# Why is benzoic acid a stronger acid than acetic acid?

Why is benzoic acid ($\mathrm{p}K_\mathrm{a} = 4.20$) a stronger acid than acetic acid ($\mathrm{p}K_\mathrm{a} = 4.76$), even though the conjugate base in case of benzoic acid is destabilized due to electron donation through resonance. Whereas there is nothing of that sort operating in acetic acid; only inductive and hyperconjugative electron donation.

• The answer lies in the resonance energy of various resonance structures of both the carbanions (formed after donating H+). – Sujith Sizon Dec 7 '15 at 17:44
• – Mithoron Dec 7 '15 at 17:54
• Among various types of resonance effects equivalent Resonance makes the most stable molecules. Here the resonance with benzene ring causes the carboxylate ion to lose its precious equivalent Resonance and thus decrease its acidity. – Sujith Sizon Dec 7 '15 at 17:58
• @SujithSizon You're second comment is wrong and vide your first comment - we have oxoanions, not carbanions here. – Mithoron Dec 7 '15 at 18:02
• @Mithoron hmm.. but isn't stability of oxanions proportional to carbanion stability here? Also isn't equivalent resonance in $\ce{COO-}$ the reason for the higher energy of separate resonance structures. Don't we use the same fact for proving that acidity: H-COOH > Ph-COOH? – Sujith Sizon Dec 7 '15 at 18:08

Acetic acid has a $\ce{-CH3}$ group - which is electron donating - so it's going to decrease the acidity, as it would destabilize the conjugate base.