Substitution at saturated carbon. Part VI. Heats of solution of tetraethyltin and mercuric chloride in methanol–water mixtures, and the enthalpy of transfer from methanol to aqueous methanol of the tetraethyltin–mercuric chloride transition state

Abstract

Calorimetrically determined heats of solution of tetraethyltin in methanol–water mixtures from χ(MeOH)= 1 to χ(MeOH)= 0·51, and of mercuric chloride in methanol–water mixtures from χ(MeOH)= 1 to χ(MeOH)= 0 are reported. When combined with the previously determined enthalpies of activation for the substitution (1) these Et4Sn + HgCl2→ EtHgCl + Et3SnCl (1), heats of solution yield values of ΔHt°(Tr), the enthalpy of transfer from methanol to aqueous methanol of the transition state in reaction (1). It is shown that the reduction in the enthalpy of activation of reaction (1) observed when solvent methanol is replaced by aqueous methanol is due to a very large increase in the enthalpy of the initial state on transfer from methanol to aqueous methanol. The enthalpy of the transition state remains constant over the solvent range χ(MeOH)= 1 to χ(MeOH)= 0·9, but from χ(MeOH)= 0·9 to χ(MeOH)= 0·51 the value of ΔHt°(Tr) increases by over 3 kcal. mole–1.