Answer the question on the basis of passage given below:
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 more than aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more effectively than in former.
\(A\) will be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → benzoic acid
Alkylbenzenes are oxidised by hot alkaline \( \ce{KMnO4} \) followed by acidification. The oxidant attacks the benzylic C–H, and the entire side chain, whatever its length, ends up as a single –COOH group on the ring, provided the benzylic carbon carries a hydrogen. The first reaction in the question shows this for toluene:
\( \displaystyle \ce{C6H5CH3 + 3[O]} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{KMnO4-KOH},\ \Delta} \ce{C6H5COOH + H2O} \)
Propylbenzene behaves in the same way; the two extra carbons are lost as \( \ce{CO2} \):
\( \displaystyle \ce{C6H5CH2CH2CH3 + 9[O]} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{KMnO4-KOH},\ \Delta} \ce{C6H5COOH + 2CO2 + 3H2O} \)
So A is benzoic acid.
Why the other options are incorrect
Option (1) 3-phenylpropanoic acid — Incorrect. Only the terminal \( \ce{CH3} \) would be oxidised; vigorous \( \ce{KMnO4} \) oxidation does not stop there.
Option (2) phenylethanoic acid — Incorrect. The chain is not shortened by one carbon only; it is cut right down to the ring carbon.
Option (4) 1-phenylpropan-2-ol — Incorrect. An alcohol is a reduction-level product, not the product of strong oxidation.
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. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected.”