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Alkyne Introduction, Isomerism and Preparation

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Alkyne

Hydrocarbons containing at least one triple bond are known as alkynes. The triple bond contains one σ bond and two π-bonds. The general formula for alkynes having only one triple bond is CnH2n-2. The first stable member of alkyne is ethyne which is also known as acetylene.

Isomers of alkynes

We have already discussed isomerism in general. We recommend you to read isomerism before reading this section.

The first two members of alkyne family — ethyne and propyne have only one structure but the higher members have more than one structure.
Example : Butyne has the following two structures :

Isomers of Butyne

isomers of c4h6 : but-1-yne
isomers of c4h6 : but-2-yne

Similarly, pentyne has three isomers : pent-1-yne, pent-2-yne and 3-methylbut-1-yne.

isomers of pentyne

Functional Isomerism

Alkynes are functional isomers with dienes.
Example : But-1-yne and buta-1,2-diene; but-1-yne and buta-1,3-diene are functional isomers.

Ring-chain Isomerism

Alkynes are ring-chain isomers with cycloalkenes.

Ring chain isomerism

Propyne

ring chain isomerism : alkynes and cycloalkenes

Cycloropene

Preparation of Alkynes

Some of the important ways of preparing alkynes are given below :

Preparation of Alkynes from Calcium Carbide

Ethyne is prepared by treating calcium carbide (CaC2) with water.

Preparation of alkynes from calcium carbide

Calcium carbide needed for the preparation of ethyne can be prepared using the following steps :

Firstly, quicklime (CaO) is obtained by heating limestone (CaCO3)

preparation of calcium carbide

Quicklime thus obtained is heated with coke to obtain calcium carbide.

preparation of calcium carbide

Preparation of Alkynes from Vicinal Dihalides

Vicinal dihalides on heating with alcoholic solution of potassium hydroxide yield alkynes.

preparation of alkynes from vicinal dihalides

Alternatively, sodamide (NaNH2) in liquid ammonia can be used instead of alcoholic KOH.

 

Revision includes earlier concepts

Let's do some revision. The reactions used in the answer are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.

Conversion : Prepare propyne from propene.

Do not forget the reactions. We will need them to do conversions in later chapters.

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