The yields of alcohol and ketone from the mutual termination reaction of sec-butyl-, sec-hexyl-, cyclopentyl-, and cycloheptylpesoxy radica!~ have been measured over the temperature range from -125 to + l o 0 "C. The ratio of alcohol/ketone departs markedly from the value of unity predicted by the Russell mechanism, being greater (ca. 2.5) at +I00 " C and considerably less (ca. 0.1) at -125 "C. -125 "C.
[Traduit par le journal]Can. J . Chem., 52, 1377 (1974) The mutual termination reactions of primary or secondary alkylperoxy radicals are generally assumed to proceed through a cyclic transition state (i.e., the Russell mechanism) (1-3) where the overall termination rate constant, 2k, = 2k, k , k,/k-, k-,. On this basis it has been concluded from thermochemical calculations that the overall activation energy, E,, should be negative (4). However, recent measurements (5) by electron spin resonance spectroscopy of 2k, for several see-alkylperoxy radicals have shown that E, is positive (2-4 kcal mol-') and that the Arrhenius plots are not linear a t low temperatures.Product studies (6-8), mainly at ambient temperature, have shown that comparable yields of alcohol and ketone are formed as predicted by the Russell mechanism (eq. 1). However, detailed studies (9) have shown recently that at ambient temperatures considerable fractions of 1-ethoxyethylperoxy radicals and 1,2-diphenylethylperoxy radicals d o not terminate by the Russell mechanism but form free alkoxy radicals by a route (eq. 2a) comparable to that postulated for the termination of tert-alkylperoxy radicals. In view of the anomalous temperature dependence of the rate constants (5) we have now made product studies over a comparable temperature range (-125 to + 100 "C) for the see-alkylperoxy radicals from butane, hexane, cyclopentane, and cycloheptane. The peroxy radicals were formed by the photolysis of an oxygenated solution of di-tert-butyl peroxide (0.25 to 4%) in the appropriate hydrocarbon (5). This method formed specific see-alkylperoxy radicals from butane, cyclopentane, and cycloheptane but gave a mixture of the 2-and 3-hexylperoxy radicals from hexane. The products were identified and their concentrations measured by gas-liquid chromatography.Detailed product analyses were made for the cyclopentylperoxy radical and appropriate control experiments were carried out. The yields of Can. J. Chem. Downloaded from www.nrcresearchpress.com by 52.183.12.225 on 04/29/19For personal use only.