ISOTOPE EFFECTS IN NUCLEOPHILIC AROMATIC SUBSTITUTION

Abstract

The activation parameters and the sulphur-34 isotope effect associated with the reaction of piperidine with 2,4-dinitrodiphenylsulphone have been measured in three different solvents, after carrying out extensive tests to establish that the reaction is uncomplicated by equilibrium processes and that the experimental procedures used to measure the isotope effect do not themselves introduce any isotopic fractionation. The rate constant ratios, have been found to be 1.013 in benzene, 1.016 in acetonitrile and 1.006 in methanol. These results establish that, for reaction in benzene and acetonitrile, carbon-sulphur bond rupture occurs in a ratedetermining step, while in methanol, the rupture of this bond is only partially rate-determining. The observed variation in magnitude of the isotope effect with change in solvent has been shown to provide strong evidence for a two-step mechanism involving a metastable intermediate.