Mechanism of electrophilic substitution at a saturated carbon atom. Part XIII. Concurrent unimolecular and bimolecular mercury exchange under anion catalysis in water

Abstract

In Part XII the occurrence of an S_E1 one-alkyl mercury exchange was inferred from the repression, caused by added inorganic mercury(II) salts, of the rate of acidolysis of 4-pyridiomethylmercury(II) salts in water. Mercury exchange by S_E2 substitution would not be indicated by this method of observation. By the use of mercury-203, total rates of exchange between 4-pyridiomethylmercury(II) salts and mercury(II) salts have now been measured. At the concentrations used, these exchange rates are considerably greater than those inferred from the retardation of acidolysis, and the kinetics show that the excess rate represents exchange by S_E2 substitution. Like the S_E1 mercury acidolysis and exchange, the S_E2 mercury exchange is accelerated by chloride ions. It is presumed that families of similar reactions arise in which the original mercuric reactants pass by pre-equilibrium chloride-ion uptake into mercurate ions, which then become the immediate reactants for the electrophilic substitutions leading to acidolysis and the concurrent processes of mercury exchange.