The Benzilic Acid Rearrangement is a classic organic reaction in which a 1,2-diketone (such as benzil) is transformed into an α-hydroxy carboxylic acid in the presence of a base and subsequent acid workup.
Barium and thallous hydroxides are more effective than sodium or potassium hydroxides. Alkoxide ions (methoxide or t-butoxide) in place of hydroxide ion gives the corresponding ester.
1,2-diketones, aliphatic, heterocyclic diketones, and o-quinones undergoe this rearrangement.
Mechanism of Benzilic Acid Rearrangement
It has been seen that the rate of reaction is proportional to the concentrations of benzil and the hydroxide ion, i.e. rate ∝ [Benzil][OH]
Following steps involved in the mechanism:
- Nucleophilic Addition: Hydroxide ion attacks one of the carbonyl carbons of the diketone, producing a tetrahedral alkoxide intermediate.
- 1,2-Migration (Rearrangement): One of the adjacent groups (usually an aryl/phenyl group) migrates from the alkoxide-bearing carbon to the other carbonyl carbon, forming a new alkoxide intermediate.
- Proton Transfer: This intermediate is deprotonated, yielding an α-hydroxy carboxylate anion.
- Acidification: On acid work-up, the carboxylate is converted to the corresponding α-hydroxy acid.
Order of Migratory Aptitude in Benzilic Acid Rearrangement
Aryl (phenyl) > Alkyl
Among alkyl groups: tertiary alkyl > secondary alkyl > primary alkyl > methyl (However, aryl groups typically dominate migration in diaryl substrates.)
Applications of Benzilic Acid Rearrangement
The reaction is a general one and can takes place with aromatic, heterocyclic, alicyclic, aliphatic 1,2-diketones, and 1,2-quinones.
