Substitution at saturated carbon XXIII. Initial-state and transition-state solvent effects of methanol, acetonitrile and DMSO on some R 4Sn/HgX 2 reactions

Abstract

The kinetics of the reaction of Me 4Sn with HgCl 2 in methanolhave been reinvestigated; the activation parameters obtained are in goodagreement with those previously reported by Abraham and Johnston. Activationparameters for a number of R 4Sn/HgX 2 reactions in methanol are comparedwith parameters in acetonitrile and DMSO; the latter two dipolar aprotic solventslower the free energy and enthalpy of activation. By determining solventeffects on the free energy, enthalpy and entropy of the reactants it is shownthat for the methanol → acetonitrile transfer the lowering of the ΔG ≠and ΔH ≠ values is mostly due to an increase in the initial-state values. For themethanol → DMSO transfer, the decrease in ΔG ≠ are due to very largetransition-state decreases combined with not-so-large initial-state decreases; this is also thecase in terms of the enthalpy function. It is shown that the solvation of the R 4Sn/HgX 2 transitionstate follows quite closely the solvation of the corresponding mercury(II) halidein methanol, acetonitrile and DMSO, so that the transition states to some extentresemble the reactants and can be described as “early” transition states.