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When is OH a good leaving group?

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saippo:
Now I know that OH usually isn't however there seems to be cases when it can leave? I remember asking about this years ago and I remember the professor mentioning something like OH being able to leave at the beta carbon when the alpha carbon is attached to a carbonyl group (esp in basic condition). I'm struggling to remember the name of the exact reaction but I think in sophomore organic chem, it often followed after the aldol reaction. My memory is a bit hazy.

If anyone does know what I'm talk about, can someone clear this up? I was confused by the explanation then and it still confuses me now.


Thanks in advance

hypervalent_iodine:
I suspect you might be talking about the dehydration that you can get following Aldol condensation upon heating under acidic conditions (the aldol reaction itself is done under basic conditions). In this case, the OH becomes a decent leaving group when it is protonated to give water.

saippo:
That might have been it. I know that's usually how OH is converted into a good leaving group however in those examples, the OH just left without being protonated.

Babcock_Hall:
I wonder whether you might be thinking of the E1cb mechanism, which includes aldol condensations.

nakhimov:
The conversion of OH groups to oxonium ions [ROH2]+ is when it's a good LG.

You could also modify the OH functionality to make it an even better LG, since in general alcohols are pretty poor LGs.

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