[bhatsinterest]

In a favorskii rearrangement of alpha-chloro cyclohexanone in the presence of NaOH, why doesnt OH- ion (or any other base) attack C-Cl carbon (nucleophilic substitution) in the first stage itself rather than abstraction of hydrogen from the alpha carbon?

One more doubt is, since the hydrogen on C-Cl carbon is more acidic (electronegative chlorine atom there), why doesn't the carbanion gets generated there?

I don't even understand the driving force for the ring contraction in this reaction!!

Whats the reason for all these could be?

[bhatsinterest]

In a favorskii rearrangement of alpha-chloro cyclohexanone in the presence of NaOH, why doesnt OH- ion (or any other base) attack C-Cl carbon (nucleophilic substitution) in the first stage itself rather than abstraction of hydrogen from the alpha carbon?

One more doubt is, since the hydrogen on C-Cl carbon is more acidic (electronegative chlorine atom there), why doesn't the carbanion gets generated there?

I don't even understand the driving force for the ring contraction in this reaction!!

Whats the reason for all these could be?

Hey, No one could explain this better??

[kriggy]

Could be steric reasons since Cl is rather large atom

[clarkstill]

In a favorskii rearrangement of alpha-chloro cyclohexanone in the presence of NaOH, why doesnt OH- ion (or any other base) attack C-Cl carbon (nucleophilic substitution) in the first stage itself rather than abstraction of hydrogen from the alpha carbon?

One more doubt is, since the hydrogen on C-Cl carbon is more acidic (electronegative chlorine atom there), why doesn't the carbanion gets generated there?

I don't even understand the driving force for the ring contraction in this reaction!!

Whats the reason for all these could be?

My thoughts:

1. Sn2 reactions at secondary alkyl halides are fairly slow, particularly where the halide is chlorine (a relatively poor leaving group)
2. It certainly will deprotonate at the methine where the chlorine is attached, but this will just occur reversibly since there is no subsequent reaction that can occur.
3. Driving force - entropically favoured since unimolecular -> bimolecular?

[orgopete]

The Favorskii rearrangement has been much studied so I doubt that I can add anything to what has already been done. I think the proposed mechanism is more of a "what else could it be" than a definitive proof. In my mind, it still leaves some aspects open to question. For example, the spirit of Baldwin's Rules is that the bond forming electrons should overlap with the leaving group. I find it difficult to understand how the pi-electrons of an enolate should enable a back-side attack of the C-Cl bond. I'm not saying it doesn't. I am saying I don't have a better mechanism either.

Regards to rates, we should examine all of the possible reactions that may occur, addition to carbonyl group, deprotonation of alpha-chloro hydrogen, deprotonation of opposed alpha-hydrogen, etc. These are all plausible steps. If the reaction path is faster for one of these steps, then we should conclude these steps are either reversible and did not lead to other products or they are slower.