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 1013.34 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 ks = (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 ks 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.