A kinetic study of the gas phase thermal decomposition of 2-chloroethylsilicon compounds.

The chemical reactivity of the halogenoalkylsilicon compounds has been reviewed and the published quantitative work on the decomposition of 2-halogenoalkylsilicon compounds outlined. The gas phase thermal decomposition of 2--chloroethyltrichloro-silane has been investigated in a static system between 356 and 417C at initial pressures of 12 - 138 mm. Hg. Use has been made of pressure measurement, gas chromatography and mass spectrometry to obtain kinetic data and to establish the mechanism of the reaction. The over-all reaction scheme is, 1. ClCH2CH2SiCl3 [right arrow] CH2=CH2+ SiCl4, 2. ClCH2CH2SiCl3 [right arrow] CH2=CH.SiCl3+HCl 3. ClCH2CH2SiCl3 [right arrow] CH2=CHCl+ HSiCl3, 4. CH2=CHCl + HSiCl3 [right arrow] CH2=CH.SiCl3+HCl, 5. CH2=CH2 + HSiCl3 [right arrow] CH3.CH2.SiCl3 where ethylene and silicon tetrachloride account for 72% of the reaction products. A brief study of the subsidiary reactions, 4 and 5, has been made and approximate kinetic data obtained. These reactions are suppressed by nitric oxide and cyclohexene. Reactions 1, 2 and 3 are shown to be simultaneous unimolecular eliminations and the Arrhenius equation for the major reaction, ethylene elimination, is: k1 (Sec.-1) = 1011.26+0.16 e-45,5000+500/RT The low pre-exponential term corresponds to ?S1 = -8.0+0.7 e.u., which suggests a cyclic transition state. In particular, a "quasiheterolytic" transition state may be involved, as in the thermal decomposition of alkyl halides and carboxylic esters, and this possibility is discussed, although accurate correlation cannot be made until more results are available on other 2-halogenoalkylsilicon compounds. An analogous study has been carried out on 2-chloroeth1-dimethlphenylsilane between 236 and 390C. It has been shown that the reaction, ClCH2CH2SiMe2Ph [right arrow] CH2=CH2 + ClSiMe2Ph accounts for more than 99% of the reaction products. However, the reaction is very sensitive to surface condition and it was not found possible to isolate the homogeneous decomposition. Suggestions for resolving this difficulty are made.