Substitution at saturated carbon. Part X. Substitution of tetraethyltin by mercuric chloride, iodide, and acetate in t-butyl alcohol–methanol mixtures. Kinetics, salt effects, and activation parameters

Abstract

Rate coefficients for reaction (1; X = Cl) have been determined with 16 different solvent mixtures of t-butyl alcohol and methanol. The large increase in ΔG^‡ observed as methanol is gradually replaced by solvent t-butyl Et₄Sn + HgX₂→ EtHgX + Et₃SnX (1) alcohol is due mainly to a decrease in the value of ΔS^‡. Positive kinetic salt effects of added tetra-n-butylammonium perchlorate have been observed with a number solvent mixtures. It is concluded that reaction (1; X = Cl) proceeds by mechanism S_E2(open) through a transition state in which there is considerable separation of charge, and that the extent of charge separation is constant over the solvent range from methanol to t-butyl alcohol. Similar experiments lead to the conclusion that reaction (1; X = l) also proceeds by mechanism S_E2(open) in solvents methanol and t-butyl alcohol. Reaction (1; X = OAc) exhibits very large positive salt effects in solvent methanol, but these may be due to anion exchange between mercuric acetate and perchlorate ion. No salt effect is observed on rate coefficients for reaction (1; X = OAc) when t-butyl alcohol is used as solvent, and in this case it is possible that reaction proceeds by mechanism S_E2(cyclic).