Abstract
The rapid exchange of methyl groups between magnetically distinct sites in CH 3HgX/CH 3HgCN mixtures in dimethylformamide (DMF) has been shown to proceed with second-order kinetics, and may involve formation of a four-center, bridged transition state. The thermodynamic parameters for the exchange are strongly dependent on halide X. The Arrhenius activation energies are 22, 15 and 9 kcal/mole for I, Cl and Br, respectively. Corresponding entropies of activation are +4, −18 and −25 cal/deg·mole. The exchanges in DMF are catalyzed slightly by mercuric halides, very markedly by tetramethylammonium bromide or iodide. Exchanges in pyridine are much faster than in DMF. The order of rates in CH 3HgX/CH 3HgCN mixtures in pyridine is in the order X Cl > Br ≈ I. Catalytic effects which are apparently due to reaction with solvent are noted. Species of the form [C 5H 5NHgCH 3] +X − may be involved.
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