Oxidation reactions silicon tetrachloride

The reinforcing fibers are usually CVD SiC or modified aluminum oxide. A common matrix material is SiC deposited by chemical-vapor infiltration (CVI) (see Ch. 5). The CVD reaction is based on the decomposition of methyl-trichlorosilane at 1200°C. Densities approaching 90% are reported.b l Another common matrix material is Si3N4 which is deposited by isothermal CVI using the reaction of ammonia and silicon tetrachloride in hydrogen at 1100-1300°C and a total pressure of 5 torr.l" " ] The energy of fracture of such a composite is considerably higher than that of unreinforced hot-pressed silicon nitride. 

An extension of the reduction-chlorination technique described so far, wherein reduction and chlorination occur simultaneously, is a process in which the oxide is first reduced and then chlorinated. This technique is particularly useful for chlorinating minerals which contain silica. The chlorination of silica (Si02) by chlorine, in the presence of carbon, occurs above about 1200 °C. However, the silica present in the silicate minerals readily undergoes chlorination at 800 °C. This reaction is undesirable because large amounts of chlorine are wasted to remove silica as silicon tetrachloride. Silica is, therefore, removed by other methods, as described below, before chlorination. Zircon, a typical silicate mineral, is heated with carbon in an electric furnace to form crude zirconium carbide or carbonitride. During this treatment, the silicon in the mineral escapes as the volatile oxide, silicon monoxide. This vapor, on contact with air, oxidizes to silica, which collects as a fine powder in the furnace off-gas handling system ... 

Titanium tetrachloride is produced on an industrial scale by the chlorination of titanium dioxide-carbon mixtures in reactors lined with silica. During the reactor operation, the lining comes into contact not only with chlorine but also with titanium tetrachloride. There appears to be no attack on silica by either of these as the lining remains intact. However, the use of such a reactor for chlorinating beryllium oxide by the carbon-chlorine reduction chlorination procedure is not possible because the silica lining is attacked in this case. This corrosion of silica can be traced to the attack of beryllium chloride on silica. The interaction of beryllium chloride with silica results in the formation of silicon tetrachloride in accordance with the reaction... 

Aminolysis of the corresponding halides is the preferred method for the synthesis of dialkylamino derivatives of boron,1 silicon,2 germanium,3 phosphorus,4 arsenic,5 and sulfur.6 (Dialkylamino) chlorosilanes are prepared stepwise by the reaction of silicon tetrachloride with dialkylamines. This method may be utilized equally well for the conversion of alkyl- or aryl-substituted halides [e.g., (CH3) SiCl4. ] or of oxide and sulfide halides (e.g., POCl3 or PSC13) to the corresponding dialkylamino compounds. 

J. L. Gay Lussac and L. J. Thenard 5 showed in 1811 that if many of the metallic oxides be intimately mixed with carbon the reaction with chlorine proceeds more readily than with the oxide alone the metal chloride and carbon monoxide or dioxide are the products of the reaction. M. le Quesneville and F. Wohler used this process for aluminium chloride, chromic chloride, silicon tetrachloride, etc., and C. Baskerville for thorium tetrachloride. 

The products of the reaction between silicon(IV) oxide and powdered magnesium can also be used to prepare silicon tetrachloride without the separation of free silicon. 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

Carbon brings about reduction of arsenious oxide at a temperature below red heat,11 while in carbon monoxide reduction begins at 60° C.12 The numerous reactions of arsenious oxide with organic compounds are described in Vol. XI, Part II, of this Series. Silicon tetrachloride heated for 30 hours at 270° to 280° C. with the oxide yields arsenic trichloride,13 whilst silicochloroform when heated with the oxide in the presence of aqueous sodium hydroxide-or sodium hydrogen carbonate... 

The most widespread technique, however, is the chlorination of titanium dioxide with chlorine or chlorine-containing substances (carbon tetrachloride, chloroform, sulftuyl chloride, phosphorus oxychloride, silicon tetrachloride). These reactions give high yield at high temperatures (800 °C and more) the chlorination with free chlorine occurs at noticeable speed only in the presence of reducing agents (e.g., coal). If there is a lack of coal, the reaction forms carbon dioxide if there is an excess of coal, it releases carbon oxide ... 

Some ceramic materials are not found widely or at all in nature, and thus are synthesized for use. To prepare more complex ceramic compositions such as perovskites of general structural formula ABO3, and ferrites, of formula MFc204, the individual oxides or salts of the cations A, B, and M are often combined as powders and then reacted at high temperature by a solid-state diffusion mechanism. Silicon nitride (Si3N4) can be manufactured from either the nitridation of silicon metal or from the reaction of silicon tetrachloride with ammonia. Silicon carbide (SiC) is obtained from the reduction of silica with a carbon containing source. 

