How many of the following compounds give benzoic acid when heated with alkaline potassium permanganate, followed by acidification?
Ethylbenzene, Propylbenzene, Butylbenzene, Isopropylbenzene, sec-Butylbenzene, tert-Butylbenzene
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5
In the vigorous oxidation of an alkylbenzene, the whole side chain is oxidised to a –COOH group, whatever its length. Alkaline \( \ce{KMnO4} \) first gives the potassium salt, and the acid added later sets free benzoic acid:
\( \displaystyle \ce{C6H5CH2CH2CH3} \xrightarrow[\text{heat}]{\ce{KMnO4}\text{-}\ce{KOH}} \ce{C6H5COOK} \xrightarrow{\ce{H3O+}} \ce{C6H5COOH} \)
Gives benzoic acid:
Ethylbenzene (primary alkyl group; only the carbon attached to the ring stays, as –COOH)
Propylbenzene (primary alkyl group)
Butylbenzene (primary alkyl group, longer chain)
Isopropylbenzene (secondary alkyl group, \( \ce{-CH(CH3)2} \))
sec-Butylbenzene (secondary alkyl group, \( \ce{-CH(CH3)CH2CH3} \))
Does not give benzoic acid:
tert-Butylbenzene — a tertiary alkyl group, \( \ce{-C(CH3)3} \), is not affected by this oxidation.
Gives benzoic acid: ethylbenzene, propylbenzene, butylbenzene, isopropylbenzene and sec-butylbenzene = 5.
NCERT (2026-27 edition):
8.7 Methods of Preparation of Carboxylic Acids – “2. 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. …”