Abstract
The reaction between mercuric chloride and tetraethyl-tin, equation (1; R=Et), has been investigated using as solvent methanol, tert-butanol/methanol mixtures, and tert-butanol itself. R?4Sn + HgC12 --k2→ R3SnC1 + RHgC1 (1) On the basis of (i) positive kinetic salt effects, (ii) an increase in the rate coefficient with increasing polarity of the solvent medium, and (iii) the variation of the activation parameters with solvent composition, it is concluded that reaction (1; R = Et) follows the S[E]2(open) mechanism of electrophilic substitution at saturated carbon in all the solvents investigated. Using solvent tert-butanol, a steric sequence of reactivity, R = Me > Et > n-Pr > n-Bu > iso-Bu > iso-Pr, was observed for reaction (1). It is suggested that this sequence of reactivity is characteristic of reactions proceeding by mechanism S[E]2(open). The reaction between mercuric iodide and tetraethyltin involves a slow bimolecular reaction followed by a fast equilibrium, Et4Sn + HgI --k2→ Et3SnI + EtHgI (2) Et3SnI + HgI2 ←K→ Et3Sn(+) + HgI3(-) (3) Reaction (2) is accelerated on addition of tetra-n-butyl-ammonium perchlorate, in both methanol and tert-butanol, and is also accelerated as the solvent polarity increases. It is concluded that reaction (2) proceeds by mechanism S[E]2(open) in the solvents methanol and tert-butanol, and consequently also in tert-butanol/methanol mixtures. The situation with regard to the reaction of mercuric acetate with tetraethyltin, Et4Sn + Hg(OAc)2 --k2→ Et3SnOAc + EtHgOAc, (4) is less clear. Although reaction (4) is accelerated as the polarity of the solvent medium is increased, from tert-butanol to methanol, no definate choice between mechanism S[E]2(open) and mechanism S[E]2(cyclic) can be made in the absence of any salt effect data.