Silicon Tetrachloride, SiCl₄

Silicon Tetrachloride (Tetrachlorosilicane), SiCl₄, was first obtained by Berzelius in 1823, and is formed by the union of its elements when dry chlorine is passed over heated amorphous or crystalline silicon. Instead of pure silicon the crude product obtained by Gattermann's method may be employed, being heated to 300°-310°C., or magnesium silicide may be gently heated in a combustion tube, through which chlorine is passed, and the silicon tetrachloride condensed in a receiver surrounded by ice. The silicon tetrachloride thus prepared will contain some Si₂Cl₆ and a little Si₃Cl₈, from which it is freed by fractional distillation. It may also be obtained by passing chlorine over ferrosilicon, containing 15 per cent, of silicon, which is heated to redness in a fire-clay retort, and also similarly from cuprosilicon.

Lastly, silicon tetrachloride may be made by passing chlorine over a heated mixture of silica and carbon:

SiO₂ + 2C + 2Cl₂ = SiCl₄ + 2CO.

This reaction is typical of a general method for preparing non-metallic chlorides, and differs essentially from that to be employed for metallic chlorides, viz. the action of hydrochloric acid upon their oxides or hydroxides. Consider, for example, the two reactions:

Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Si(OH)₄ + 4HCl ← SiCl₄ +4H₂O.

The difference between the metal and the non-metal is signalised by the direction of the reaction. This difference, however, is seldom absolute, and with elements of intermediate character it disappears. In the case of stannic tin, for example, the reaction is reversible:

Sn(OH)₄ + 4HCl ⇔ SnCl₄ + 4H₂O;

and stannic chloride can be prepared either by dissolving stannic hydroxide in hydrochloric acid, or by passing chlorine over heated tin or a mixture of stannic oxide and carbon.

Silicon tetrachloride is a heavy, colourless, fuming liquid. There is some discrepancy between different observations of its boiling-point, probably owing to the difficulty of obtaining the compound free from higher chlorides. The results lie between 59° C. and 56.9° C. at 760 mm. pressure. The latter temperature, observed by J. Meyer-and Becker, is probably most nearly correct. This compound solidifies at - 89° C.

Numerous observations have been made upon the density, the mean of four closely agreeing results being 1.524. Dumas found the vapour density to be 5.9390; Regnault found it to be 5.86, the calculated value being 5.8833. The critical temperature is 230° C. and the heat of formation 157,640 calories. The vapour is not found to dissociate at the softening-point of porcelain.

Like acid chlorides in general, but unlike carbon tetrachloride, silicon tetrachloride suffers hydrolytic decomposition by water, forming hydrochloric acid, and the acid corresponding to the chloride, in this case silicic acid; consequently, when silicon tetrachloride reacts with water gelatinous silica separates. The heat of this reaction has been measured by Berthelot and Thomsen, who found it to be 69,000 and 69,300 calories per gram-molecule respectively. When steam reacts with silicon tetrachloride vapour at a red heat crystallised silica is formed, and if metallic oxides are present crystallised artificial silicates may be prepared. Silicon tetrachloride reacts with dry air at a white heat to form oxychloride; it may be distilled from sodium or potassium, since it does not react with these metals below 200° C., though at a red heat it yields its chlorine to them, amorphous silicon being separated; zinc and silver, and to a less extent copper and iron, react similarly, but in no case is a lower chloride of silicon formed.

Hydrogen sulphide at a red heat forms silicon trichlorohydro-sulphide, SiCl₃SH, and hydrogen bromide and iodide effect a change of halogen with the formation of mixed silicon halides (q.v.). Phosphoric oxide forms phosphoryl chloride and silica; sulphur trioxide forms pyrosulphuryl chloride, S₂O₅Cl₂, and concentrated sulphuric acid produces silicic acid with evolution of hydrogen chloride. Silicon tetrachloride also reacts with many metallic oxides, forming silica and metallic chlorides or oxychlorides; with nitrous acid it forms nitrosyl chloride, and it reacts similarly with fatty acids, forming their chlorides, while with alcohol it forms ethyl silicate.

Silicon tetrachloride is the starting-point for the preparation of organic derivatives of silicon, which are now known in large numbers. The chlorine atoms may be exchanged for organic radicles through the agency of (i) zinc alkyls, (ii) sodium and alkyl bromides or iodides (Fittig's reaction), or (iii) by Kipping's application of the more recently discovered and valuable Grignard's reagent, in which magnesium reacts with an alkyl or aryl bromide in ethereal solution, producing magnesium alkyl or aryl bromide, , which can then exchange its organic radicle for a chlorine atom of silicon tetrachloride, thus:

+ SiCl₄ = + SiCl₃X.

Besides these substitution products, silicon tetrachloride also forms some addition compounds. It forms with ammonia a white, infusible mass having the composition SiCl₄.6NH₃; and absorbs phosphine at low temperature, forming substances of indefinite composition. Thus at -20° C. silicon tetrachloride absorbs twenty times, and at -50° C. forty times, its volume of hydrogen phosphide. By strong compression at low temperature silicon tetrachloride and hydrogen phosphide can be made to form an unstable crystalline compound whose composition is unknown. Silicon tetrachloride also forms additive compounds with organic bases.