17.5 Benzilic acid Rearrangement

  Aldehydes and Ketones

The addition of a strong base to a carbonyl group results in the formation of an anion. The reversal of the anionic charge may cause expulsion of the attached group X, e.g.

However, in a 1,2-diketone the group X may migrate to the adjacent electron-deficient carbonyl carbon forming $\alpha $-hydroxy acid.

Thus, benzil on treatment with a strong base forms benzilic acid (salt), whence the name benzilic acid rearrangement.

Barium and thallous hydroxides are more effective than sodium or potassium hydroxides. Alkoxide ions (methoxide, t-butoxide etc.) in place of hydroxide ion give the corresponding esters.

Phenoxide ions are too weak a nucleophile to attack. Besides aromatic 1,2-diketones, aliphatic and heterocyclic diketones as also o-quinones undergo this rearrangement.

Mechanism: It has been seen the rate of reaction is proportional to the concentration of benzyl and the hydroxide ion, i.e., rate $\alpha \left[ {{C_6}{H_5}COCO{C_6}{H_5}} \right]\left[ {O{H^--}} \right]$

It has also been found that when the reaction is carried out in the presence of ${H_2}^{18}O,$ benzyl exchanges $^{18}O$ faster than it rearranges.

On the basis of the above observations, it has been suggested that a fast reversible nucleophilic attack occurs at the carbonyl carbon in the first step. The second step is the rate determining step in which the migration occurs. Finally, a rapid proton transfer completes the process.

The rearrangement is analogous to intramolecular Cannizzaro reaction of glyoxal.

The carbonyl group which is attached to the less electron releasing of the two aryl groups is relatively more positively charged and, hence, is attacked by OH–. Consequently, the less electron donating aryl group migrates to the other carbonyl group.