Silicofluoroform, SiHF₃

The other three compounds of the type SiHX₃ are formed by the action of the halogen hydracid on silicon; silicofluoroform, however, has not been prepared in this way, but is derived from silicochloroform by the replacement of its chlorine by fluorine. This may be done by heating the latter substance to 220° C. with tin tetrafluoride or to 100°-120° C. if titanium tetrafluoride is employed:

4SiHCl₃ + 3TiF₄ = 4SiHF₃ + 3TiCl₄.

The operation is carried out in a sealed copper flask or gun-barrel; and silicofluoroform is formed as a colourless gas which on strong cooling condenses to a liquid boiling at -80° C., from which a crystalline solid melting at -110° C. can be obtained. The vapour density of silico-fluoroform (air = 1) has been found to be 3.02, the theoretical value being 2.99.

Gaseous silicofluoroform is not very stable, but decomposes, slowly at atmospheric temperature, quickly on heating to 420° C. in accordance with the reaction: 4SiHF₃ = 3SiF₄ + 2H₂ + Si.

It burns with a bluish flame, producing amorphous silicic acid, and forms an explosive mixture with air. Water and dilute alkalis decompose the gas thus:

2SiHF₃ + 4H₂O = Si(OH)₄ + H₂SiF₆ + 2H₂;

absolute alcohol produces, similarly, ethylorthosilicate:

2SiHF₃ + 4C₂H₅OH = Si(OC₂H₅)₄ + H₂SiF₆ + 2H₂

(cf. reaction with silicochloroform), while ether yields the ester of the corresponding acid, orthosilicoformic acid:

SiHF₃ + 3(C₂H₅)₂O = SiH(OC₂H₅)₃ + 3C₂H₅F.