Consider the following statements about the acidity of carboxylic acids.
(A) Carboxylic acids evolve hydrogen with electropositive metals.
(B) The smaller the pKa value of an acid, the weaker the acid.
(C) Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols.
(D) Electron withdrawing groups decrease the acidity of carboxylic acids by stabilising the conjugate base.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
A carboxylic acid ionises in water to give a carboxylate anion and a hydronium ion. The more stable the anion, the more the acid ionises and the larger its Ka. The strength is usually given as pKa:
\( \displaystyle \mathrm{p}K_a = -\log K_a \)
(A) Carboxylic acids evolve hydrogen with electropositive metals. — Correct statement. Like alcohols, they give a salt and hydrogen gas with a metal such as sodium:
\( \displaystyle \ce{2CH3COOH + 2Na -> 2CH3COONa + H2 ^} \)
(B) The smaller the pKa value of an acid, the weaker the acid. — Incorrect statement. Because of the minus sign, a larger Ka gives a smaller pKa. So the smaller the pKa, the stronger the acid; for example, benzoic acid (pKa 4.19) is stronger than acetic acid (pKa 4.76).
(C) Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols. — Correct statement. Hydrochloric acid has pKa −7.0, acetic acid 4.76 and ethanol about 16.
(D) Electron withdrawing groups decrease the acidity of carboxylic acids by stabilising the conjugate base. — Incorrect statement. Stabilising the conjugate base favours ionisation, so electron withdrawing groups increase the acidity. It is electron donating groups that decrease it, by destabilising the conjugate base.
NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols.”
“For convenience, the strength of an acid is generally indicated by its pKa value rather than its Ka value. pKa = – log Ka The pKa of hydrochloric acid is –7.0, where as pKa of trifluoroacetic acid (the strongest carboxylic acid), benzoic acid and acetic acid are 0.23, 4.19 and 4.76, respectively.”
“Smaller the pKa, the stronger the acid (the better it is as a proton donor).”
“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).”
“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.”