Substitution at saturated carbon. Part IV. Bimolecular electrophilic substitution of tetraethyltin by mercuric chloride in methanol–water mixtures. Kinetics, mechanism, and activation parameters

Abstract

Second-order rate coefficients and activation parameters for the substitution of tetraethyltin by mercuric chloride in solvent methanol–water are reported over the range of solvent composition χ(MeOH)= 1 to χ(MeOH)= 0·51. Although ΔG‡ varies regularly with solvent composition, both ΔH‡ and ΔS‡ fluctuate markedly, with the value of ΔS‡ passing through a minimum at a solvent composition χ(MeOH)≃ 0·85. It is thought that the fluctuations in ΔH‡ and ΔS‡ could be due to a combination of solvent effects on the reactants and solvent effects on the transition state. Salt effects, with the solvent χ(MeOH)= 0·91, have been investigated and indicate that the positive kinetic salt effect of lithium perchlorate is due to destabilisation of the reactants by the added salt. The reactivity sequence towards the substrate tetraethyltin has been found to be Hgl₂ < HgCl₂ < Hg(OAc)₂ for the three solvent mixtures with χ(MeOH)= 1, 0·91, and 0·72. It is concluded that the substitution of tetraethyltin by mercuric chloride, in the various methanol–water mixtures used, proceeds by the S_E2(open) mechanism of electrophilic substitution at saturated carbon through a transition state in which considerable charge separation has occurred.