Which hydrogen of the molecule shown is the most acidic?

AThe carboxylic acid O-H
BThe phenolic O-H
CAn aromatic C-H
DThe C-H next to the ring

Answer: A

The drawn compound is 4-hydroxybenzoic acid. Carboxylic acids (pKa about 4.5) are far more acidic than phenols (pKa about 10), because the carboxylate negative charge is spread over two oxygens. Aromatic C-H bonds are essentially non-acidic.

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