Favorskii Rearrangement
Transformation of α-haloketones having an α-hydrogen on the non halogenated side of the ketone to esters with rearranged carbon skeleton by the treatment with alkoxide ions (used as base) is called Favorskii Rearrangement. Alkali hydroxides or amines in place of metal alkoxides gives acids or amides respectively.
Cyclic α-haloketones gives esters with ring contraction.
Mechanism of Favorskii Rearrangement
Mechanism of Favorskii Rearrangement involves the following steps:
In the first step, the base abstracts an 𝞪'-H atom and form an enolate ion.
In the second step, 𝞪'-carbanion attacks on the 𝞪-carbon atom and the halide ion leave to produce a cyclopropanone intermediate by intramolecular SN2 displacement of halogen by 𝞪'-carbanion.
In the third step, the cyclopropanone intermediate undergoes the ring-opening to form a stable carbanion. In the case of an unsymmetrical cyclopropanone, the direction of ring-opening is determined by which is the more stable carbanion of the two possible carbanions that can be formed.
In the final step, the carbanion abstracts a proton from the solvent to form corresponding carboxylic acids or carboxylic acid derivatives.
Favoraskii Rearrangement with Cyclic Ketones
Favoraskii Rearrangement with Unsymmetrical Ketones
Stereochemistry Involved in the Favorskii Rearrangement
Favorskii rearrangement involves the SN2 displacement of the halogen atom. Hence, an inversion of configuration occurs at the centre containing a halogen atom and the reaction is considered to be a stereospecific reaction.
