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 acid do not give the characteristic reaction of carbonyl group because:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → carbonyl carbon of carboxyl group is less electrophilic
The characteristic reactions of the carbonyl group begin with a nucleophile attacking the electrophilic carbonyl carbon. In a carboxyl group a lone pair on the –OH oxygen is delocalised into the C=O group by resonance (structures below), which partly cancels the positive charge on the carbon. So the carboxyl carbon is less electrophilic than the carbonyl carbon of an aldehyde or ketone, and nucleophiles do not add to it in the usual way.
Why the other options are incorrect
Option (1) the oxygen atom of the OH group does not contain lone pair of electrons — Incorrect. The –OH oxygen has two lone pairs; it is one of them that is delocalised into the C=O group and causes the effect.
Option (2) carbonyl carbon of carboxyl group is more electrophilic — Incorrect. A more electrophilic carbon would make nucleophilic addition easier, not prevent it.
Option (3) carbonyl carbon of carboxyl group is more nucleophilic — Incorrect. The carbonyl carbon is an electron-poor centre in every carbonyl compound; resonance only makes it less positive, it does not turn it into a nucleophile.
NCERT (2026-27 edition):
Structure of Carboxyl Group
