Question
Explain why carboxylic acids (e.g., CH₃COOH) are more acidic than phenols (C₆H₅OH), even though both form stable conjugate base anions through resonance.
Solution — Step by Step
Both carboxylate ion (CH₃COO⁻) and phenoxide ion (C₆H₅O⁻) are stabilised by resonance — the negative charge is delocalised beyond the oxygen atom. This is why both carboxylic acids and phenols are more acidic than simple alcohols. But the degree of stabilisation differs, and that’s what determines which is more acidic.
In the carboxylate ion (CH₃COO⁻), the negative charge is delocalised over two equivalent oxygen atoms. The two resonance structures are exactly equivalent — the charge is shared equally between two oxygens.
This symmetric delocalisation over two identical atoms is extremely effective — the charge density on each oxygen is halved.
In the phenoxide ion (C₆H₅O⁻), the negative charge is delocalised over the benzene ring through resonance — but the negative charge spreads onto carbon atoms, which are less electronegative than oxygen.
The resonance structures place the negative charge on carbon atoms at ortho and para positions. Carbon is less able to accommodate a negative charge compared to oxygen. So the delocalisation is present but less effective than in the carboxylate.
Carboxylate ion: negative charge on two equivalent, electronegative oxygens → very stable.
Phenoxide ion: negative charge spread onto carbon atoms (less electronegative) → less stable by comparison.
More stable conjugate base → stronger acid. Therefore:
Carboxylic acids () are about 10⁵ times more acidic than phenols ().
Why This Works
The key principle: effective delocalisation onto electronegative atoms produces more stable anions. Oxygen is more electronegative than carbon, so placing negative charge on oxygen is more stable than placing it on carbon. The carboxylate anion achieves this perfectly — both resonance contributors have the charge on oxygen.
There’s also an inductive component: the carbonyl carbon (C=O) in carboxylic acid has an electron-withdrawing effect through the sigma bonds, further stabilising the carboxylate by pulling electron density away from oxygen. Phenoxide doesn’t benefit from this effect.
Alternative Method
Using values directly: lower = stronger acid. Acetic acid () < Phenol () — carboxylic acid is stronger. For theoretical reasoning, the resonance quality argument above is essential.
CBSE Class 12 Chemistry (Aldehydes, Ketones and Carboxylic Acids chapter) frequently asks this comparison. The explanation must cover: (1) both have resonance stabilisation, (2) carboxylate has charge on two oxygens (same electronegativity), (3) phenoxide has charge partially on carbon (lower electronegativity). Three points = full marks.
Common Mistake
Students sometimes say “carboxylic acids are more acidic because they have two oxygens” — this is incomplete. Phenol also has resonance spreading over the ring. The correct reason is the quality of resonance: charge on two electronegative oxygens (carboxylate) vs. charge partially on carbon (phenoxide). Always mention electronegativity in your answer.