Substitution at saturated carbon. Part II. Kinetics and mechanism of the electrophilic substitution of several tetra-alkyltins by mercuric iodide in solvent 96% methanol–4% water

Abstract

The substitution of tetra-alkyltins by mercuric iodide in solvent 96% methanol–4% water proceeds by the rate-determining bimolecular reaction R₄Sn + Hgl₂→ RHgl + R₃Snl, followed by the rapid reversible reaction R₃Snl + Hgl₂ R₃Sn⁺+ Hgl₃ ^–. Second-order rate coefficients, at 25 and at 40°, of reaction (1) have been obtained for the substitution of tetramethyl-, tetra-n-propyl-, tetra-n-butyl-, tetraisobutyl-, and methyltri-n-butyl-tin, and the various activation parameters determined. The second-order rate coefficient at 40° has also been obtained for the substitution of tetra-isopropyltin. The rate coefficients decrease markedly along the series (for the symmetrical tetra-alkyltins): R = Me ≫ Et > Prⁿ Buⁿ > Buⁱ≫ Prⁱ. Consideration of the rate coefficient for the substitution of the methyl group in methyltri-n-butyltin leads to the conclusion that the above sequence arises mainly from transition-state interactions of the alkyl group undergoing substitution with both the entering electrophile (Hgl₂) and the leaving group (SnR₃). It is suggested that these substitutions, in solvent 96% methanol–4% water, proceed by the S_E2 (open) mechanism of electrophilic substitution at saturated carbon.