Reaction of Sulfoxides with Acylating Reagents. I. Mechanism of Oxygen Exchange Reaction of Diaryl Sulfoxides with Acetic Anhydride

Racemization and oxygen exchange reactions were carried out by heating both optically active and 18O-labeled diaryl sulfoxides in a large excess of acetic anhydride. The rate of racemization of p-tolyl phenyl sulfoxide was found to be twice the rate of oxygen exchange, suggesting the reaction to involve a Walden inversion at the rate-determining step. When the reaction was carried out in a mixture of acetic anhydride and chlorobenzene, the rate of racemization was found to be first order with respect to both the sulfoxide and acetic anhydride. The remarkably small substituent effect is also in keeping with the SN2 type mechanism. The marked retardation of the rate of racemization by the addition of common salt is similar to well-known cases in the SN2 reaction of alkyl halides, and the remarkably large acceleration of the rates of both racemization and oxygen exchange reactions by the addition of sodium perchlorate seems to be caused by a special salt effect.