Relative acidic strength of Carboxylic acids and their derivatives
Carboxylic acids are weak acids. Mineral acids like HCl or HNO3 are very strong acids and non-edible, but many carboxylic acids are edible acids. Carboxylic acids are more acidic than phenol. The conjugate base of carboxylic acid is stabilized by two equivalent resonating structures in which the negative charge is at the more electronegative oxygen atom. Electron withdrawing group (EWG) increases the acidity of carboxylic acids while electron donating group (EDG) decreases the acidic strength of carboxylic acids. The acidic nature of carboxylic acid is also affected by direct attachment of groups such as phenyl or vinyl to the carboxylic acid. They increase the acidity of corresponding carboxylic acids.
It is due to the greater electronegativity of sp2 hybridized carbon to which carboxylic carbon is attached. This acidic character of carboxylic acid is based on its pKa values. The smaller the pka values, the stronger the acid. The effect of the following groups in increasing acidic order is
Ph < I < Br < Cl < F < CN < NO2 < CF3
Among the following aromatic acids, which one is the least acidic in nature?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → D
Carboxylic acids are more acidic than phenols. In a carboxylate ion, the negative charge is shared equally by two oxygen atoms, through two equivalent resonance structures. In a phenoxide ion, the charge spreads onto the less electronegative carbon atoms of the ring, and the resonance structures are not equivalent. So the carboxylate ion is much more stable.
Substituents change acidity further: an electron withdrawing group (such as –\( \ce{NO2} \)) stabilises the anion and increases acidity. An electron donating group (such as –\( \ce{OCH3} \)) destabilises the anion and decreases acidity.
A, B and C are carboxylic acids, while D (4-methoxyphenol) is a phenol. So D is weaker than all three acids, and its electron donating –\( \ce{OCH3} \) group lowers its acidity further. Hence D is the least acidic.
Why the other options are incorrect
Option (1) A — Incorrect. A is benzoic acid, a carboxylic acid, so it is more acidic than any phenol.
Option (2) B — Incorrect. B carries an electron withdrawing –\( \ce{NO2} \) group, so it is even more acidic than benzoic acid.
Option (3) C — Incorrect. C carries two –\( \ce{NO2} \) groups, which makes it the strongest acid of the four.
Note: It seems that there is a mistake in the original NTA paper. In structure B, the group at the top is printed as “COOOH”; it should be “COOH”, so that B is 4-nitrobenzoic acid.
NCERT (2026-27 edition):
Reactions Involving Cleavage of O–H Bond – “… Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (pKa is ~16 for ethanol and 10 for phenol). In fact, carboxylic acids are amongst the most acidic organic compounds you have studied so far.”
Effect of Substituents on the Acidity of Carboxylic Acids
