Silicon Oxychlorides

Of the oxyhalides of silicon which might be expected to exist, several oxychlorides alone have been obtained, and of these the simplest, SiOCl₂, is unknown. Our knowledge of silicon oxychlorides is due to the researches of Friedel and Ladenburg, and of Troost and Hautefeuille, and the subject appears scarcely to have been investigated since 1881.

The following oxychlorides have been prepared, the molecular formulae of several of them having been established by vapour-density determinations:

	Boiling-point.	Vapour Density. (Air = 1.)
Si2OCl6	137-138	10.05 (theory = 9.86)
Si4O3Cl10	152-154	?
Si4O4Cl8	198-202	15.5 (theory = 15.9)
Si8O10Cl12	c. 300	31.2 (theory = 28.0)
(Si2O3Cl2)n	above 400	-
(Si4O7Cl2)n	above 440	-

The oxychloride Si₂OCl₆ is formed when a mixture of chlorine and oxygen is passed over crystallised silicon heated to 800° C., when the vapour of silicon tetrachloride is passed over felspar heated to whiteness in a porcelain tube, and by the interaction of silicon tetrachloride and sulphur trioxide at 50° C.:

2SiCl₄ + 2SO₃ = Si₂OCl₆ + S₂O₅Cl₂.

It is a colourless liquid which fumes in the air and reacts with water, forming hydrochloric and silicic acids, whilst with alcohol, and also with zinc ethyl, it yields the ester Si₂O(OC₂H₅)₆. The higher oxy-chlorides were obtained by passing this oxychloride over heated felspar, or by leading a mixture of it and oxygen through a red-hot porcelain tube, the product being then submitted to fractional distillation.

All of these oxychlorides consist of polymerised SiO₂ molecules, some of whose oxygen atoms have been replaced by atoms of chlorine. Moreover, they contain chains or rings of alternative silicon and oxygen atoms thus:



which aye so characteristic of the element silicon, especially in the natural silicates. Although there is no evidence as to the actual constitution of these compounds, it is reasonable to suppose that Si₄O₄Cl₈ possesses a cyclic constitution, to which, it is interesting to note, a phenyl derivative prepared by Kipping from SiCl₄ corresponds:



Si₄O₃Cl₁₀, however, is most easily represented as a chain compound: Cl₃Si-O-SiCl₂-O-SiCl₂-O-SiCl₃, while Si₈O₁₀Cl₁₂ may possibly consist of two eight-membered rings linked through oxygen, thus:



Whether these constitutional formulae adequately represent the facts or not, it is plain that the existence of these complex oxychlorides is connected with that of the similarly complex silicic acids of which the natural silicates are salts.