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I've heard some people say that benzoic acid has a $+\ce{M}$ effect due to the benzene ring which destabilizes the conjugate base. Also, this would be the dominating factor over the weak $-\ce{I}$ effect of the phenyl ring. This makes sense and also explains that benzoic acid is less acidic than methanoic acid (which has no significant effects). But this fails to explain why alkanoic acids (ethanoic and onwards) are less acidic than benzoic acid. The former has a $+\ce{I}$ effect, so is it that the overall electron donating effect of benzoic acid is less than that of alkanoic acids (i.e. the $-\ce{I}$ of phenyl reduces the EDG nature of? If this is the case, that surprises me because generally resonance is a strong enough factor to overcome a weak inductive effect.

Any help is appreciated!

AVS
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  • There was quite a bit of questions on this topic. And you know what? In practice, you can either believe whatever lie for student you want, or better ignore all lame explanations and just remember facts. Some +M or +I certainly qualify as lame. – Mithoron Dec 21 '23 at 18:42
  • Fair enough, I guess. So you're saying this particular fact can't really be explained well theoretically? – AVS Dec 21 '23 at 19:32
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    I guess if you used some software, maybe you could get satisfying results. Otherwise I don't think so. – Mithoron Dec 21 '23 at 19:37
  • https://chemistry.stackexchange.com/questions/7591/comparing-acidities-of-substituted-and-aromatic-carboxylic-acids – Mithoron Dec 25 '23 at 23:05
  • But solvation likely isn't the explanation - I heard HCOOH is stronger even in gas phase. – Mithoron Dec 25 '23 at 23:07

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