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Which of the following carboxylic acids is the strongest acid? (A) CH3CH2CH(Cl)CO2H (2-chlorobutanoic acid) (B) ClCH2CH2CH2COOH (4-chlorobutanoic acid) (C) CH3COOH (acetic acid) (D) HCOOH (formic acid)

1. CH3CH2CH(Cl)CO2H
2. ClCH2CH2CH2COOH
3. CH3COOH
4. HCOOH

Correct answer: CH3CH2CH(Cl)CO2H

Solution

Inductive effect of Cl on carboxylic acid acidity decreases rapidly with distance. Alpha-chloro acid (Cl on C-2, adjacent to COOH) has the strongest electron withdrawal and hence the lowest pKa (highest acidity). Gamma-chloro (C-4) is much weaker. Formic acid lacks the methyl group's slight electron donation, making it more acidic than acetic, but still less acidic than alpha-chloro acid.

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