Reactions of carbonyl compounds in basic solutions. Part 20. Mechanism of the base-catalysed rearrangement of symmetrically substituted benzils

Abstract

Rate coefficients have been measured for the base-catalysed rearrangement of a series of symmetrically 2,2′-, 3,3′- and 4,4′-disubstituted benzils at 30.0 and 60.0 °C in 70%(v/v) aqueous dimethyl sulfoxide (DMSO), as well as at 40.0 and 50.0 °C for benzil itself. The enthalpies and entropies of activation have been evaluated. For benzil itself, rate coefficients have been measured at 60.0 °C in a series of aqueous DMSO and dioxane mixtures. The effects of meta/para-substitution have been assessed by means of the Hammett equation to give, using σ(and not 2σ), a ρ value of about 5.7. The role of these effects for the migrating and non-migrating phenyl groups have been separated. The steric effects from ortho-substitution have been evaluated. The limiting conditions for the observation of the base-catalysed rearrangement and two types of fission for benzils are delineated. The results are discussed in terms of mechanistic paths for the rearrangement and fission processes and the structures of the transition states.