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Benzilic Acid Rearrangement

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$\alpha$-diketones on treatment with ethanolic potassium hydroxide undergo rearrangement involving 1,2 phenyl shift to give $\alpha$-hydroxy acids. The reaction usually referred to as benzilic acid rearrangement involves the nucleophilic attack of the hydroxide ion at the carbonyl carbon. The intermediate oxyanion then undergoes 1,2-nucleophilic shift of phenyl group to the adjacent carbonyl carbon.

The transformation of $\alpha$-diketones to the salts of $\alpha$-hydroxy carboxylic acid, in the presence of bases is known as benzilic acid rearrangement. The best known example is the conversion of benzil into benzilic acid anion.

Benzilic Acid Rearrangement

Mechanism

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The mechanism of benzilic acid rearrangement is an example of 1,2-shift intra molecular rearrangement.

This arrangement involves three steps.
  1. Addition of -OH group
  2. Migration of a phenyl group
  3. Migration of a proton
Benzilic Acid Rearrangement Mechanism

The benzilic acid rearrangement is an irreversible process. The first step of the mechanism is the addition of the nucleophile across the C=O bond to give a tetrahedral intermediate.

Applications

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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.

Benzilic Acid Rearrangement Applications

The reaction may be used for the preparation of $\alpha$-hydroxy acids from the easily accessible starting materials.

Benzilic Acid Rearrangement Extension

It is important to note that as the ethoxide and isopropoxide ions reduce benzil to benzoin these reagents do not carry out benzilic rearrangement.