Prepd by hydrolyzing dimethyldichlorosilane or its esters followed by oxidation with air and a catalyst to the desired CHj/Si ratio Hyde, DeLong, ibid. I ]94 by hydrolysis of dimethyldichlorosilane mixed with methyltrich]oro. silane or silicon tetrachloride followed by cocondensation of the products Rochow, Gilliam, loc. elf, by partially methylating silicon tetrachloride to the desired CH3/Si ratio and hydrolyzing the reaction mixture U.S, pat. 2,258,218 (1941). 

The reduction of zirconium tetrachloride in a carrier salt with sodium as a reducing agent may be examined next. Again, as described before, complete deoxidation of the bath before reduction is the essential condition for success, if ductile metal is wanted. Zirconium tetrachloride is soluble in sodium chloride or potassium chloride (48) and a salt with about 25% zirconium tetrachloride can be melted without excessive zirconium chloride losses. Such a bath can even be obtained from powdery commercial zirconium silicide and iron dichloride, which react when heated and deliver a stream of zirconium tetrachloride, contaminated with some titanium tetrachloride and silicon tetrachloride. The gas so produced can be condensed in a fused salt bath such as potassium chloride-sodium chloride, in which only the zirconium tetrachloride dissolves (47), To obtain a low oxide metal after reduction with sodium, the conditions for fluo salt deoxidation must be observed. This process of zirconium production has no special interest, except for obtaining powder for getter purposes. A carrier salt, which might introduce oxide, is not wanted, as the reaction itself liberates sodium chloride. 

Silicon accounts for 27.7% of the mass of the Earth s crust and occurs in a wide variety of silicates with other metals, clays, micas, and sand, which is largely Si02. The element is obtained on a small scale by the reduction of silicon(IV) oxide (Si02) by carbon or calcium carbide. For semiconductor applications very pure silicon is produced by direct reaction of silicon with an HCI/CI2 mixture to give silicon tetrachloride (SiCb), which can be purified by distillation. This is then decomposed on a hot wire in an atmosphere of hydrogen. For ultra-pure samples zone refining is used. Unlike carbon, silicon does not form allotropes but has only the diamond type of structure. 

Silicon nitride (Si3N4) is a major industrial material which is produced extensively by CVD for electronic and structural applications. It is an excellent electrical insulator and diffusion barrier (to sodium and water vapor) and has replaced CVD oxides in many semiconductor devices.l l Silicon nitride coatings are produced by the reaction of silicon tetrachloride (SiCl4) with ammonia ... 

It has been also found that chiral phosphine oxides (5)-BINAPO catalyzed silicon tetrachloride-mediated, enantioselective phosphonylation of aldehydes with trialkyl phosphites, led to formation of optically active a-hydroxyphosphonates with low to moderate enantioselectivities (ee 22-52%)7 This reaction constituted the first example of asymmetric Abra-mov-type phosphonylation of aldehydes with trialkyl phosphites catalyzed by chiral Lewis bases. 

A hypervalent silicon complex, generated from silicon tetrachloride and a chiral phosphine oxide, acts as an enantioselective organocatalyst of the MBH reaction, by asym- q. metric delivery of a chloride anion as a nucleophile. ... 

Some other processes involve the reaction of unsaturated compounds [5-7] ethylene oxide, or carbon monoxide with silicon tetrachloride [Eqs. (7)-(10)]. 

Preparative Methods two different routes have been developed for the preparation of methyl trichlorosilyl ketene acetal. One route is through bis(tributyltin) oxide-catalyzed transmetala-tion of methyl tributylstannyl acetate (2) with silicon tetrachloride (eq 1).3 When the reaction is complete, excess silicon tetrachloride is removed and the product is purified by fractional distillation under reduced pressure. 

In addition, this class of chiral phosphoramides could catalyze the enantioselective ring opening of epoxides with silicon tetrachloride (SiCfl) [4]. For instance, the reaction of ds-stilbene oxide with SiCl, proceeded smoothly in the presence of 1 to afford the desired chlorohydrin in an enantiomericaUy enriched form (Scheme 7.3). The reactive ion pair shown in the scheme was proposed to be generated through the activation of SiCl, by the coordination of one phosphoramide 1, serving to activate the epoxide it was confirmed that bis-phosphoramide 2 was far less selective in this reaction. 

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