Alkenes and Alkynes
Methods of Preparation of Alkynes
(i) By dehydrohalogenation of dihalides :
Vinyl halide being less reactive & thus to get better yield a stronger base sodalime NaNH2 is used in II step.
(ii) By dehalogenation of tetrahalides :
(iii) By haloform : only for acetylene
CHX3 + 6Ag + X3HC CH≡CH + 6AgX
(iv) By Kolbe's electrolytic method :
(v) Laboratory method : Acetylene is prepared in laboratory by the action of water on calcium carbide.
CaC2 + 2H2O → Ca(OH)2 + C2H2 (Wohler reaction)
The air of the flask in which acetylene is to be prepared is displaced with oil gas as acetylene forms an explosive mixture with air. Acetylene evolved is collected over water. The acetylene so prepared is contaminated with small amounts of impurities such as PH3, H2S, AsH3 and NH3 which are removed by passing the mixture through CuSO4 solution before its collection over water.
(vi) Manufacture :
old method : 2C + H2 C2H2 (Bertholots reaction)
Modern method : 6CH4 + O2 2CH≡CH + 2CO + 10H2
or 2CH4 CH≡CH + 3H2
(vii) Preparation of higher alkynes : Higher alkynes may be obtained b acetylene.
(a) CH≡CH + Na CH≡CNa + 1/2H2
CH≡CNa + RX → CH≡CR + NaX
(b) CH≡CH + RMgX → XMg-C≡CH → R-C≡CH + MgX2