Mechanism of Oxygen Exchange Reaction of Diaryl and Alkyl Aryl Sulfoxides in Sulfuric Acid

The mechanistic investigations of the concurrent oxygen exchange and racemization reactions of 18O-labeled and optically active methyl p-tolyl sulfoxide (I) and phenyl p-tolyl sulfoxide (II) in sulfuric acid of various concentrations have been carried out by the determination of detailed kinetics. The kinetic data were analyzed in the lights of kex⁄krac values, activation parameters, correlations between the rates and acidity functions, solvent isotope effects, polar effects of substituents, etc.. A gradual change of mechanism of the reaction from an SN2 type process to a predominant A-1 like one (in above 95% sulfuric acid) with the increase of the concentration of sulfuric acid was observed. All these observations suggest that the A-1 like reaction takes place through a cation radical (–+·S–) or a dication (–++S–) intermediate at higher concentrations of sulfuric acid, while the SN2 type reaction that involves a water molecule as nucleophile in the rate-limiting step is the predominant path in less concentrated sulfuric acid.