Benzilic Acid Rearrangement

Theory and Defination :

Benzilic Acid Rearrangement is the rearrangement reactions of 1, 2-diketones to give alpha hydroxy carboxylic acids. 1, 2-Diketones can be converted into the salt of an alpha hydroxy caboxylic acid upon treatment with alkali hydroxide after acidic workup, the free  alpha hydroxy carboxylic acid is obtained.
A well-known example is the rearrangement of benzil into 2-hydroxy-2, 2-diphenyl acetic acid. The substituent should not bear hydrogen to the carbonyl group, in order to avoid competitive reactions.
 The conversion of benzil (α-diketone) into the salt of α-hydroxy acid by means of base treatment is generally referred to as the benzilic acid rearrangement or benzil-benzilic acid rearrangement. This rearrangement is normally carried out in the favored solvents of water and aqueous ethanol, and also in other aqueous organic solvents, such as in aqueous dioxane or even in solid state. This rearrangement has been reported to complete within a few hours under refluxing condition. The counterion of the base affects the reaction rate of the rearrangement when the reaction is carried out in aqueous organic solvents. The coordination of metal cation also helps this rearrangement.

General Reaction :

Illustration as below ,

 

 

 

Mechanism:

• Reaction is induced by nucleophilic addition of the hydroxide anion to one of the two carbonyl groups.
• The aryl substituent migrates with the bonding electrons to the adjacent carbon atom.
• Electrons excess at the center is avoided by the release of a pair of $\pi$-electrons from the carbonyl group to the oxygen.

Finally, a proton transfer leads to the formation of carboxylate anion. The benzilic acid rearrangement of cyclic diketones are of particular interest, since these reactions leads to ring contraction.

Example and Application: 

 

1) The reaction is general one and can take place with aromatic, heterocyclic, alicyclic, and aliphatic 1, 2 – diketones as also 1,2 quinones.

 

 

2) Doering extended the reaction to the formation of the corresponding ester by replacing the normal alkali by alkoxides. Thus benzil may directly be converted into alkyl benzilate by treatment with sodium alkoxide. 

3) The reaction may be used for the preparation of αα-hydroxy acids from the easily accessible starting materials.

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