My professor emphasizes the concept of the carboxylate "cage" or the idea that electrons participate in resonance within a carboxylate group such as in the benzoate anion but the electrons in the carboxylate group cannot leave this "cage" - i.e. participate in resonance with the phenyl substituent.
Is this idea valid?
I've already poked a hole into it by pointing out that electron donation to the "cage" is possible - i.e. consider carbonic acid with one hydroxyl group replaced by oxygen connected to a phenyl group. Valid resonance structures can be drawn in which electron density from the oxygen in the ester linkage participates in resonance with the electrons "in" the carboxylate group. So donation into the "cage" is possible. Now, this begs the question - is withdrawal from the cage possible as well? He insists no. Now, he might be hiding some edge cases for pedagogical purposes, but I don't care - if there is an exception - I want to know about it (because chemistry always gets interesting at the exceptions).