Arrange the following acids in increasing order of their acidic stren…

The correct answer is Option (4) → HCOOH < ClCH₂COOH < FCH₂COOH < NO₂CH₂COOH

An electron withdrawing group on the α-carbon pulls electron density away from the carboxylate ion, spreads its negative charge and stabilises it, so the acid becomes stronger. The stronger the electron withdrawal, the stronger the acid.

• \( \ce{HCOOH} \): no electron withdrawing group next to –COOH, first (weakest acid)

• \( \ce{ClCH2COOH} \): −I effect of chlorine: second

• \( \ce{FCH2COOH} \): fluorine is more electronegative, stronger −I effect: third

• \( \ce{NO2CH2COOH} \): the nitro group withdraws electrons even more strongly, last (strongest acid)

Shortcut: HCOOH is the weakest and the nitro acid the strongest. Only Option (4) starts with HCOOH and ends with \( \ce{NO2CH2COOH} \).

NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “… The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO₂ < CF₃. Thus, the following acids are arranged in order of increasing acidity (based on pK_a values): CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NC-CH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH”

Note: The same/similar question has been asked in different years by NTA as per the details given below: