Kinetic applications of ESR spectroscopy to some reactions of the t-butoxy radical in solution

Abstract

Kinetic studies of free radical reactions in solution are often laborious and difficult to carry out over a wide range of temperature. As a result, the literature contains relatively few reports of Arrhenius parameters for these reactions. We have tried to devise simple ESR techniques for studying key steps in radical reactions with a view to measuring activation parameters. The study of unimolecular radical decompositions, which are often not readily accommodated by conventional techniques, is particularly suited to the ESR method. We would like to illustrate the ESR method with three examples. Bimolecular homolytic substitution (S,2) reactions occurring at metal centres in organometallic compounds have been the subject of a good deal of recent interest (I, 2). This prompted us to attempt a kinetic study of the reactions of t-butoxy radicals with organoboron compounds, with a view to elucidating some of the factors which influence the rates of these substitution processes. t-Butoxy radicals may be generated in the cavity of the ESR spectrometer by photolysis of di-t-butyl peroxide (3) or thermolysis of di-t-butyl hyponitrite (4) solutions (Eqs. [l and 21). In the presence of an alkylboron compound a rapid displacement reaction takes place, producing a readily observable concentration of alkyl radicals (Eq. [3]). These reactions provide a facile method of generating specific alkyl radicals for ESR study [5].