Reactions with elemental silicon

In practice vapours of the hydrocarbon halide, e.g. methyl chloride, are passed through a heated mixture of the silicon and copper in a reaction tube at a temperature favourable for obtaining the optimum yield of the dichlorosilane, usually 250-280°C. The catalyst not only improves the reactivity and yield but also makes the reaction more reproducible. Presintering of the copper and silicon or alternatively deposition of copper on to the silicon grains by reduction of copper (I) chloride is more effective than using a simple mixture of the two elements. The copper appears to function by forming unstable copper methyl, CUCH3, on reaction with the methyl chloride. The copper methyl then decomposes into free methyl radicals which react with the silicon. 

Another important aspect is the very simple preparation of the silyltriflates. Systematic investigations of the cleavage of the silicon element bond (Si-E) by CF3SO3H have shown that the reaction rate decreases in the sequence (E=) a-naphthyl > phenyl > Cl > H > alkyl [3]. Therefore especially pure silyltriflates result from protodesilylation of arylsilanes with CF3SO3H. On the basis of these general results the synthesis of a large number of variously substituted silyltriflates [4,5] can be planned. This is of particular interest in the chemistry of oligosilanes. 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

With Silicon.—If a solution of caustic soda is brought to contact with elemental silicon, chemical reaction ikes place, with the production of sodium silicate and y drogen. The following equation was supposed to .present the reaction —... 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

CARBIDES. A binary solid compound of carbon and another element. The most familiar carbides are those of calcium, tungsten, silicon, boron, and iron (cemcntitc) Two factors have an important bearing on the properties of carbides (1) the difference in electronegativity between carbon and the second elemenl. and (2) whether the second element is a transition metal. Saltlike carbides of alkali metals are obtained by reaction with acetylene. Those ohlained from silver, copper, and mercury sails are explosive. See also Carbon and Iron Metals, Alloys, and Steels. 

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

Until 1982, most alkoxysilanes had been produced from chlorosilanes and alcohols. Hydrochloric acid was therefore still a problem. In 1982, a process was developed in which TMOS could be made directly from elemental silicon and methanol [5]. In the production of silicate coatings, TMOS is first converted to TEOS by an alcoholysis reaction with ethanol. This prevents toxic methanol vapors from escaping from the curing coating. The TEOS is partially hydrolyzed with the rest of the hydrolysis occurring at the time of application. This is therefore a way to produce silicates without chlorine. (If a practical method for converting alkoxysilanes to alkylsilanes could be found, there would also be a nonchlorine method of production of silicones.)... 

S- and 6-coordinated silicon compounds appear often as reaction intermediates. In several cases such compounds are isolable. The known structure frameworks with carbon- and silicon element bonds, which are stable under normal conditions, are represented in Table 2. 

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