Four carboxylic acids are shown beside the options (the compound drawn under the question is only a reference acid). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

Electron-withdrawing groups near the carboxyl stabilise the carboxylate. Fluoroacetic acid (pKa about 2.66) is stronger than chloroacetic acid (2.86), acetic acid (4.76) and propanoic acid (4.87), the last being weakened by the +I ethyl group.

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