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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.

Martin - マーチン
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Abhirikshma
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  • The answer lies in the resonance energy of various resonance structures of both the carbanions (formed after donating H+). – Sujith Sizon Dec 07 '15 at 17:44
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    related http://chemistry.stackexchange.com/questions/7309/resonance-of-benzoic-acid – Mithoron Dec 07 '15 at 17:54
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    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 07 '15 at 17:58
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    @SujithSizon You're second comment is wrong and vide your first comment - we have oxoanions, not carbanions here. – Mithoron Dec 07 '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 07 '15 at 18:08
  • @Mithoron the questions that you have cited are in no way same as what I had asked. Interestingly the two questions in the links even contradict each other. – Abhirikshma Dec 07 '15 at 18:09
  • Aromatic ring is inductive acceptor and mesomeric donor - together it makes benzoic acid stronger then acetic which is only inductive donor. @SujithSizon I don't get your point – Mithoron Dec 07 '15 at 18:19
  • @Mithoron you cannot probably say that aromatic ring is inductive acceptor straight away. What if we add a fluorine group to it ? – Abhirikshma Dec 07 '15 at 18:22
  • @Mithoron When we try to show that formic acid is more acidic than benzoic acid, all these points regarding Inductive and Mesomeric effect are immediately flawed. Sooner or later Abhirikshma will face the same situation, thats why i was trying to give him hint regarding resonance energies. – Sujith Sizon Dec 08 '15 at 01:14
  • I found a solution on YouTube – 5 Dots Feb 14 '22 at 07:03
  • https://youtu.be/wH3dVLxxoYs (in HINDI) – 5 Dots Feb 14 '22 at 07:03

1 Answers1

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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.

Martin - マーチン
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A.K.
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    The phenyl ring is donating electron to the carboxylic acid group through resonance, that is what I am talking about. – Abhirikshma Dec 07 '15 at 17:57
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    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. – A.K. Dec 07 '15 at 18:02
  • How is that the ortho effect in substituted benzoic acids increase the acidic strength ? – Abhirikshma Dec 07 '15 at 18:10
  • please elaborate. – A.K. Dec 07 '15 at 18:13
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    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. – Abhirikshma Dec 07 '15 at 18:18
  • @Abhirikshma http://chemistry.stackexchange.com/questions/7683/ortho-effect-in-substituted-aromatic-acids-and-bases Please refrain fro prolonged discussions in comments. There's chat for that – Mithoron Dec 07 '15 at 18:20
  • @A.K. I agree that because of the stability due to aromaticity, phenyl ring donating electrons to the carboxylate is not a major resonance structure and that electron withdrawing nature of phenyl group stabilizes the negative charge, but in that case how will we explain that formic acid(pKa=3.751) is more acidic than benzoic acid(pka=4.204). Before reading your comment, I believed that it was because of the destabilization caused by phenyl ring to caboxylate. But now how to explain it? – Arishta Dec 08 '17 at 18:01
  • OP is on the point, to satisfy one condition(benzoic acid>acetic acid or formic acid>benzoic acid) other will contradict instantly – 5 Dots Feb 14 '22 at 06:53