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 \(pK_a\) value:
(A) Benzoic acid
(B) 2-Nitrobenzoic acid
(C) 4-Nitrobenzoic acid
(D) 4-Methoxybenzoic acid
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → B < C < A < D
The stronger an acid, the larger its Ka and the smaller its pKa (pKa = −log Ka). So increasing pKa is the order of decreasing acid strength.
A substituent on the ring changes the stability of the benzoate ion (the conjugate base):
An electron-withdrawing group such as –\( \ce{NO2} \) (−I and −R) spreads the negative charge, stabilises the anion and increases acidity. Its inductive pull is strongest when it is closest to –COOH, so the ortho (2-) isomer is the strongest acid.
An electron-donating group such as –\( \ce{OCH3} \) at the para position (+R) pushes electron density towards the carboxylate, destabilises it and decreases acidity.
Acid strength: 2-nitrobenzoic > 4-nitrobenzoic > benzoic > 4-methoxybenzoic acid. The pKa values run the other way:
2-Nitrobenzoic (2.17) < 4-Nitrobenzoic (3.41) < Benzoic (4.19) < 4-Methoxybenzoic (4.46)
Shortcut: the 4-methoxy acid (D), with an electron-donating group, is the weakest acid and has the highest pKa, so it comes last. Options (2) and (4) end with D, and only Option (4) starts with the strongest acid B.
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.”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 10-06 S3 01-06 S3 | – |