Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more efficiently than in the former.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Benzoic acid, C6H5COOH
Alkylbenzenes are oxidised by hot alkaline (or acidic) potassium permanganate to aromatic carboxylic acids. The attack starts at the benzylic carbon, and the entire side chain is oxidised to a –COOH group, whatever its length. That is why toluene gives benzoic acid in the given reaction.
Propylbenzene (\( \ce{C6H5CH2CH2CH3} \)) has a primary alkyl side chain, so it behaves in the same way: the side chain is oxidised down to one carbon and the other two carbons are lost.
\( \ce{C6H5CH2CH2CH3} \) → (\( \ce{KMnO4} \)/KOH, heat; then \( \ce{H^{+}} \)) → \( \ce{C6H5COOH} \)
So A is benzoic acid.
Why the other options are incorrect
Option (1) C6H5CH2CH2COOH — Incorrect. This would need only the terminal \( \ce{CH3} \) to be oxidised; permanganate attacks the benzylic carbon and does not stop at 3-phenylpropanoic acid.
Option (2) C6H5CH2COOH — Incorrect. Phenylethanoic acid still has a \( \ce{CH2} \) between the ring and COOH; the benzylic carbon itself becomes the COOH carbon.
Option (4) C6H5CH2CH(OH)CH3 — Incorrect. This is an alcohol, not an oxidation product of the type formed here; strong permanganate would oxidise it further.
NCERT (2026-27 edition):
From Alkylbenzenes – “Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain.”