Abstract
The substitution of tetraethyltin by mercuric salts (HgX2: X = Cl, I, OAc) in solvent acetonitrile at 25°C and in some cases (X = Cl, I) at 40 and 60°C has been studied. The stoichiometry of reaction is represented by Et4Sn + HgX2 → Et3SnX + EtHgX In the case of mercuric iodide reaction is followed by a rapid reversible reaction of the form Et3SnI + HgI2 → Et3Sn(+) + HgI3(-) The reactions have been shown to follow second order kinetics, first order in each reactant, and some kinetic salt effect studies and polar co-solvent studies suggest that reaction proceeds by mechanism Sg2(open). The reaction sequence found, Hg(OAc)2 > HgI2 > HgCl2 > (HgI3(-) = 0) is different from that used by previous workers as evidence for mechanism SE2(open), and it is concluded that the prediction of mechanism from reactivity sequences is unreliable. The substitution of tetraalkyltins (R4Sn: R = Me, Et, Pr n, Bu n, Pr i, Bu i) by mercuric chloride in solvent acetonitrile at 25°C and in some cases (R = Me, Et, Bu i) at 60°C have been studied# From qualitative and quantitative product analyses the stoiciohmetry of the reactions has been shown to be R4Sn + HgCl2→ R3 SnCl + RHgCl. The reactions have been shown to follow second order kinetics, first order in each reactant. The order of reactivity was found to be Me>> Et > Prn ~ Bu n > Bui >>Pri, qualitatively and quantitatively very similar to the sequences found in previous work for the substitution of tetraalkyltins in solvent 96% aqueous methanol by mercuric iodide and mercuric chloride.These latter reactions have been shown to proceed by mechanism SE2(open) and by analogy this mechanism has been proposed for the reactions in solvent acetonitrile. From a semi-quantitative treatment of steric effects the transition state geometries were demonstrated to be consistent with mechanism SE2(open) throughout the series of substitution reactions. When determined, activation parameters were shown to be not inconsistent with this mechanism. These substitution reactions in solvent acetonitrile complied with the SE2 rule of Abraham and Hill. A method of calculating steric effects has been developed and has been applied to some bimolecular electrophilic substitution reactions. In two cases it has enabled a choice to be made between transition states, and has provided some insight into factors contributing to the rates of sterically controlled reactions.