In aldehydes, the carbonyl group is bonded to a carbon and hydrogen, while in ketones it is bonded to two carbon atoms. The compounds in which the carbonyl group is bonded to oxygen are known as carboxylic acids and their derivatives.
Which series represents compounds in decreasing order of acidity?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → CF3COOH > NO2CH2COOH > ClCH2COOH > CH3COOH
The electron withdrawing groups on these four acids rank as \( \ce{CF3} \) > \( \ce{NO2} \) > Cl, while \( \ce{CH3} \) donates electrons; the acids follow the same order:
CF3COOH — three F atoms pull electrons strongly; it is the strongest carboxylic acid (pKa 0.23).
NO2CH2COOH — \( \ce{NO2} \) is a strong −I group (pKa about 1.7).
ClCH2COOH — one Cl, weaker −I effect (pKa about 2.9).
CH3COOH — \( \ce{CH3} \) donates electrons (+I); the weakest acid of the four (pKa 4.76).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids
