Formation from methylchlorosilanes

In this section, we will first demonstrate the formation of methylchlorosilanes from CH3 + a monolayers on CuaSi surfaces. The effects of promoters and the effect of surface segregation on the reaction rate and selectivity are scussed subsequently. 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

In the case of unpromoted CuaSi surfaces, the effect of this copper enrichment on methylchlorosilane formation appears to be relatively minor. The majority of methylchlorosilanes are evolved at 400 - 650 K as on the unpromoted surface. There is, however, a small yield of methylchlorosilanes with a peak temperature of -370 K. By contrast, for trimethylsilane formation from pure methyl monolayers, copper enrichment by low temperature sputtering shifts the dominant product peak from... 

The reactions of CH3 radicals and CI2 alone with CujSi have also been investigated. On pure Cu3Si, the dominant silane product from CH3 adsorption is SiH(CH3>3 and the temperature at which the surface is sputtered prior to methyl adsorption has a dramatic effect on the reaction rate (see section 3.3). The CI2 reaction gives SiCU evolution, and the reaction temperature is close to that for methylchlorosilane formation. 

It follows from all the above-mentioned facts that the direct synthesis of methyl-, ethyl and phenylchlorosilanes is a complex heterophase process which depends on many factors and forms a compex reactive mixture. For example, in the direct synthesis of methylchlorosilanes there are about 130 compounds found and characterised. This does not mean, however, that in this or other definite synthesis all the 130 products are formed. The composition of the mixtures formed and the transformation degree of alkyl-chlorides and chlorobenzene in the synthesis of methyl-, ethyl and phenylchlorosilanes depend on the synthesis conditions, the type of the reactor used and many other factors. In spire of the complexity of the process and the variety of its products, the reaction of direct synthesis can nevertheless be directed (towards a preferential formation of a main product), changing the conditions for the preparation of contact mass, introducing various promoters into contact mass and changing the reaction conditions. 

The first step (starting at the bottom) is the formation of the r -phase, which then catalyzes the main reaction shown at the top (methylchlorosilane formation). The asterisks denote active states of the element. Then we model the formation of r -phase and develop an equation for predicting the size distribution of products from a given size distribution of the reactant solid. 

The higher boiling residues obtained in the Direct Synthesis of methylchlorosilanes (from elementary silicon and methyl chloride) include a variety of compounds which contain silicon-silicon bonds. A particularly useful fraction (b.p. 150°-160°C) contains methylchlorodisilanes of the type, (CH3) Cl6-nSi2. The use of this fraction for the preparation of disilanes has been described (2, 62-65) however, few applications leading to the formation of higher polysilanes have been reported. 

A synthesis of dihydrojasmone (493) and cis-jasmone (486) has been reported which centres on the formation of 1,4-diketones.Compound (492) was derived from acyloin condensation of methyl levulinate ethylene acetal in the presence of tri-methylchlorosilane the subsequent sequence is outlined in Scheme 27. By means of... 

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