Which is More Acidic: Phenol or Benzoic Acid?

Introduction

Phenol and benzoic acid both feature a benzene ring and an acidic O-H. But one is a carboxylic acid (COOH) and the other is a simple alcohol attached to the ring. Both conjugate bases have resonance - so which one benefits more?

Acidic Protons

Phenol donates its proton from the O-H directly on the benzene ring. Benzoic acid donates from the carboxylic acid O-H, which is connected to the ring through a carbonyl carbon (C=O).

Governing Factor: Mixed

Multiple factors are at play here. Benzoate has two equivalent resonance structures that distribute the charge equally between two oxygen atoms. Phenoxide has resonance into the ring, but the structures are not equivalent - placing negative charge on carbon is less favorable than on oxygen.

Conjugate Base Stability

Benzoate spreads its charge 50/50 between two identical oxygens in the carboxylate group. Phenoxide delocalizes into the ring, but the primary charge carrier remains the oxygen. The equivalent resonance in benzoate provides stronger stabilization.

pKa Comparison

Benzoic acid has a pKa of 4.2 and phenol has a pKa of 10. The 5.8-unit difference means benzoic acid is nearly 600,000 times more acidic, primarily because of the superior resonance stabilization in the carboxylate group.

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