Reaction of Methylchlorosilanes

The discovery that the methylchlorosilanes in the vapor phase will react with many types of surfaces to produce water-repellent films1 has led to one of the most important developments in the organo-silicon field. The effect was an entirely unexpected outcome of the preparation and handling of large volumes of methylchlorosilanes intended for methyl silicone and serves as an excellent illustration of how the availability of new or of previously scarce intermediates may lead to valuable developments which are entirely unrelated to the original purpose of the investigation. 

The same reaction is found to occur with cotton cloth and with wood, and in a less pronounced way with wool, silk, leather, and many other materials. Glass and other ceramic surfaces also react readily with the methylchlorosilane vapor to give very effective water-repellent films,2 but only if the surface had upon it an adsorbed film of water. Completely dry glass, baked out under vacuum, does not become distinctly water-repellent upon treatment. Glass which has stood at ordinary room conditions usually has on its surface a film of 

Measurements which have been made on the water-repellent films 

Millions of steatite parts have been treated with methylchlorosilane vapor to make them permanently water-repellent and so to maintain 

The water which condenses on a treated or an untreated steatite surface will evaporate as the piece warms to the surrounding tem- 

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Table 1. Optimum reaction temperature and activation energy of the reaction of methylchlorosilanes with isolated silanol groups of the silica surface.

In 1940 Rochow discovered the direct process, also cabed the methylchlorosilane (MCS) process, in which methyl chloride is passed over a bed of sibcon and copper to produce a variety of methylchlorosilanes, including dim ethyl dichi oro sil a n e [75-78-5] (CH2)2SiCl2. Working independently, Mbber made a similar discovery in Germany. Consequently, the process is frequently cabed the Rochow process and sometimes the Rochow-Mbber reaction. 

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 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. 

The products from the reaction of CH3 + Q monolayers on the promoted Cu3Si surface are the same as those for pure Cu3Si, but both the absolute rates and the selectivities are significantly different. In experiments analogous to those described in section 3.1, methylchlorosilanes are evolved from the promoted CusSi surface between 300 and 450 K. This temperature is 200 K lower than that from the pure Cu3Si surface. This 200 K difference in reaction temperature corresponds to a difference of six orders of magnitude in rate (if the rates are extrapolated to a common reaction temperature of 500 K assuming standard and equivalent pre exponential factors for the reactions on these two surfaces [10]). 

We have now tried to make new larger cyclosilanes by the reductive dehalogenation of mixtures of methylchlorosilanes and methylchlorodisilanes. After a separation of the reaction product mixture by means of GC/MS, we found some new cyclosilanes (Fig. 1) [12]. 

Clogston(Ref 2) reported that a bomb contg a mixt of Al carbon tetrachloride(CCl ) exploded violently on heating for 53 min at 152°, The expl decompn of several chlorinated methyl-siloxanes Si chlorinated methylchlorosilanes when heated to ca 200° was reported by Zimmermann(Ref 3). A previously undescribed exothermic reaction of chlorinated rubber with zinc oxide was held responsible for an expln that leveled themanufg area of Dayton Chemical Products Laboratories(Ohio) in April... 

Raw stock for the direct synthesis of methylchlorosilanes, methylchlo-ride, has such impurities as moisture, methyl alcohol, oxygen, sulfur dioxide, methylenechloride, dimethyl ether, carbon oxide and dioxide, etc. Most of them negatively affect the synthesis of methylchlorosilanes harmful impurities are chemisorbed on the active centres of contact mass and foul the copper catalyst, which naturally inhibits the reaction of methyl-chloride with contact mass. A similar situation is observed in the direct synthesis of ethylchlorosilanes. 

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. 

Thus, the methylation of dimethyldichlorosilane becomes very significant in the processes of direct synthesis, whereas the total amount of trimethylchlorosilane, which is formed according to the reactions of dis-proportioning and methylation, may reach 60-65%. Thus, in the direct synthesis of methylchlorosilanes the introduction of significant amounts of AI or its compounds into contact mass reduces the yield of dimethyldichlorosilane and respectively increases the yield of trimethylchlorosilane. 

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. 

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 chlorination of methylchlorosilanes is realised by free chlorine in the liquid or gaseous phase. The reaction takes place by the radical chain mechanism its initiators (substances capable of generating free radicals) can be peroxides (benzoyl, kumyl, etc.), the dinitrile of 2,2 -azobis(isobutyric) acid, fluorine, 60Co y-rays, as well as UV rays. 

The chlorination of methylchlorosilanes with UV rays happens readily and forms a whole range of chlorinated methylchlorosilanes. For example, apart from chloromethyltrichlorosilane CIO FSiCF the reaction also forms dimethyltrichlorosilane Cl2CHSiCI3 and trimethyltrichlorosilane Cl3CSiCl3 ... 

Stannyllithiums are synthetic equivalent of stannyl anions. The reaction of 2,6-di- 7 /-butyl-4-rncthylphcnyl(UI IT) esters with BuLi, followed by the addition of Bu3SnLi, and trapping of the resulting lithium enolate with phenyldi-methylchlorosilane leads to silyloxyvinylstannanes, which are allowed to the subsequent coupling with vinyl iodides (Equation (110)).279... 

If attention at first is confined to the production of methyl silicone from the previously accepted raw materials, the chemical processes must include reduction of silica to silicon, preparation of the methyl chloride from methane or methanol, reaction of the methyl chloride with silicon, and hydrolysis of the methylchlorosilanes. If the same conventions are used as in the discussion of the,Grignard method, and the methanol process for methyl chloride is elected, the steps are ... 

For production of methylchlorosilanes, copper equivalent to about 10 per cent of the weight of the silicon is preferred as a catalyst. This may be added as a powder to the pulverized silicon and the mixture sintered in a hydrogen furnace, or it may be added in other ways. On a molar basis, this copper requirement represents 0.049 mole of copper per mole of silicon entering into the reaction, and all of it remains behind as the silicon is consumed. It Is at least theoretically possible to recover the copper after the reaction has run its course, but the limited recovery value of this small amount allows only the simplest methods to be considered. 

Although the direct reaction of elemental silicon with methyl chloride shown in Eq. (3) looks simple, it is a complicated reaction and gives many kinds of byproducts.7,8 The yield of methylchlorosilane obtained from the direct reaction varies, and depends upon the reaction conditions such as temperature, pressure, flow rate of reactants, and other processing conditions including particle size and impurities of elemental silicon, catalyst, promoter, reactor type, etc.7... 

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

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