<P> A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to the hydroxyl anion 2 . The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group in a concerted step with reversion of the hydroxyl group back to the carbonyl group . This sequence resembles a nucleophilic acyl substitution . Calculations show that when R is methyl the charge build - up on this group in the transition state can be as high as 0.22 and that the methyl group is positioned between the central carbon carbon at a separation of 209 pm . </P> <P> The carboxylic acid in intermediate 4 is less basic than the hydroxyl anion and therefore proton transfer takes place to intermediate 5 which can be protonated in acidic workup to the final α - hydroxy--carboxylic acid 6 . Calculations show that an accurate description of the reaction sequence is possible with the participation of 4 water molecules taking responsibility for the stabilization of charge buildup . They also provide a shuttle for the efficient transfer of one proton in the formation of intermediate 5 . </P> <P> A variation of this reaction occurs in certain steroids . In the so - called D - Homo Rearrangement of Steroids a cyclopentane ring expands to a cyclohexane ring with added base . </P>

Synthesis of benzilic acid from benzil reaction mechanism