Read the passage carefully and answer the following questions.
Aldehydes, carboxylic acids and ketones are widespread in the animal and plant kingdoms. They play a pivotal role in various biochemical processes of life. Their presence in nature adds fragrance and flavor. These compounds are widely used in food products, pharmaceuticals, paints, resins and other important product industries. These compounds are prepared by various laboratory methods, which mainly include oxidation, formylation, acylation and reduction. Due to the polar nature of the carbonyl group in aldehydes and ketones, they can exhibit different reactions like nucleophilic addition. They do exhibit redox and various condensation reactions which lead to the formation of various important compounds. On the other hand, the carboxylic acids are mainly prepared by oxidation and hydrolysis of different compounds. The carboxylic acid consists of a carbonyl group and the hydroxy group (attached to the carbonyl carbon atom). This makes it possible for the carboxylic acid to participate in various chemical reactions which involve cleavage of the C-OH bond and the O-H bond along with the reactions involving the complete -COOH group.
The reagent which doesn't react with both acetone and benzaldehyde is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Fehling's solution
Aldehydes and ketones both carry the polar C=O group, so reagents that work by nucleophilic addition to the carbonyl carbon react with both of them. Oxidation is different: an aldehyde has an H atom on the carbonyl carbon and is oxidised easily, while a ketone is not oxidised by mild oxidising agents. Tollens’ and Fehling’s reagents are such mild oxidising agents and are used to tell the two classes apart.
Fehling’s solution (alkaline copper(II) tartrate) oxidises aliphatic aldehydes only, giving a reddish brown precipitate of copper(I) oxide:
\( \displaystyle \ce{RCHO + 2Cu^2+ + 5OH^- -> RCOO^- + Cu2O v + 3H2O} \)
Acetone is a ketone, so it does not reduce Fehling’s solution. Benzaldehyde is an aldehyde, but aromatic aldehydes do not respond to Fehling’s test. So Fehling’s solution is the reagent that reacts with neither acetone nor benzaldehyde.
Option (1) Sodium hydrogensulphite — Reacts with both. It adds to the C=O group of both compounds and gives crystalline addition products:
\( \displaystyle \ce{(CH3)2C=O + NaHSO3 -> (CH3)2C(OH)SO3Na} \)
\( \displaystyle \ce{C6H5CHO + NaHSO3 -> C6H5CH(OH)SO3Na} \)
Option (2) Phenyl hydrazine — Reacts with both. It reacts with both by addition followed by loss of water, giving phenylhydrazones:
\( \displaystyle \ce{(CH3)2C=O + H2N-NHC6H5 -> (CH3)2C=N-NHC6H5 + H2O} \)
\( \displaystyle \ce{C6H5CHO + H2N-NHC6H5 -> C6H5CH=N-NHC6H5 + H2O} \)
Option (3) Fehling's solution — Reacts with neither. Acetone is a ketone, and benzaldehyde is an aromatic aldehyde; neither reduces Fehling’s solution.
Option (4) Grignard reagent — Reacts with both. It adds to both; after hydrolysis acetone gives a tertiary alcohol and benzaldehyde a secondary alcohol:
\( \displaystyle \ce{(CH3)2C=O} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CH3MgBr}} \ce{(CH3)3C-OH} \)
\( \displaystyle \ce{C6H5CHO} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CH3MgBr}} \ce{C6H5CH(OH)CH3} \)
NCERT (2026-27 edition):
Fehling’s Test
