Methylchlorosilane, formation

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. 

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... 

Because direct synthesis in this case is very complex, the mechanism of this process has not been fully established. However, the following way of methylchlorosilane formation with the catalytic effect of copper on the reaction of methylchloride with silicon is the most probable. 

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. 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

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. 

The example of the first category is the formation of alkyl- and arylchlorosilanes in the so-called direct process (DP). The process was discovered over 60 years ago by Rochow in the United States, and, independently, by Muller in Germany, and it is still the most important reaction in organosilicon chemistry. In fact, it is at the very basis of the silicone industry, being the primary source of organochlorosilane precursors (mostly methylchlorosilanes, comprising over 90% of the total) in the production of silicone oligomers and polymers. 

Selectivity of contact mass. Since the most important products in the synthesis of organochlorosilanes are diorganodichlorosilanes, it is natural that the increase of their yield receives much attention. The selective formation of diorganodichlorosilanes when organic chlorine derivatives interact with contact mass is connected with the purity of the reactants, silicon above all. High yield of dimethyldichlorosilane in the direct synthesis of methylchlorosilanes largely depends on the presence of noticeable quantities of aluminum in contact mass. The yield of dimethyldichlorosilane in the presence of aluminum as a rule decreases due to the formation of trimethylchlorosilane in the reaction with pure (semiconductor) silicon in the presence of copper trimethylchlorosilane is virtually not formed. 

This example shows the importance of so-called activators, or promoters, for increasing the activity and selectivity of contact mass. These additives can sharply activate the reaction and shift it in a certain direction. Various substances have different effects on the activity of contact mass. For example, antimony has a positive effect on the direct synthesis of organochlorosilanes and increases the total yield of methylchlorosilanes, whereas lead and bismuth reduce the formation of these substances. How-ever, the positive effect of a promoter manifests itself only in a certain concentration, exceeding which transforms a positively acting additive into poison or an inhibitor of the reaction. For example, in a 0.002—0.005% concentration antimony is a promoter of the direct synthesis of methylchlorosilanes on the other hand, in a concentration higher than 0.005% it becomes poison. 

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. 

Scheme 1. Silylene insertion reaction leading to the formation of methylchlorosilanes.

The proposed silylene mechanism gives an explanation for the high selectivity of (CH3)2SiCl2 formation in the "Direct Synthesis" of methylchlorosilanes (Miiller-Rochow process). Via an oxidative addition of CH3CI to methylsilylenes on the surface of a Cu/Si catalyst, (CH3)3SiCl2 is produced in a kinetically controlled process (Scheme 2). 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

Insertion of silylenes into C-Cl bond forms methylchlorosilanes Scheme 1. The silylene-mechanism for the formation of methylchlorosilanes. 

TABLE 7. Standard enthalpies of formation of methylchlorosilanes (comparison)... 

TABLE 8. Recommended standard enthalpies of formation of methylchlorosilanes and methyl(hydrido)chlorosilanes... 

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... 

Hexafluoroacetone has been used as a photochemical source of trifluoro-methyl radicals in kinetic studies dealing with attack of the latter on ethylene," di-isopropyl ketone, methylchlorosilanes, the methylsilanes MejSi, MejSiF, MeaSiFj, and MeSiFj, methyl acetate and deuteriated methyl acetates, dimethyl and di-isopropyl ether, tetramethyltin (less precise data were obtained with McsB, Me Si, and Me Ge), methyl formate, and hydrogen sulphide, deuterium sulphide, hydrogen chloride, and deuterium chloride. The photolysis of hexafluoroacetone alone has received further detailed attention and trifluoromethyl radicals thus generated have been shown not to attack sulphur hexafluoride even at temperatures up to 36S °C. ... 

---