Rate constants and activation energies for the reactions of triphenylmethyl and 9-arylthioxanthyl radicals with dioxygen

Triphenylmethyl radical reacts with dioxygen in dichloromethane with a second-order rate constant of 1.1 x 10(7) dm(3) mol(-1) s(-1) at 298 K and an Arrhenius activation energy of -4.1 kcal mol(-1). 4-Phenyl-substituted thioxanthyl radicals are somewhat less reactive toward dioxygen in acetonitrile (second-order rate constants from 1.4 x 10(5) to 3.2 x 10(5) dm3 mol(-1) s(-1) at 298 K with Arrhenius activation energies of about -3 kcal mol(-1)). A weak substituent effect (p=-0.45) was observed for the latter reaction series. The major products of these reactions are the corresponding carbonyl compounds, benzophenone and thioxanthone, believed to form by rearrangement after rate determining adduct formation between the radicals and dioxygen. The mechanism of adduct formation is believed to occur in two steps involving the formation of a complex in a pre-equilibrium followed by bond formation.