Gas phase reaction between iodine and tetramethylsilane. Part 2.—Kinetics and the bond dissociation energy D(Me3SiCH2—H)

Abstract

The kinetics of the title reaction, which leads to the formation of small quantities of trimethyl-silymethyl iodide and HI at equilibrium, have been studied in the temperature range 609–649 K. From absorbance measurements on I2 the approach to equilibrium was found to be characterised by the rate law –d[I2]//dt=k[I2]½[Me4Si]{1–[Me3SiCH2I][HI]//K[I2][Me4Si]}, where K is the experimentally determined equilibrium constant. This expression is consistent with an iodine atom abstraction mechanism and for the step I·+Me4Si[graphic omitted]Me3SiCH2+HI, log(k1/dm3 mol–1 s–1)=(11.82 ± 0.30)–(122.2 ± 3.6 kJ mol–1)/RT ln 10 has been deduced. From this the bond dissociation energy D(Me3SiCH2—H)= 415 ± 5 kJ mol–1(99.2 kcal mol–1) is obtained. A more limited study of the reaction between I2 and neopentane indicates that DC—H(Me4C)-DC—H(Me4Si)= 1.8 ± 0.7 kJ mol–1.Some thermochemical implications of the absence of alternative reaction pathways are pointed out.