Which of the following chlorobutanoic acids is the strongest acid?

A $CH X_{3} CH X_{2} CH X_{2} COOH$
B $ClCH X_{2} CH X_{2} CH X_{2} COOH$
C $CH X_{3} CHClCH X_{2} COOH$
D $CH X_{3} CH X_{2} CHClCOOH$

Answer: D

The -I effect of chlorine decreases rapidly with distance from the carboxyl group. 2-Chlorobutanoic acid has Cl on the alpha carbon, closest to COOH, so it stabilises the carboxylate most (pKa about 2.8, against about 4.0, 4.5 and 4.8 for the others).

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Which of the following chlorobutanoic acids is the strongest acid?

Answer: D

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