Effect of protic cosolvents on the photooxygenation of diethyl sulfide

The sluggish (kr < 10kq) photooxygenation of diethyl sulfide in both benzene and other aprotic solvents such as acetone and acetonitrile is made efficient by addition of small amounts of alcohols and, with a much more conspicuous increase, of phenols and carboxylic acids (<<0.1% additive is sufficient in this case). A kinetic analysis shows that the effect is accounted for by interaction of the protic additives with the first formed intermediate, the persulfoxide, in competition with cleavage to the components. The thus obtained rate constants kH linearly correlate with the acid strength of the additives, and the effect is rationalized as a general acid catalysis. Hydrogen bonding of the persulfoxide under this condition accounts in an economic way for the observed data, including co-oxygenation of Ph2SO in mixed solvents.