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All the carbon and oxygen atoms are sp2 hybridised and can have a p orbital in the correct plane yet only conjugation of the COO- group occurs. I can't think why. I can't draw resonance structures for the negative charge being delocalised around the ring but if you look at it from the orbital perspective (sp2 hybridisation) it seems clear that this conjugation should occur.

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Here are some of the resonance structures we can draw for the carboxylate anion of benzoic acid.

enter image description here

You're correct that we can't delocalize the negative charge from the oxygen into the benzene ring. Even though there is continuous p-orbital overlap between the oxygen and benzene carbons, there is just no way to push the electrons so that a negative charge winds up in the benzene ring. However, as the resonance structures show, we can move electrons the other way and place a second negative charge on the oxygens and leave a positive charge delocalized around the ortho and para positions in the benzene ring. This is why the carboxylate anion of benzoic acid will not react with electrophiles (electrophilic aromatic substitution) at those positions in the aromatic ring.

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  • $\begingroup$ Why can't we have a resonance structure in which the Oxygen donates 2 of its electrons to the carbon and thus is slightly positive with only 6 electrons, this will push electrons into the ring. $\endgroup$ – user1084113 Aug 23 '15 at 3:21
  • $\begingroup$ Oxygen doesn't like to be positive because of its electronegativity and in the benzoate anion there is actually a formal negative charge on oxygen. But maybe I am not understanding your question. If so, please ask it as a new question with a drawing of your proposed resonance structure. $\endgroup$ – ron Aug 23 '15 at 13:03

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