Four chlorinated acids are shown beside the options (the drawn compound is propanoic acid, for reference). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The inductive effect of Cl decreases with distance. 2-Chlorobutanoic acid (pKa about 2.9) is the strongest; 3-chlorobutanoic (4.05), 4-chlorobutanoic (4.5) and unsubstituted butanoic acid (4.8) are progressively weaker.

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