Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C-H Bonds and Evaluation of O-H Bond Dissociation Energy of N-Hydroxyphthalimide

摘要

The reactions of the phthalimide N-oxyl (PINO) radical with several hydrocarbons having different C-H bond dissociation energies (BDEs) were investigated in HOAc at 25 ℃. The slope of the Evans-Polanyi plot is 0.38, which indicates that the reactions are mildly exothermic or almost thermoneutral. This finding is supported by the O-H BDE of N-hydroxyphthalimide (NHPI), 375 ± 10kJ mol~(-1), obtained by means of a thermodynamic cycle. The observed kinetic isotope effects (KIEs) are related to the reaction free-energy changes, which can be explained by a Marcus-type equation. The comparison of the PINO radical reactions with analogous hydrogen atom abstraction reactions of the dibromide radical implies that HBr_2 · reactions are more exothermic than the PINO radical reactions. This factor is put forth to explain the different KIEs.