In the benzoate ion, does the carboxylate group show a $-I$ or a $+I$ effect?

I learnt that the carboxylate group is a $-I$ group, but according to a website the carboxylate group in benzoate shows a $+I$ effect.

M and I effects of benzoate

So, which one is correct? Does the carboxylate group show a positive or a negative inductive effect?

Please neglect mesomeric effect when answering.

  • 4
    $\begingroup$ To the close voters, I think this question shows enough thought and reasoning not to close it. To the OP, please make sure to edit and cite your source. $\endgroup$ – M.A.R. Nov 24 '16 at 5:58
  • $\begingroup$ the carboxylic acid group always shows -I effect due to the presence of electronegative oxygen atoms, however the carboxylate group(COO-) shows +I effect due to excess negative charge. $\endgroup$ – Abhimanyu Jun 22 '20 at 2:33

The carboxy group is electron-withdrawing both by inductive and by mesomeric mechanisms. I have drawn two depictions in the scheme below. The upper one is how the mesomeric effect of the carboxy group can be explained. This effect is only relevant if the $\ce{COOH}$ fragment is in the same plane as the ring.

depictions of the inductive and mesomeric effects of a carboxy group

In the case of 2,6-di(tert-butyl)benzoic acid, the bulky tert-butyl groups prevent the mesomerism depicted above. The carboxy group is perpendicular to the ring. Yet, this group still exercises an electron-withdrawing inductive effect. It can be explained by a mesomeric structure of the $\ce{C=O}$ double bond with charge separation:

$$\ce{C=O <-> C^+-O^-}$$

This positive charge is, naturally, electron-withdrawing and can be used to explain the $-I$ effect of carboxy groups. No matter which mesomeric depiction you choose, you can never get a negative charge on the carboxy carbon, thus it will always remain partially positive. That also makes sense considering it is bonded to two electron-withdrawing, strongly electronegative oxygen atoms.


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