Bimolecular self-reactions of ketyl radicals of acetophenone and diphenylaminyl radicals

Abstract The rate constants of self-reactions of ketyl radicals of acetophenone in n -heptane [2 k = (3.2 ± 0.5) × 10 9 M −1 s −1 ] and diphenylaminyl radicals in toluene [2 k = (3.3 ± 0.5) × 10 7 M −1 s −1 ] have been determined at 298 K using the flash photolysis technique. The rate constant of ketyl radicals is equal to the calculated diffusion constant and, therefore, this reaction is diffusion-controlled. The aminyl radical recombination rate is independent of the viscosity of the toluene/vaseline oil binary mixture (0.55 ⩽ η ⩽ 12 cP) and this reaction is activation-controlled. Reactivity anisotropy averaging due to the cage effect has been considered for ketyl and some other radicals. On the basis of the analysis it has been proposed that ketyl recombination involves formation of not only pinacol, but also iso-pinacols.