Arrange the following compounds in decreasing order of their acidity.…

The correct answer is Option (1) → (D), (C), (B), (A)

A carboxylic acid is stronger when its conjugate base (carboxylate ion) is more stable. Electron-withdrawing groups pull electron density away from the \( \ce{-COO^-} \) group, spread out its negative charge and stabilise it, so they increase the acidity. Chlorine is strongly electronegative and withdraws electrons by its −I effect; each extra chlorine on the α-carbon adds to the effect:

    \( \displaystyle \ce{CH3COOH <=> CH3COO^- + H^+} \)

    \( \displaystyle \ce{Cl3C-COO^-}\ \text{(charge most spread out)} \gt \ce{Cl2CH-COO^-} \gt \ce{ClCH2-COO^-} \gt \ce{CH3-COO^-} \)

The methyl group of acetic acid has a small +I effect, so acetic acid is the weakest. The pK_a values agree (lower pK_a = stronger acid): trichloroacetic acid 0.7, dichloroacetic acid 1.3, chloroacetic acid 2.9, acetic acid 4.8. In decreasing order of acidity:

    \( \displaystyle \ce{Cl3CCOOH} \gt \ce{Cl2CHCOOH} \gt \ce{ClCH2COOH} \gt \ce{CH3COOH} \)

Shortcut: Trichloroacetic acid is the strongest and acetic acid the weakest, so the order starts with (D) and ends with (A): Options (1) and (4). Since two chlorines withdraw more than one, (C) comes before (B), which leaves Option (1).

NCERT (2026-27 edition):
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. … 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”