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 D_C—H(Me₄C)-D_C—H(Me₄Si)= 1.8 ± 0.7 kJ mol^–1.

Some thermochemical implications of the absence of alternative reaction pathways are pointed out.