We all know that hydroxide will never be a leaving group in an SN2 reaction, no matter how strong the nucleophile. Yet an intermediate containing R-O- is created when a nucleophile adds to a carbonyl compound. It looks sort of like a leaving group, but it's still attatched to the molecule. What is the apparently huge difference between these two reactions that allows this to happen? The only difference I can see is that SN2 involves a pair of SP3 electrons and the addition to a carbonyl involves a pair of P orbital electrons being shifted. By VSEPR the intermediate has its new electron pair in an SP3 orbital, but I know lone pair orbitals can be unpredictable.