Liquid-phase reactions of CCl3 radicals with trimethylsilane and triethylsilane

The γ-radiation-induced chain reactions in liquid solution of silane (R'3SiH) in CCl4-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'3SiCl, CHCl3, and RCl. By kinetic analyses of product distribution, the rate constants of hydrogen abstraction from the two silanes by CCl3 radicals were competitively determined vs. hydrogen abstraction from cyclohexane. The relative rate constants combined with the known Arrhenius parameters of the reference systems gave the following Arrhenius parameters for the reaction CCl3 + R'3SiH → CHCl3 + R'3Si: log A4 (L mol-1s-1) = 8.49 and E4 = 8.70 kcal mol-1 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 Cl3SiH in the H-atom transfer reactions with CCl3 radicals. This observation is rationalized in terms of the polar effects of the alkyl substituents