Which of the following compounds will show maximum acidic character?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4-nitrobenzoic acid (structure 4)
A carboxylic acid is acidic because its anion, the carboxylate ion, is stabilised by resonance. Anything that stabilises this anion further makes the acid stronger:
The four compounds are benzoic acid and its para-substituted derivatives. In order of increasing acid strength (pKa in brackets; lower pKa = stronger acid):
\( \displaystyle \text{4-methoxy (4.47)} < \text{4-methyl (4.37)} < \text{benzoic (4.19)} < \text{4-nitro (3.41)} \)
The –\( \ce{NO2} \) group withdraws electrons by both the inductive and the resonance effect, so 4-nitrobenzoic acid shows the maximum acidic character.
Why the other options are incorrect
Option (1) 4-methylbenzoic acid — Incorrect. The –\( \ce{CH3} \) group donates electrons (+I, hyperconjugation), so it is weaker than benzoic acid.
Option (2) 4-methoxybenzoic acid — Incorrect. The –\( \ce{OCH3} \) group at the para position donates electrons by resonance (+R), so it is the weakest of the four.
Option (3) benzoic acid — Incorrect. It has no substituent to help stabilise the anion, so it is weaker than 4-nitrobenzoic acid.
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids
