Four para-substituted benzoic acids are shown beside the options (the drawn compound is benzoic acid, for reference). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: D

A para nitro group withdraws electrons by -I and -M and stabilises the carboxylate: p-nitrobenzoic acid has pKa about 3.44. p-Toluic acid (4.34), p-anisic acid (4.47) and p-aminobenzoic acid (4.92) are weaker than or close to benzoic acid (4.20) because of +I or +M donation.

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