Aldehydes and ketones are two important classes of organic compounds which are widely used in the synthesis of a variety of other organic compounds. Due to the polarity of the carbonyl group, they undergo nucleophilic addition reactions. Broadly speaking, these addition reactions are of two types- one in which simply the addition of the nucleophilic reagent occurs across \(>C=O\) bond and the other in which addition is followed by the elimination of a molecule of water. A number of ammonia derivatives such as hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, and semicarbazide belong to the second category of reactions. These derivatives are used for the identification and characterization of aldehydes/ketones. Both types of nucleophilic addition reactions are influenced by steric and electronic factors. In general, aliphatic aldehydes are more reactive than aromatic aldehydes. The aliphatic aldehydes reduce both Fehling's solution and Tollen's reagent but aromatic aldehydes reduce only Tollen's reagent. Like aldehydes, aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition reactions.
Phenols and carboxylic acids are acidic in nature. Both dissolve in \(NaOH\) solution and turn blue litmus red. However carboxylic acids are much stronger acids than phenols and decompose \(NaHCO_3\) with the evolution of \(CO_2\) but phenols do not. Both electron-donating and electron-withdrawing substituents influence the acid strength of aliphatic as well as aromatic acids. Unlike all other aliphatic acids, formic acid has reducing properties and also does not show reactions of the alkyl group.
Carboxylic acids do not give the characteristic reaction of the carbonyl group because of:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Resonance
In the carboxyl group the lone pair on the oxygen of the –OH group is delocalised onto the C=O group by resonance. This reduces the positive charge on the carbonyl carbon, so the carboxylic carbon is less electrophilic than the carbonyl carbon of aldehydes and ketones. That is why carboxylic acids do not give the typical nucleophilic addition reactions of the carbonyl group (with HCN, \( \ce{NaHSO3} \), \( \ce{NH2OH} \), 2,4-DNP, etc.).
Why the other options are incorrect
Option (1) Polar nature — Incorrect. Aldehydes and ketones also have a polar C=O group, and that polarity is the reason they do undergo nucleophilic addition.
Option (3) Symmetrical structure — Incorrect. The –COOH group is not symmetrical; symmetry has nothing to do with the loss of carbonyl character.
Option (4) Attached alkyl group — Incorrect. Ketones carry two alkyl groups and still give carbonyl reactions; an alkyl group alone cannot remove the carbonyl character.
NCERT (2026-27 edition):
Structure of Carboxyl Group
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 | – |