Halogen Derivatives of Silico-ethane, Si₂H₆

Hexachlorosilico-ethane, Si₂Cl₆, is formed by the action of chlorine upon the corresponding iodide, Si₂I₆, or by gently heating this compound with mercuric chloride; much heat is evolved and hexachlorosilico-ethane, Si₂Cl₆, distils. Troost and Hautefeuille obtained this compound by the union of silicon and its tetrachloride, by passing the vapour of the latter over white-hot silicon; and Gattermann and Weinlig4 found that it was formed, together with silicon tetrachloride, when chlorine acted on crude silicon, and separated the two compounds by fractional distillation.

Hexachlorosilico-ethane is a colourless, fuming liquid which solidifies at -1° C. and boils at 146°-148° C. Its density at 0° C. is 1.58, and its vapour density at 239° C. is 9.7 (theory requires 9.92).

Above 350° C. the vapour of hexachlorosilico-ethane begins to dissociate into silicon tetrachloride and silicon, and this dissociation continues up to 1000° C., above which temperature the vapour becomes stable again. Consequently, when silicon tetrachloride vapour is passed over silicon heated above 1000° C. in a porcelain tube hexachlorosilico-ethane is formed, and is decomposed again in a cooler part of the tube; by this means silicon is transported along the tube from the hotter to the cooler part (Troost and Hautefeuille). Hexachlorosilico-ethane reacts with water to form silico-oxalic and hydrochloric acids, thus:

+ 4H₂O = + 6HCl.

Solutions of ammonia and potash produce silicic acid or a silicate with evolution of hydrogen, since silico-oxalic acid is decomposed by alkalis:

Si₂Cl₆ + 6H₂O = 2H₂SiO₃ + 6HCl + H₂.

With gaseous ammonia there is formed the solid compound Si₂Cl₆.10NH₃, which begins to lose ammonia at 100° C. and is slowly decomposed by water.

Phosphine does not unite with hexachlorosilico-ethane, but is reduced by it, even at -10° C., to the solid hydrogen phosphide.

This compound is formed by the action of bromine on a carbon disulphide solution of the corresponding iodide, the liberated iodine being removed by shaking with mercury. It is also among the products of the action of the silent electric discharge on silicobromoform, SiHBr₃. It forms tabular crystals, which melt at 95° C. and boil at 265° C. Hexabromosilico-ethane resembles the corresponding chlorine compound in its behaviour towards water and alkalis.

The manner of preparation of hexa-iodosilico-ethane is of interest because this compound is the source of hexachlorosihco-ethane and hexabromosilico-ethane, and in its formation the linkage of silicon atoms takes place. The process is one of condensation, to which numerous analogies are furnished in organic chemistry. Thus Friedel and Ladenburg obtained hexa-iodosilico-ethane by heating the tetra-iodide with finely divided silver at 290°- 300° C. for some hours:

SiI₄ + 2Ag = + 2AgI.

Unchanged tetra-iodide was then removed from the product by washing it with carbon disulphide, in which the tetra-iodide is much more soluble than hexa-iodosilico-ethane, and the latter was then crystallised from hot carbon disulphide. Hexa-iodosilico-ethane forms colourless, hexagonal, doubly refractive plates, which melt in vacuo at 250° C. with partial decomposition, silicon tetra-iodide and a lower iodide, supposed to be silicon di-iodide, SiI₂, being produced. Hexa-iodosilico-ethane fumes in the air, and resembles the corresponding chlorine and bromine compounds in its behaviour towards water and alkalis.