General Methods of Preparation of Alkynes

(1) From alkenes, in two steps: (i) halogenation of the alkene to a vicinal dihalide (e.g. ethene + Br2 --> 1,2-dibromoethane); (ii) double dehalogenation of the vicinal dihalide, first with alcoholic KOH to bromoethene, then with NaNH2 (losing NaBr then NH3) to acetylene. (2) From gem-dihalides: a gem-dihalide (both halogens on the SAME carbon; Latin 'gemini' = twins) gives an alkyne on heating with alcoholic KOH, via a vinyl-halide intermediate -- e.g. 1,1-dichloropropane --alc. KOH--> 1-chloroprop-1-ene --alc. KOH--> prop-1-yne. (3) Kolbe's electrolytic method: electrolysing the sodium or potassium salt of an unsaturated dicarboxylic acid (maleic or fumaric acid) causes decarboxylation at both ends, giving an alkyne directly -- e.g. potassium maleate --electrolysis--> acetylene + 2 CO2 + 2 electrons. (4) Industrial preparation of ethyne: calcium carbide reacts with water on a large scale to give acetylene, CaC2 + …