Rate constants for reactions of perhaloalkylperoxyl radicals with alkenes

Abstract

Trichloro- and tribromomethylperoxyl radicals were produced by radiolytic reduction of CCl[sub 4] and CBr[sub 4] in various aerated solvents. Rate constants for the reactions of these radicals with unsaturated organic compounds were determined in methanol solutions by competition kinetics, using mainly chloropromazine as a reference. The rate constants were in the range k = 10[sup 4]-10[sup 7] L mol[sup [minus]1] s[sup [minus]1], and a good correlation was obtained between log k and the Taft substituent constants [sigma]*. Steric effects were also noticeable in some cases. The reactivities of the radicals were in the order CBr[sub 3]O[sub 2][center dot] < CCl[sub 3]O[sub 2][center dot] < C[sub 4]F[sub 9]O[sub 2][center dot], which is in contrast with the order observed in electron-transfer reactions. The rate constants for reaction of CCl[sub 3]O[sub 2][center dot] with 2,3-dimethyl-2-butene (DMB) and cyclohexene (CH) were studied in 12 solvents. In all cases DMB is considerably more reactive than CH, indicating that at least in the case of DMB the reaction is predominantly via addition. The rate constant for addition of CCl[sub 3]O[sub 2][center dot] to DMB varies by a factor of 7 in the different solvents. No correlation was found between these rate constants and thermodynamic or polaritymore » parameters of the solvents, although viscosity appears to have an effect. 19 refs., 2 figs., 2 tabs.« less