Silicoformic Anhydride, H₂Si₂O₃

Silicoformic Anhydride, H₂Si₂O₃, was obtained in an impure form by Buff and Wohler, and pure by Friedel and Ladenburg, by decomposing silicochloroform with ice-cold water. The formation of the anhydride is probably preceded by that of the free acid, which then loses water, thus:

2HSiCl₃+ 6H₂O → 2 HSi(OH)₃ + 6HCl → + 3H₂O

Although orthosilicoformic acid is unstable, its ethyl ester is formed by the action of alcohol on silicochloroform, thus:

HSiCl₃ + 3C₂H₅OH = HSi(OC₂H₅)₃ + 3HCl.

For the less reactive chloroform to yield the corresponding ortho-formic ester, sodium ethoxide must be employed thus:

HCCl₃ + 3C₂H₅ONa = HC(OC₂H₅)₃ + 3NaCl.

Formic anhydride, (HCO)₂O, corresponding to (HSiO)₂O, is unknown, for any attempt to dehydrate formic acid yields carbon monoxide:

2HCOOH → (HSiO)₂O + H₂O → 2CO + 2H₂O.

Silicoformic anhydride is an insoluble white powder, which should be dried in vacuo over sulphuric acid. It may be heated to 300° C. without decomposition, but at a higher temperature it evolves water and silicane, SiH₄, and leaves a residue of silica. It inflames when heated to a high temperature in the air, and emits a phosphorescent light; it is stable towards acids, but reacts with alkalis, forming silicates and hydrogen, thus, e.g.:

(HSiO)₂O + 4NaOH = 2Na₂SiO₃ + H₂O + 2H₂.

Towards various substances in solution it behaves as a reducing agent; thus it reduces permanganic and chromic acids, and separates sulphur from sulphurous acid, and mercury, silver, gold, and platinum from their salts.

These reducing properties are traceable to the hydrogen atom in silicochloroform, which persists in silicoformic anhydride. Like silicic acid, which it somewhat resembles in physical properties, this compound no doubt consists of polymerised molecules, (H₂Si₂O₃)_n.