Kinetics of self-decomposition and hydrogen atom transfer reactions of substituted phthalimide N-oxyl radicals in acetic acid

Kinetic data have been obtained for three distinct types of reactions of plathalimide N-oxyl radicals (PINO.) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO. which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-Nhydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO., e.g., PINO. + 4-Me-NHPI reversible arrow NHPI + 4-Me-PINO.. The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO. from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO. is more electron donating. Analysis by the Hammett equation gave p = 1.1 and. 1.8 for the reactions of PINO. with p-xylene and toluene, respectively.