You’ve already learned the most important mechanism in carbonyl chemistry: addition. (C-O Pi bond breaks, C-(nucleophile) bond forms).
Today let’s talk about the second most important mechanism in carbonyl chemistry.
Since you’ve already learned addition, this is pretty easy to remember:
it’s the exact opposite. C-O pi bond forms, C-(leaving group) bond breaks.
This is called “elimination” (sometimes I call it, “1,2-elimination”. The “1,2” isn’t so important right now).
What factors might make this reaction favorable?
- It’s faster when the oxygen is negatively charged. Think of the oxygen as the “nucleophile” in this reaction. The more electron-rich it is, the faster the reaction will occur. That’s why the second example (conversion of cyanohydrins to ketones) goes faster when a strong base is added.
- It’s faster when there’s a good leaving group on carbon. In order for elimination to occur, you have to have at least a half-decent leaving group! Remember what makes something a good leaving group? Good leaving groups are weak bases!
This mechanism is a key part of acetal formation and imine/enamine formation.
Thanks for reading! James