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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.

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  • $\begingroup$ The answer lies in the resonance energy of various resonance structures of both the carbanions (formed after donating H+). $\endgroup$ – Sujith Sizon Dec 7 '15 at 17:44
  • $\begingroup$ related chemistry.stackexchange.com/questions/7309/… $\endgroup$ – Mithoron Dec 7 '15 at 17:54
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    $\begingroup$ 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. $\endgroup$ – Sujith Sizon Dec 7 '15 at 17:58
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    $\begingroup$ @SujithSizon You're second comment is wrong and vide your first comment - we have oxoanions, not carbanions here. $\endgroup$ – Mithoron Dec 7 '15 at 18:02
  • $\begingroup$ @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? $\endgroup$ – Sujith Sizon Dec 7 '15 at 18:08
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It is all about the electron withdrawing/ donating nature of the acid's functional groups. The more electron withdrawing the group, the stronger the acid will be. The more electron donating the weaker the acid will be. A phenyl ring is electron withdrawing, while a methyl group is electron donating. I don't know why resonance would destabilize an organic molecule or ion. if you are referring to the carboxylate donating electrons to the phenyl ring, I don't see how this could be possible and not violate the octet rule.

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  • $\begingroup$ The phenyl ring is donating electron to the carboxylic acid group through resonance, that is what I am talking about. $\endgroup$ – Abhirikshma Dec 7 '15 at 17:57
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    $\begingroup$ The aromatic ring is so thermodynaically stable that the phenyl ring donating electrons to the carboxylate is not a major resonace structure. Though it is enough to dictate meta directed electrophilic aromatic substitution. $\endgroup$ – A.K. Dec 7 '15 at 18:02
  • $\begingroup$ How is that the ortho effect in substituted benzoic acids increase the acidic strength ? $\endgroup$ – Abhirikshma Dec 7 '15 at 18:10
  • $\begingroup$ please elaborate. $\endgroup$ – A.K. Dec 7 '15 at 18:13
  • $\begingroup$ When we add a bulky substituent at the ortho position it throws the carboxylic acid group out of plane and thus successfully inhibits the phenyl ring from donating electron through resonance, and increases the acidic strength. $\endgroup$ – Abhirikshma Dec 7 '15 at 18:18
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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.

Benzoic acid has a phenyl (benzene ring) as substituent. Now, the benzene ring donates electrons by resonance, and this leads to inductive effect (it will pull the electrons and it will decreases the electron density over oxygen atom of the carboxylic acid so it can be protonated more easily) so it increases the acidity.

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