Reactions of cation radicals of EE systems. 9. Kinetics and mechanism of the reaction of thianthrene cation radical with nitrate ion

The stoichiometry and dynamics of the reaction of thianthrene cation radical with nitrate ion have been studied in acetonitrile, butyronitrile, nitromethane, nitrobenzene, and methylene chloride. In all cases, the reaction is found to yield thianthrene 5-oxide exclusively, with no detectable thianthrene being regenerated during the reaction. The observed rate law in all solvents studied was found to be second order in thianthrene cation radical and first order in nitrate ion. The mechanism proposed for this reaction is analogous to the half-regeneration pathway but differs in that a bridged intermediate comprised of two cation radicals and a nitrate ion decomposes to form two molecules of the oxide and NO⁺. The observed rate constants, corrected for association equilibria involving nitrate ion, exhibit a linear inverse dependence on Dimroth-Reichardt solvent polarity (E_T) parameters, thereby suggesting formation of the bridged intermediate as the rate-determining step in the reaction.