Why does the methyl group at the ortho-position stabilize the carboxylate ion? I recognize that there will be a steric clash between the carboxyl group and the methyl group but I don't understand why this would help increase acidity. If there was any resonance occurring then this steric clash would DESTABILIZE the anion and make it LESS acidic given that the carboxylate group would not be in the same plane as the ring and thus delocalization, which would stabilize negative charge, would be impossible. Clearly there is no conjugation with the aromatic ring in "benzoic acid based compounds" from the carboxylate group. So why does having a methyl group at the ortho-position increase acidity?
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2 Answers
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Having one methyl group ortho to the carboxylate group will not prevent the carboxylate group from becoming reasonably planar with the aromatic ring. If you had a second methyl group in the other ortho position, then planarity would be a problem.
A methyl group attached to a benzene ring is electron releasing in both an inductive and a resonance sense.
- Inductively, the electrons in an $\ce{sp^3}$ hybridized methyl group prefer to shift towards the lower energy (more s-character in the orbital) $\ce{sp^2}$ aromatic carbon.
- The methyl group is also electron releasing in a resonance sense due to carbon-hydrogen hyperconjugation (see figure below).
The resonance structure pictured below on the left is one of many that we can draw for the o-toluic carboxylate anion. Note that it places a positive charge on the methyl bearing ortho carbon. Both inductively and through resonance (see the hyperconjugated resonance structure below on the right) the methyl group will stabilize this resonance structure. It is this stabilization provided by the methyl group that makes o-toluic acid a stronger acid (pKa=3.91) than benzoic acid (pKa=4.19).
Note: As pointed out by both @Mithoron and @GaurangTandon, this answer does not explain why p-toluic acid is a weaker acid than o-toluic acid or benzoic acid. The answers in this earlier question better answer this question.
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o-toulic acid is stronger than benzoic acid because the conjugate base of o-toulic acid is stabilized by hydrogen bonding (C–H···O interaction).
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2$\begingroup$ Wow. That is a very untypical hydrogen bond. $\endgroup$ Commented Dec 2, 2016 at 11:46
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1$\begingroup$ I downvoted this answer because this is the same answer as Klaus' answer from more than two years earlier $\endgroup$ Commented Feb 22, 2018 at 8:49