Arrange the following compounds in increasing order of their acidic strength?
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B), (C), (D), (A)
Note: None of the given options was correct in the NTA question paper. To provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
The acidity of a substituted benzoic acid depends on how stable its carboxylate ion is after the proton leaves:
An electron-withdrawing group spreads out the negative charge of the carboxylate ion, stabilises it and makes the acid stronger than benzoic acid.
An electron-donating group pushes more electron density towards the carboxylate ion, destabilises it and makes the acid weaker than benzoic acid. The stronger the donor, the weaker the acid.
Matching the given acids to these two classes:
(A) p-nitrobenzoic acid: \( \ce{-NO2} \) withdraws electrons strongly by both −R and −I effects; it is the strongest acid of the four.
(B) p-aminobenzoic acid: \( \ce{-NH2} \) donates its lone pair into the ring by resonance (+R), a strong electron-donating effect, much stronger than the hyperconjugation and +I effect of a methyl group; it is the weakest acid of the four.
(C) p-toluic acid (p-methylbenzoic acid): \( \ce{-CH3} \) donates electrons only weakly, by hyperconjugation and +I effect; a little weaker than benzoic acid.
(D) p-chlorobenzoic acid: \( \ce{-Cl} \) withdraws electrons by its −I effect, which outweighs its weak +R effect; stronger than benzoic acid, but weaker than the nitro acid.