Q.CH2=CH2 + Br2 --> A --(alc. KOH)--> B --(NaNH2)--> C. Find A, B and C.

Preparation and Reactions of Alkynes

Preparation builds the C-C triple bond mainly by DOUBLE elimination: (1) an alkene is first halogenated to a vicinal dihalide, then that dihalide undergoes two successive dehydrohalogenations (alcoholic KOH, then the stronger base NaNH2, each removing one H-X) to reach the alkyne; (2) a gem-dihalide (both halogens on one carbon) similarly loses two moles of HX with alcoholic KOH, via a vinyl-halide intermediate; (3) Kolbe's electrolytic method applied to an unsaturated DIcarboxylate (maleate or fumarate) decarboxylates both ends directly to the alkyne; (4) industrially, ethyne is made in bulk from calcium carbide + water (itself made by heating quicklime with coke at 3273 K).

Reactions, alongside the terminal-alkyne acidity covered separately, include: ADDITION of H2 (Pt catalyst, stepwise through the alkene to the alkane), of halogens (Br2/CCl4, stepwise through a 1,2-dihaloalkene to a 1,1,2,2-tetrahaloalkane -- and, like alkenes, decolourising bromine as a test for unsaturation), and of hydrogen halides (Markovnikov regiochemistry, stepwise to a gem-dihalide); HYDRATION (HgSO4/dil. H2SO4, 333 K, Markovnikov addition of water to give an unstable enol that immediately tautomerises to a ketone or, for a terminal alkyne, to acetaldehyde); OZONOLYSIS (ozone cleaves the triple bond, and the ozonide hydrolyses directly to two carbonyl/carboxylic-acid-type fragments, useful for locating the triple bond's position); and POLYMERISATION, either LINEAR (ethyne + CuCl/NH4Cl gives vi …