Liquid-phase reactions of CCl3 radicals with trimethylsilane and triethylsilane

Abstract

The y-radiation-induced chain reactions in liquid solution of silane (R',SiH) in CC14-c-C6H12(RH) mixtures were investigated over the temperature range 303-423 K for R' = Me and 335-423 K for R' = Et. In both systems the main products are R',SiCl, CHCl,, and RC1. By kinetic analyses of product distribution, the rate constants of hydrogen abstraction from the two silanes by CCl, radicals were competitively determined vs. hydrogen abstiraction from cyclohexane. The relative rate constants combined with the known Arrhenius parameters of t h e reference systems gave the following Arrhenius parameters for the reaction CC13 + Rt3SiH -CHC13 + R',Si: log A4 (L mol-' s-l) = 8.49 and E4 = 8.70 kcal mol-l when R' = Me, and log A4 = 8.62 and E4 = 8.06 when R' = Et. These results indicate that alkyl-substituted silanes are considerably more reactive than C13SiH in the H-atom transfer reactions with CCl, radicals. This observation is rationalized in terms of the polar effects of the alkyl substituents.

Structural and Chemical Properties of Silyl Radicals

“…For Me3SiH and Et3SiH, cyclohexane was used as the reference with the C13C* radical; the relative rate constants were combined with previously available Arrhenius parameters in the liquid phase of the reference systems. 90 For the other silanes, the relative rates of hydrogen atom abstractions by the C13C* radical97 were combined with the rate constant for Et3SiH. 90 The two rate constants at 77 °C obtained for the reaction of Cl3SiH with the C13C' radical, either with the above method in CCI4 or from the available gas-phase Arrhenius parameters,102 are identical.…”

“…90 For the other silanes, the relative rates of hydrogen atom abstractions by the C13C* radical97 were combined with the rate constant for Et3SiH. 90 The two rate constants at 77 °C obtained for the reaction of Cl3SiH with the C13C' radical, either with the above method in CCI4 or from the available gas-phase Arrhenius parameters,102 are identical. The kinetic data for perfluoron-heptyl radicals were obtained by competition of the appropriate silane with the addition to 1-hexene.…”