Reactions of methyl radicals. IV. Hydrogen abstraction from tetramethylsilane by methyl radicals produced by the photolysis of both acetone and azomethane

Abstract

Studies of hydrogen abstraction by CH3 radicals from (CH3),Si with two different radical sources, acetone and azomethane, have yielded data on the reactions CH₃ + Ch₃COCH₃ → CH₄ + CH₃COCH₂ (1) CH₃ + CH₃NNCH₃ → CH₄ + CH₃ NNCH₂ (2) Ch₃ + (Ch₃)₄Si → Ch₄ + (CH₃)₃SiCH₂

Their rate constants, based on the value of 10^13.34 cm³ mol^-1 s^-1 for the recombination of CH₃ radicals, together with the corresponding experimental temperature ranges, are given by (k in cm³ mol^-1 s^-1, E in J mol^-1):

log k₂ = (11.54±0.02) - (40 000±220)/19.145T 398-522 K (2)

log k₈ = (10.98±0.02) - (32 550±160)/19.145T 334-463 K (8)

log k₈' = (11.03±0.02) - (32 920±180)/19.145T (8')

With acetone as the radical source:

log k₅ = (11.85±0.06) - (44 810±160)/19.145T 395-523 K (5)

With azomethane as the radical source:

log k_s = (11.88±0.03) - (44 610±260)/19.145T 363-463 K (5) log k₅' = (12.02±0.04) - (45 630±310)/19.145T (5') where k₅' and k₅' are the values for k_s and k₅, respectively, obtained after allowance is made for the formation of C₂H₆ by way of the intramolecular decomposition of azomethane. From a consideration of these and previously reported data on CH₃ and CD₃ attack on (CH₃)₄Si, we conclude that the source of CH₃ radicals, and the intramolecular formation of C₂H₆ in the photolysis of azomethane, both have a negligible effect on the rate constants for the reaction, and that CH₃ and CD₃ radicals are of equal reactivity. We also recommend best values for the Arrhenius parameters for the reaction.