Q.Identify the compound A, B, C and D in the following series of reactions
CH3-CH2-Br $\xrightarrow{\text{alc. KOH}}$ A $\xrightarrow{Cl_2/CCl_4}$ B
A $\xrightarrow[\text{ii) Zn/H}_2O]{\text{i) O}_3}$ C
B $\xrightarrow{NaNH_2}$ D

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 vinylacetylene, a dimer) or CYCLIC (three molecules of ethyne trimerise over a red-hot iron tube to benzene -- the same reaction used as a preparation of benzene itself).

"Preparation of alkynes from calcium carbide and vicinal dihalides" and "alkyne reactions important questions class 11" are frequently searched topics in the Hydrocarbons chapter of the NCERT-aligned CBSE Class 11 Chemistry curriculum, tested consistently in JEE Main, JEE Advanced and NEET. The Markovnikov-selective hydration step highlighted here, which routes through an unstable enol to a carbonyl product, is a recurring mechanism-based question in competitive organic chemistry papers.