Match List-I with List-II.
| List-I (Observation) | List-II (Reason) |
|---|---|
| (A) Chloroacetic acid is a stronger acid than ethanoic acid. | (I) The carboxyl carbon is attached to a more electronegative sp2 hybridised carbon atom. |
| (B) Benzoic acid is a stronger acid than 4-methoxybenzoic acid. | (II) An electron donating group destabilises the carboxylate anion. |
| (C) Benzoic acid is a stronger acid than ethanoic acid. | (III) An electron withdrawing group stabilises the carboxylate anion. |
| (D) Ethanoic acid is a stronger acid than phenol. | (IV) In the conjugate base, the negative charge is delocalised over two electronegative oxygen atoms. |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A-III, B-II, C-I, D-IV
A carboxylic acid is stronger when its conjugate base, the carboxylate anion, is more stable. Anything that spreads out the negative charge of the anion makes the acid stronger; anything that pushes more electron density onto it makes the acid weaker.
(A) Chloroacetic acid is a stronger acid than ethanoic acid. — This matches (III). Chlorine is an electron withdrawing group; by its inductive effect it pulls electron density away from \( \ce{ClCH2COO^-} \) and stabilises the anion.
(B) Benzoic acid is a stronger acid than 4-methoxybenzoic acid. — This matches (II). The methoxy group on the ring of 4-methoxybenzoic acid is electron donating; it destabilises the carboxylate anion, so 4-methoxybenzoic acid is the weaker of the two acids.
(C) Benzoic acid is a stronger acid than ethanoic acid. — This matches (I). In benzoic acid the –COOH group is joined directly to a ring carbon, which is sp2 hybridised and more electronegative than the sp3 carbon of the methyl group in ethanoic acid.
(D) Ethanoic acid is a stronger acid than phenol. — This matches (IV). The carboxylate ion has two equivalent resonance structures with the charge spread over two oxygen atoms; in the phenoxide ion the charge is spread over one oxygen atom and less electronegative carbon atoms.
NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.”
“Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, … This is because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached.”
“…the negative charge is delocalised over two electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalised over one oxygen atom and less electronegative carbon atoms in phenoxide ion … Thus, the carboxylate ion is more stabilised than phenoxide ion, so carboxylic acids are more acidic than phenols.”