Alkynes

Topic Content:

Alkynes are homologous series of unsaturated hydrocarbons with a general formula of C_n H_2n-2 . Because of the presence of carbon-to-carbon triple bonds, alkynes show a higher degree of unsaturation than the alkenes and are therefore more reactive than the corresponding alkenes and alkanes. 

Ethyne:

Ethyne is the first member of the alkyne series. It has a molecular formula C₂ H₂ and a structural formula of CH ≡ CH.

Laboratory Preparation of Ethyne:

Ethyne, C₂ H₂ , is prepared in the laboratory by the action of cold water on calcium carbide, CaC₂ . The gas evolved is passed through an acidified CuSO₄ solution to remove phosphine, PH₃ present as an impurity.

Because the reaction evolves a great amount of heat, it is carried out in a heap of sand inside the flask to prevent the flask from cracking. The set-up for the preparation is shown below.

\(\scriptsize CaC_{2(s)} \: +\: 2H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} \: + \: C_2H_{2(g)}\)

Physical Properties of Ethyne:

1. It is a colourless gas with a characteristic sweet smell when pure.