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Which hydrogen of the compound shown is the most acidic?

Athe ring hydrogens ortho to the OH
Bthe O-H hydrogen of the phenol
Cthe ring hydrogens ortho to the COOH
Dthe O-H hydrogen of the carboxyl group

Answer: D

The compound is 4-hydroxybenzoic acid. The carboxylic acid O-H (pKa about 4.6) is far more acidic than the phenolic O-H (about 9.3) because the carboxylate negative charge is delocalised over two oxygens; aromatic C-H hydrogens are not acidic.

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