In the Favorskii rearrangement, why doesn't the base grab the alpha hydrogen of the carbon bearing the halogen?

Question

In the Favorskii rearrangement, the base (like hydroxide) always abstracts the alpha hydrogen of the carbon not bearing the halogen. But to me the hydrogens on the carbon directly connected to the halogen should be more acidic due to the electron pulling inductive effect of the halogen (which indeed does happen in the haloform reaction). This should result in an aldol condensation type reaction. Why is this not so?

Answer 1

I think there are a couple of issues here. You bring up a good point that an alpha hydrogen that is also alpha to a halogen is more acidic, but this doesn't mean it will lead to a product.

1) Aldol addition where the ketone acts as an electrophile has a very small equilibrium constant. Only a tiny percentage of molecules will do this. A condensation product (alpha/beta unsaturation) might only form if water is rigorously removed.

2) Deprotonation of either alpha carbon is an equilibrium, reversible process.

3) Three membered rings form surprising quickly via Sn2, even though they are unstable, due to entropy factors.

4) The formation of the final product in Favorskii rearrangement is not reversible. Once it forms, it is trapped and will not go back.