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My book says that benzoic acid does not show resonance as the carboxylate anion and the benzene ring are not in the same plane due to steric effects.

But there aren't any large groups in the ortho positions to cause steric inhibition of resonance. Then, is the info given in the book wrong? If not, where does the steric effect come from?

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I have never heard of this effect occurring with an ordinary, unsubstituted benzoic acid. There is, however, a phenomenon known as the ortho effect that occurs when bulky groups occupy the ortho positions on the ring. In that case, the steric hindrance between the bulky groups and the carboxylic acid moiety forces the carboxylic acid group to rotate out of the plane of the aromatic ring, thereby breaking the conjugation.

One of the consequences of this, which may not be readily apparent, is that this often produces a substantial increase in the acidity of the carboxylic acid, even when the substituents are not electron-withdrawing (viz. 2,6-dimethylbenzoic acid, which is disubstituted at the two ortho positions, having a $\mathrm{p}K_\mathrm{a}$ roughly one unit lower than ordinary benzoic acid, while similar para- and meta-substituted benzoic acids have higher $\mathrm{p}K_\mathrm{a}$ values — that is, are less acidic).

While I've seen references to this effect in multiple sources, I've never actually found a thorough explanation for it. I think it could be argued that the benzene ring is electron-donating within the $\pi$-system (i.e., by resonance), which would only serve to destabilize the carboxylate conjugate base already bearing a negative charge. By rotating out of the plane, the electron-donating effect is removed. Rather than consider the stability of the resulting carboxylate, I suppose one could also consider the stability of the carboxylic acid directly. Here, I'd expect that the reactivity of the carboxylic acid flanked by bulky substituents should be greater, as the loss of $\pi$-conjugation with the carboxylic acid moiety is inherently destabilizing. My reasoning may be flawed, though, so if anybody has some corrections or a better explanation to offer, that would be entirely welcome.

Whether this effect occurs with ordinary benzoic acid or not, I can't be certain. At least one good reason I can think of to doubt that it does is the observed regioselectivity of electrophilic aromatic substitution reactions on benzene rings conjugated to carbonyl groups. They are known to be deactivating (i.e., they withdraw electron density) via their conjugation with the $\pi$ system of the aromatic ring. If they were not coplanar with the ring, the conjugation would be impossible, and the deactivating effect would not be observed.

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  • $\begingroup$ So, you are saying that the $COOH$ group experiences steric hindrance (and hence, rotates out of plane) when there are bulky groups in the ortho positions, right? $\endgroup$ – Ris97 Dec 12 '13 at 4:08
  • $\begingroup$ @Ris97, to the best of my knowledge, yes. $\endgroup$ – Greg E. Dec 12 '13 at 12:53
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    $\begingroup$ @GregE. You are correct in saying that only if there are bulky groups on o-position, than the carboxylic group goes out of plane . "Electrophilic aromatic substitution reaction will take place mainly in 3-position due to the electron-withdrawing carboxylic group; i.e. benzoic acid is meta directing." this from wikipedia suggests meta direction so resonance is present . Also if only inductive effect had been responsible for this , para would have been major . Anyway your answer covers it all . +1 $\endgroup$ – Rajesh Mar 7 '15 at 4:39

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