An organic rearrangement reaction in which benzil (1,2-diphenylethan-1,2-dione) is treated with hydroxide and then acid to give benzilic acid (2-hydroxy-2,2-diphenylethanoic acid):
In the reaction a phenyl group (C6H5–) migrates from one carbon atom to another. The reaction was discovered in 1828 by Justus von Liebig; it was the first rearrangement reaction to be described.