule. In the dehydration of alcohols, the water molecule is obtained by the separation of the OH group present on one carbon atom and the hydrogen atom present on another adjacent carbon atom.

According to Saytzeff’s rule, hydrogen is obtained from the nearest carbon atom which has less number of hydrogen atoms already present.

Example: The main product of dehydration of 2-butanol is 2-butene.

Preparation-of-Alkenes

By dehydrohalogenation of alkyl halides: Alkene is obtained by heating alkyl halides with an alcoholic solution of caustic potash. In this process one molecule of hydrogen halide is removed from one molecule of alkyl halide. This irreversible process is called dehydrohalogenation.

Example :

Preparation of Alkenes

If the dehydrohalogenation of an alkyl halide can yield more than one alkene, the reaction follows Saytzeff’s rule.

Dehydrohalogenation-of-Alkyl-Halides

By dehalogenation of Dihalo alkanes :- Dihalo alkanes in which two halogen atoms are present on two adjacent carbon atoms are called vicinal dehalides.

On heating such dihalos with zinc powder, alkene is obtained.

Example :-

dehalogenation-of-Dihalo-alkanes

By electrolysis of dicarboxylic acids :-

Kolbes reaction :- Alkenes are obtained at the anode by electrolysis of aqueous solutions of sodium or potassium salts of dicarboxylic acids.

Example: Electrolysis of aqueous solution of potassium succinate gives ethylene at the anode.

electrolysis of dicarboxylic acids

By partial reduction of alkynes: The end product of the addition reaction of alkynes with hydrogen is alkane. In this reaction, Ni is used as a catalyst and the temperature is kept at 250 – 300°C. If alkynes are taken in large quantity and the reaction is carried out at low temperature, then alkene is also obtained as a result of the reaction.

example:-

CHCH + H2 CH2 = CH2

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