Oxygen Exchange and Racemization Reactions of Sulfoxides in Acetic and Chloroacetic Acid

Oxygen exchange reaction of sulfoxides has been performed mainly in mineral acids such as sulfuric acid, aqueous hydrochloric and hydrobromic acids. We have found that concurrent oxygen exchange and racemization reactions take place also in carboxylic acids such as acetic, mono- and poly-chloroacetic acids. The pseudo first order reaction rates of both oxygen exchange and racemization reactions of p-tolyl phenyl sulfoxide have been determined using both optically active and 18O-labeled p-tolyl phenyl sulfoxides in these acids. This reaction is found to be mainly dependent on the strengths of the acids; namely, the reaction velocity is faster in an acid of smaller pKa value. Together with the fact that k(rac.)/k(exch.) is unity and the substituent effect is small, the reaction is suggested to proceed through an unimolecular type S–O bond cleavage, probably involving radical cation intermediate (II).