Calculations of steric effects. Part 3. The anion-catalysed substitution of alkylmercury(II) bromides by mercury(II) bromide in ethanol

Abstract

The steric effect of a neopentyl (Pe) group compared with that of the methyl group has been calculated for both the one-bromide ion and two-bromide ion catalysed reactions of alkylmercury(II) bromides with mercury(II)bromides in ethanol. For the case of one-anion catalysis, calculations were carried out with three model transition states, an open model, an unsymmetrical cyclic model with one bridging bromide ion, and the symmetrical bicyclic model with two bridging bromide ions suggested by Jensen and Rickborn. It is shown that only the Jensen and Rickborn model yields calculated rate ratios k^Pe/k^Me that are in accord with experiment. Calculations on the two-anion catalysed reactions were carried out on a symmetrical cyclic transition state with one bridging bromide ion, and good agreement between calculated and observed rate ratios was obtained.