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Chemistry Question 65 – JEE-MAIN 2026

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An optically active alkyl bromide C4H9Br, reacts with ethanolic KOH to form major compound [A] which reacts with bromine to give compound [B]. Compound [B] reacts with ethanolic KOH and sodamide to give compound [C]. One molecule of water adds to compound [C] on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form compound [D]. The functional group in compound D will be confirmed by :

Trace the sequence of reactions starting from the optically active alkyl bromide to identify the final product's functional group.

Step 1: Determine the structure of the starting material and intermediate compounds.✦ Active

An optically active alkyl bromide C4H9Br must be 2-bromobutane (CH3−CH(Br)−CH2−CH3). Reaction with ethanolic KOH (E2 elimination) gives major product [A] as But-2-ene (CH3−CH=CH−CH3). Addition of Br2 to [A] gives [B] as 2,3-dibromobutane (CH3−CH(Br)−CH(Br)−CH3). Double dehydrohalogenation of [B] with ethanolic KOH and NaNH2 gives [C] as But-2-yne (CH3−C≡C−CH3).

Step 2: Determine the structure of compound [D].○ Expand

Hydration of [C] (But-2-yne) with HgSO4/H2SO4 adds water across the triple bond. This forms an enol intermediate which tautomerizes to a ketone.

CH3−C≡C−CH3+H2O→HgSO4/H2SO4[CH3−C(OH)=CH−CH3]⇌CH3−C(=O)−CH2−CH3

Therefore, compound [D] is Butan-2-one (CH3−C(=O)−CH2−CH3).

💡 Teacher's Secret Hint

Remember that internal alkynes hydrate to form ketones, while terminal alkynes form methyl ketones (via Markovnikov's rule).

Step 3: Identify the test to confirm the functional group of compound [D].○ Expand

Compound [D] is Butan-2-one, which is a methyl ketone (contains the CH3−C(=O)− group). The Haloform test (specifically, the iodoform test) is used to detect methyl ketones. Lucas test is for alcohols. Silver mirror test (Tollens' test) and Benedict test are for aldehydes and alpha-hydroxy ketones. Butan-2-one is none of these. Thus, the Haloform test will confirm the functional group in compound [D].

💡 Teacher's Secret Hint

Recall the specific functional groups detected by each qualitative test.

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