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Chemistry Question 66 – JEE-MAIN 2025

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An optically active alkyl halide C4H9Br [A] reacts with hot KOH dissolved in ethanol and forms alkene [B] as major product which reacts with bromine to give dibromide [C]. The compound [C] is converted into a gas [D] upon reacting with alcoholic NaNH2. During hydration 18 gram of water is added to 1 mole of gas [D] on warming with mercuric sulphate and dilute acid at 333 K to form compound [E]. The IUPAC name of compound [E] is :

Trace the transformation of the starting material through each reaction step to identify the intermediate and final products.

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Ninja Strategy•Functional Group and Reaction Product Analysis

Recognize that alkyne hydration with HgSO4/H2SO4 produces a ketone, eliminating options that are not ketones or are intermediate products.

● Video Walkthrough
Step 1: Identify Compound [A], [B], [C], and [D]✦ Active

An optically active alkyl halide C4H9Br [A] must be 2-bromobutane (CH3−CH(Br)−CH2−CH3) as it contains a chiral center at C2. Reaction of 2-bromobutane [A] with alcoholic KOH (E2 elimination) yields But-2-ene (CH3−CH=CH−CH3) [B] as the major product (Zaitsev's rule). Addition of Br2 to But-2-ene [B] gives 2,3-dibromobutane (CH3−CH(Br)−CH(Br)−CH3) [C]. Double dehydrohalogenation of 2,3-dibromobutane [C] with alcoholic NaNH2 forms But-2-yne (CH3−C≡C−CH3) [D].

CH3−CH(Br)−CH2−CH3→alc. KOHCH3−CH=CH−CH3→Br2CH3−CH(Br)−CH(Br)−CH3→alc. NaNH2CH3−C≡C−CH3
Step 2: Identify Compound [E]○ Expand

Compound [D] (But-2-yne) undergoes hydration with 1 mole of water (18 gram) in the presence of mercuric sulphate and dilute acid (HgSO4/H2SO4) at 333 K. This is an acid-catalyzed hydration of an alkyne. For internal alkynes like But-2-yne, hydration leads to the formation of a ketone via an enol intermediate.

CH3−C≡C−CH3+H2O→HgSO4/H2SO4[CH3−C(OH)=CH−CH3]⇌CH3−CO−CH2−CH3
💡 Teacher's Secret Hint

Remember that enols are unstable and rapidly tautomerize to their more stable keto forms.

Step 3: Determine the IUPAC Name of [E]○ Expand

The final product [E] is CH3−CO−CH2−CH3, which is a ketone with four carbon atoms and the carbonyl group at the second carbon. Its IUPAC name is Butan-2-one.

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