Carboxylic acids are stronger acids than phenols. Which of the following explains this?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The carboxylate ion is stabilised by two equivalent resonance structures in which the negative charge is on the more electronegative oxygen atom.
The strength of an acid depends on how stable its conjugate base is. On losing H+, a carboxylic acid gives the carboxylate ion and a phenol gives the phenoxide ion:
\( \displaystyle \ce{RCOOH + H2O <=> RCOO^- + H3O^+} \)
In the carboxylate ion the negative charge is spread over two oxygen atoms through two equivalent resonance structures. In the phenoxide ion the resonance structures are not equivalent and place the charge on less electronegative carbon atoms.
The carboxylate ion is therefore more stable, and the carboxylic acid is the stronger acid.
Why the other options are incorrect
Option (2) The carboxylate ion is stabilised by non-equivalent resonance structures in which the negative charge is spread over the carbonyl carbon and the two oxygen atoms. — Incorrect.
The two resonance structures of the carboxylate ion are equivalent, and in both of them the negative charge is on an oxygen atom, not on carbon.
Non-equivalent structures with the charge on carbon belong to the phenoxide ion.
Option (3) In the phenoxide ion, the negative charge cannot be delocalised into the benzene ring, whereas in the carboxylate ion it is delocalised over two oxygen atoms. — Incorrect.
The negative charge of the phenoxide ion is delocalised into the ring; this is why phenols are more acidic than alcohols.
Its resonance structures are non-equivalent and put the charge on the less electronegative ring carbon atoms, so they stabilise the ion less than the carboxylate ion is stabilised.
Option (4) Carboxylic acids exist as hydrogen-bonded dimers, which weakens the O–H bond and makes the proton easier to remove than in phenols. — Incorrect. The dimers are held together by intermolecular hydrogen bonds; this association explains the high boiling points of carboxylic acids, not their acid strength. Acid strength depends on the stability of the conjugate base.
NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “…The conjugate base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom. The conjugate base of phenol, a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom. Therefore, resonance in phenoxide ion is not as important as it is in carboxylate ion. … Thus, the carboxylate ion is more stabilised than phenoxide ion, so carboxylic acids are more acidic than phenols.”