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.
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
Example : Butyne has the following two structures :
Isomers of Butyne
Similarly, pentyne has three isomers : pent-1-yne, pent-2-yne and 3-methylbut-1-yne.
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.
Alkynes are ring-chain isomers with cycloalkenes.
Ring chain isomerism
Some of the important ways of preparing alkynes are given below :
Ethyne is prepared by treating calcium carbide (CaC2) with water.
Vicinal dihalides on heating with alcoholic solution of potassium hydroxide yield alkynes.
Alternatively, sodamide (NaNH2) in liquid ammonia can be used instead of alcoholic KOH.
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.