Which is More Acidic: Phenol or Acetic Acid?

Introduction

Phenol and acetic acid both have O-H groups that can donate a proton, and both conjugate bases benefit from resonance. But one is dramatically more acidic. The quality of resonance structures matters, not just the quantity.

Acidic Protons

Phenol donates its proton from the hydroxyl group attached to the benzene ring. Acetic acid donates from its carboxylic acid O-H. Both lose a proton from oxygen.

Governing Factor: Resonance

The key factor is resonance equivalence. Acetate has 2 resonance structures that are perfectly equivalent - the negative charge is shared equally between two identical oxygen atoms. Phenoxide has 5 resonance structures, but they are not equivalent since placing charge on carbon is less favorable than on oxygen.

Conjugate Base Stability

Acetate distributes its charge 50/50 between two identical oxygens. Phenoxide spreads charge across oxygen and ring carbons, but the structures with charge on carbon are higher in energy. Equivalent resonance structures provide superior stabilization.

pKa Comparison

Acetic acid has a pKa of 4.8 and phenol has a pKa of 10. The 5.2-unit difference means acetic acid is over 100,000 times more acidic, primarily because acetate has equivalent resonance structures.

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