Which of the following reagents reacts with neither acetone nor benza…

The correct answer is Option (3) → Fehling’s solution

Acetone, \( \ce{CH3COCH3} \), is a ketone and benzaldehyde, \( \ce{C6H5CHO} \), is an aromatic aldehyde. Both have a C=O group, so both take part in the nucleophilic addition reactions of carbonyl compounds; the reagent wanted must leave both a ketone and an aromatic aldehyde unchanged.

Option (1) Sodium hydrogensulphite — Reacts with both. \( \ce{NaHSO3} \) adds to aldehydes and ketones to form the hydrogensulphite addition products.

Option (2) Phenylhydrazine — Reacts with both. It condenses with the C=O group of aldehydes and ketones to give a phenylhydrazone.

Option (3) Fehling’s solution — Reacts with neither. Fehling’s reagent is a mild oxidising agent that oxidises aliphatic aldehydes; ketones such as acetone are not oxidised by it, and aromatic aldehydes such as benzaldehyde do not respond to this test.

Option (4) Grignard reagent — Reacts with both. RMgX adds to the carbonyl group of aldehydes and ketones; hydrolysis then gives a secondary alcohol from benzaldehyde and a tertiary alcohol from acetone.

NCERT (2026-27 edition):
8.4 Chemical Reactions: 3. Oxidation – “The mild oxidising agents given below are used to distinguish aldehydes from ketones: … (ii) Fehling’s test: … On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.”