See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the starting material: Toluene (methylbenzene), which has a benzylic CH3 group. Step 2 – Reaction with Br2 under light (free-radical benzylic bromination): Light initiates a free-radical chain reaction. The weakest, most reactive C–H bonds are the benzylic C–H bonds of the CH3 group. Bromination occurs selectively at the benzylic position, converting –CH3 into –CH2Br. Product A is benzyl bromide (PhCH2Br). Step 3 – Reaction of A with KCN (SN2 nucleophilic substitution): Cyanide ion (CN–) is an excellent nucleophile. It attacks the benzylic carbon of PhCH2Br in an SN2 reaction, displacing bromide and forming phenylacetonitrile (PhCH2CN), also called benzyl cyanide. This step extends the carbon chain by one carbon. Step 4 – Hydrolysis with H3O+, heat (acid hydrolysis of nitrile): The nitrile group (–CN) is hydrolyzed under acidic aqueous conditions with heat. The nitrile is first converted to an amide and then to a carboxylic acid. PhCH2CN + H3O+/heat → PhCH2CO2H (phenylacetic acid). Product B is phenylacetic acid: a benzene ring bearing a –CH2CO2H group. Step 5 – Match to options: (a) 2-methylbenzoic acid – requires ring functionalization, not benzylic chain extension; incorrect. (b) Benzoic acid (PhCO2H) – would require oxidation of the entire methyl directly to COOH without chain extension; incorrect. (c) 4-methylbenzoic acid – ring carboxylation, not this pathway; incorrect. (d) Phenylacetic acid (PhCH2CO2H) – exactly matches the product of benzylic bromination → SN2 with KCN → acid hydrolysis; correct. Why other options fail: Options (a), (b), and (c) involve a carboxylic acid directly attached to the ring (ArCO2H), which would require either electrophilic aromatic substitution or oxidation of a methyl group directly attached to the ring. The KCN/H3O+ sequence on a benzylic halide gives a one-carbon-extended carboxylic acid (ArCH2CO2H), not a direct ring carboxylic acid. Therefore, the correct answer is D.