The Second Most Important Carbonyl Mechanism

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?

Two things.

1. 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.
2. 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