The kinetics of the reactions of silicon compounds. Part VIII. The gas-phase thermal decomposition of trifluoro-1,1,2,2-tetrafluoroethylsilane

Abstract

The gas-phase thermal decomposition of trifluoro-1,1,2,2-tetrafluoroethylsilane at 140–200 °C and 20–150 Torr initial pressure is a first-order reaction with a rate constant which is independent of pressure and of the nature and extent of the surface, and is given by equation (i). The rate-determining step (ii) is concluded to be the unimolecular log₁₀k/s^–1=(13·11 ± 0·10)–(137·8 ± 0·9) kJ mol^–1/2·303 RT(i) transfer of α-fluorine to silicon by a three-centre transition state to give tetrafluorosilane and difluoromethylfluorocarbene. In the absence of other compounds the carbene isomerises quantitatively to trifluoroethylene, but in the presence of additives, or in the later stages of the reaction, it may undergo characteristic reactions, such as addition CHF₂· CF₂· SiF₃→ [graphic omitted] →CHF₂· CF: + SiF₄(ii) to olefins to give cyclopropane derivatives, or insertion into the SiH bond in trimethylsilane. The decomposition of reactant proceeds at the same rate independently of the presence of the additives, and this observation, together with the nature of the products formed, indicates clearly the two-step nature of the reaction. The reaction with trimethylsilane gave up to 95% of insertion product, demonstrating the virtual absence of any β-elimination.