Abstract
The reactions between tetraalkyltins, R4Sn (R=Me, Et, n-Pr, n-Bu, iso-Bu and iso-Pr) and mercuric chloride in various methanol-water mixtures have been studied at 25°C and 40°C. From product analyses the general reaction has been shown to be: R[4]Sn + HgCl[2] [k]2 ? R[3]SnCl + RHgCl. The reactions have been shown to follow second order kinetics, first order in each reactant, and are considered to proceed by a single rate-determining bimolecular reaction. Good second-order plots of k[2]t vs. t were obtained in all cases, except for the reaction of tetramethyltin in solvent 85% methanol where a secondary equilibrium reaction was observed. A steric order of reactivities was observed, the sequence being: R = Me > Et > n-Pr > n-Bu > iso-Bu >> iso-Pr. This sequence was shown to be indicative of an SE[2] mechanism, further confirmation being obtained from positive kinetic salt effects and increasing rate constant with increasing polarity of the medium. The activation parameters for all the reactions were shown to be consistent with a bimolecular reaction proceeding from relatively non-polar reactants to a polar transition state. The activation parameters for the reaction of mercuric chloride with tetraethyltin in various methand-water mixtures were further analysed by their dissection into ground state and transition state contributions. This full thermodynamic analysis of solvent effects is the first instance of such a procedure being used for a bimolecular reaction. The free energies of transfer of mercuric chloride, tetraethyltin and the mercuric chloride - tetraethyltin transition state were further dissected into their "neutral" and "electrical" contributions. A comparison of the ''electrical" contri-butions to the free energy of transfer confirmed, on a more quantitative bases than has previously been possible, that the reaction between tetraalkyltins and mercuric chloride in various methanol-water mixtures proceeds by an "open", S[E]2 mechanism of electrophilic substitution at a saturated carbon atom.