Answer the question on basis of passage given below:
Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (\(pK_a\) is ∼16 for ethanol and 10 for phenol). The higher acidity of carboxylic acids as compared to phenols can be understood similarly. 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. Further, the negative charge is delocalized over two electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalized 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.
Effect of substituents on the acidity of carboxylic acids: substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. 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.
Arrange the following acids in order of their increasing acidity:
A. 4-Methoxybenzoic acid
B. 4-Nitrobenzoic acid
C. 4-Chloro benzoic acid
D. 4-Methylbenzoic acid
E. Benzoic acid
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, D, E, C, B
For para-substituted benzoic acids, an electron withdrawing group on the ring increases the acidity and an electron donating group decreases it.
(A) 4-Methoxybenzoic acid (pKa 4.46) — \( \ce{OCH3} \) donates electrons strongly by +R; the weakest acid.
(D) 4-Methylbenzoic acid (pKa 4.37) — \( \ce{CH3} \) donates electrons by +I and hyperconjugation, less strongly than \( \ce{OCH3} \).
(E) Benzoic acid (pKa 4.19) — no substituent.
(C) 4-Chlorobenzoic acid (pKa 3.98) — the −I effect of Cl outweighs its +R effect, so Cl withdraws electrons.
(B) 4-Nitrobenzoic acid (pKa 3.41) — \( \ce{NO2} \) withdraws electrons by −I and −R; the strongest acid.
Shortcut: Only Option (3) starts with A (the methoxy acid, the only +R donor).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “… The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.”
