Organic: How is nucleophilic addition to a carbonyl carbon possible?

[Spirochete]

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.

 

[GCT]

OH- is not a good substitution group for either SN3 or nucleophilic addition eliminations - however species of RO- can become involved in transesterifications which is a type of nucleophilic addition eliminations. I'm not quite certain what your question is exactly.

 

[Spirochete]

well somebody else gave me an explanation that was satisfactory. A carbonyl carbon has much more partial positive charge because it loses electron density to oxygen through resonance, whereas a tetrahedral carbon can only lose electron density inductively which is generally a weaker force.

 

[Gannon]

OH- can be a leaving group if a stronger base such as NH3 attacks the carbonyl compound under acidic conditions.

 

[kalery]

The carbonyl carbon is electrophilic due to induction. The oxygens pull electron density away from this carbon, making it more vulnerable to nucleophilic attack. The stability of the tetrahedral intermediate is less relevant, as this is what you get after the initial kinetic reaction of negative and positive sites.