The transformation of alpha-haloketones to esters with rearranged carbon skeleton by the treatment with alkoxide ions is called Favorskii rearrangement. Alkali hydroxides or amines in place of metal alkoxides give acids or amides respectively.
The subjects of this section are two reactions that do not actually involve carbocation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, alpha halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, alpha halo sulfone gives an alkene
Cyclic a-haloketones give esters with ring contraction.
Mechanism : The mechanism of the rearrangement has been subject of much Investigation. It was observed that both the isomeric ketones on treatment with CH3O– Ions.
There is also a mechanism that can operate in the absence of an acidic hydrogen. This process, called the semibenzilic rearrangement, is closely related to the pinacol rearrangement. A tetrahedral intermediate is formed by nucleophilic addition to the carbonyl group and the halide serves as the leaving group.
The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanism may be fairly closely balanced. Cases of operation of the semibenzilic mechanism have been reported even for compounds having a hydrogen available for enolization.
Among the evidence that the cyclopropanone mechanism operates is the demonstration that a symmetrical intermediate is involved.