The Higher Chlorides of Silicon

The most satisfactory way for the preparation of these chlorides has been by the chlorination of silicon alloys, especially of magnesium4,5 and iron,7 details having been given in the latter case for a large-scale laboratory preparation. 

In order to prevent the decomposition of the higher chlorides of silicon, it is very important that only a small part of the reaction tube should be heated at a time. When the chlorine is first passed through the reaction tube, the temperature of the heating coil is about 250°C. when the reaction is well started, it is lowered to about 150°C. The best rate of flow of chlorine is less than two bubbles per second. Under these conditions, in about 12 or 14 days, all the calcium-silicon will be used up, and about 700 ml. of liquid silicon chlorides will be collected. 

When the preparation is carried out as described above, keeping the temperature low (150°C.) and the rate of flow of chlorine slow (not over 100 bubbles per minute), a 

Under ordinary conditions, the chlorides of silicon are colorless liquids, with the exception of Si6Cli4, which is a white solid. They all hydrolyze with great ease, fuming with the moisture of the air. When heated, the vapors of the higher chlorides inflame in air. 

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Martin, G. Researches on Silicon Compounds. Part VI. Preparation of Silicon Tetrachloride, Disilicon Hexachloride, and the Higher Chlorides of Silicon by the Action of Chlorine on 50 per cent. Ferrosilicon, Together with a Discussion on Their Mode of Formation. J. chem. Soc. [London] 105, 2836 (1914). 

The higher chlorides of silicon also may be prepared by an electric-discharge method,27 by the action of SiCl4 on silicon,28 and by substitution of chlorine for iodine in hexaiododisilane.29... 

These higher chlorides of silicon hydrolyze in the same way as does the tetrachloride, and they likewise undergo similar reactions with Grignard reagents and with sodium alkyls or aryls.30 Under some conditions, however, the silicon-silicon bond is cleaved by sodium.31 The silicon-silicon chains are destroyed by the action of aqueous alkalies, liberating hydrogen at the rate of one molecule of H2 per silicon-silicon bond, just as are the higher silanes. 

The fluid material remaining in the distilling flask consists of higher homologous chlorides of silicon which may be collected as follows ... 

Just as the chemistry of the silicon hydrides is not limited to that of monosilane, so the silicon halides are not represented solely by the simple tetrahalides so far considered. In addition to these there are several series of higher halides, such as the higher chlorides Si2Cl6,... 

Allyl chloride did not react at temperatures below 220 °C. The total amount of products and the selectivity for allyldichlorosilane gradually increased as the temperature increased to 260 °C, but decreased beyond 280 °C. Reaction temperatures were significantly lower than those for unactivated alkyl chlorides, a reflection of the higher reactivity of allyl chloride. As hydrogen chloride content increased from 50 to 75 mol% in the mixture of allyl chloride and hydrogen chloride, the yield of allyldichlorosilane increased from 31 to 41% and that of diallyldichlorosilane decreased from 1.9 to 0.2%. However, the amount of trichlorosilane, formed from the reaction of elemental silicon with hydrogen chloride, also increased. 

As shown in Table XIV, the reactivity of (trichloromethyl)silanes varied depending upon the substituent on silicon. The reactivity and yields of (trichloromethyl)-methyldichlorosilanes were slightly higher than those of (trichloroinethyl)tri-chlorosilanes in the aluminum chloride-catalyzed alkylation as similarly observed in the alkylations with (ai-chloroalkyl)silanes and (dichloroalkyl)silanes. The electron-donating methyl group on the silicon facilitates the alkylation more than the electron-withdrawing chlorine. The minor products, (diphenylmethyl)chloro-silanes, were presumably derived from the decomposition of (triphenylmethyl)-chlorosilanes. 

Vanadium Subsilicide, V2Si, is obtained by fusing a mixture of vanadium trioxide, V2Os, and silicon, with the addition of either a large excess of vanadium or carbon or copper. The carbide or copper alloy produced is decomposed at the temperature employed.11 The silicide forms metallic prisms, of density 5-48 at 17° C., the m.pt. of which is higher than in the ease of the disilicide. It is attacked by the halogens, hydrogen chloride gas, and fused sodium or copper, but hydrochloric acid, nitric acid and sulphuric acid are without action. 

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