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.
The correct order of decreasing acid strength of acetic acid (A), formic acid (B), Benzoic acid (C) is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B, C, A
The strength of a carboxylic acid depends on how well its carboxylate ion is stabilised. An electron donating (+I) alkyl group destabilises the ion and weakens the acid; a phenyl group attached directly to COOH increases the acidity because its sp2 carbon is more electronegative than an sp3 carbon.
(B) Formic acid, HCOOH (pKa 3.75) — no alkyl group on the carboxyl carbon; the strongest of the three.
(C) Benzoic acid, \( \ce{C6H5COOH} \) (pKa 4.19) — the phenyl ring makes it stronger than acetic acid but weaker than formic acid.
(A) Acetic acid, \( \ce{CH3COOH} \) (pKa 4.76) — the +I effect of \( \ce{CH3} \) makes it the weakest.
Shortcut: Only Option (3) starts with B and ends with A.
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – …
