Correct order of the following acids in order of their increasing acidic strength is
(A) \( CCl_3COOH \)
(B) \( ClCH_2COOH \)
(C) \( CH_3COOH \)
(D) \( CHCl_2COOH \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (B), (D), (A)
A carboxylic acid is acidic because it loses \( \ce{H+} \) to give the carboxylate ion, which is stabilised by resonance. Any group that pulls electron density away from the carboxylate ion spreads its negative charge further and makes the acid stronger. Chlorine is strongly electronegative and withdraws electrons by its −I effect; each extra chlorine on the α-carbon adds to the effect.
(C) \( \ce{CH3COOH} \): no chlorine; the methyl group even pushes electrons towards the carboxylate (+I). The weakest acid (pKa about 4.76).
(B) \( \ce{ClCH2COOH} \): one chlorine; stronger (pKa about 2.86).
(D) \( \ce{CHCl2COOH} \): two chlorines; stronger still (pKa about 1.29).
(A) \( \ce{CCl3COOH} \): three chlorines; the strongest acid of the four (pKa about 0.65).
Shortcut: Acetic acid, with no chlorine, must come first. Only Option (2) starts with (C).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – …
