Four nitro- and methyl-substituted benzoic acids are shown beside the options (the drawn compound is benzoic acid, pKa 4.20). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: D

o-Nitrobenzoic acid (pKa about 2.2) gains from the strong -I effect of the adjacent NO2 plus the ortho effect (COOH twisted out of plane). p-Nitrobenzoic acid is 3.44, m-nitrobenzoic acid 3.45 and o-toluic acid 3.91.

Practice General Organic Chemistry II: Acids and Bases More on Keto-enol tautomers and ortho effect in drawn compounds

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