Stirred flow thermolysis of hexamethyldisilane.

A novel type of flow technique, for the study of gas phase kinetics is described. This is based on the passage of a pulse of reactant vapour through a stirred flow reactor, and subsequently a gas chromatograph, using a single carrier gas stream. In this way, a chromatogram of unreacted starting material and reaction products is produced, a matter of minutes after the injection of reactant into the system. Consideration of first order and n th order chemical reactions, under the non-steady state conditions existing in these experiments, has lead to the derivation of expressions for rate constants in terms of known parameters. This technique has been applied to a study of the gas phase thermolysis of hexamethyldisilane, (i) in the absence of inhibitors, and (ii) in the presence of excess m-Xylene. In the first series of experiments the following main reaction products were found: trimethylsilane, tetramethylsilane, 2,2,4-trimethyl-2,4-disilapentane (the rearrangement isomer of hexamethyldisilane), and 1,1,3,3-tetra-methyl-l,3-disilacyclobutane, However, complex kinetic behaviour was observed, the formation of some products being non-first order and variable. A large number of experiments were therefore conducted in the presence of the m-Xylene, thus simplifying the kinetic behaviour, Mechanisms are proposed which rationalise all the kinetic observations, and from the observed rate of formation of trimethylsilane in the presence of m-Xylene, the dissociation rate constant for: Me3SiSiMe3 ? 2 Me3Si is identified as: 10g10k / Sec-1 = 17.23 + 0.25 - (336.9 + 4.0) kJ mol-1/2.303 RT. From the rate parameters evaluated in this, and other work, several thermochemical quantities are estimated. Previous work on hexamethldisilane thermolysis is discussed in the light of the present findings.