Reactivity of thioesters with respect to nucleophilic attack

Why are thioesters relatively reactive with regard to nucleophilic attack? Prof says to wait until pchem 3 when we learn about orbital symmetry. He also said that sulfur’s d-orbitals (?!) don't have the correct symmetry to participate in resonance with the carbonyl carbon.

Wait. I thought that sulfur didn’t utilize its d-orbitals. Was it previously taught that sulfur’s lone pairs were in d-orbitals? Why would sulfur even need d-orbitals when in a thioester? Can't we just explain this based on the poor overlap between carbon's 2p and sulfur's hypothetical 3p orbitals?

• Apply the concepts discussed here to thioesters – ron Mar 10 '15 at 14:36
• @ron So would you chalk a thioester's reactivity to the 2p-3p bond between the C and S? – Dissenter Mar 10 '15 at 14:58
• Yes, similar to an acid chloride. – ron Mar 10 '15 at 15:15
• @Ron so d orbitals are irrelevant? – Dissenter Mar 10 '15 at 18:32
• Some do, some don't ($\ce{d_{z^2}}$), it also depends on the relative orientation of the orbitals. For example an s orbital has the proper symmetry to interact with a p orbital if the interaction is along the long axis of the p-orbital and at the "end" of the p orbital. On the other hand, due to symmetry, interaction along the long axis but at the center of the p-orbital cannot result in constructive overlap. – ron Mar 10 '15 at 18:46