Silico-oxalic Acid, (H₂Si₂O₄)_n

This compound, the silicon analogue of oxalic acid, is formed by the hydrolysis of hexachlorosilico-ethane, Si₂Cl₆, or hexa-iodosilico-ethane, Si₂I₆, by means of ice-cold water:

(SiCl₃)₂ + 4H₂O = (SiOOH)₂ + 6HCl.

Hexa-iodosilico-ethane also reacts with absolute alcohol thus:

Si₂I₆ + 6C₂H₅OH = (SiOOH)₂ + 6C₂H₅I + 2H₂O.

Silico-oxalic acid is conveniently prepared by allowing moist air to act slowly upon hexachlorosilico-ethane contained in a platinum capsule cooled by ice, and then washing the product with cold water till it is free from chloride, and drying it in a vacuum. It is a white insoluble powder which explodes feebly when heated or rubbed. It dissolves in alkalis, forming silicates with evolution of hydrogen, thus:

H₂Si₂O₄ + 4NaOH = 2Na₂SiO₃ + 2H₂O + H₂;

consequently it is a reducing agent; it reduces permanganic acid quickly, and chromic acid slowly, but does not precipitate gold from cold solution. That silico-oxalic acid possesses but half the reducing power of silicoformic anhydride is indicated by the equations:

H₂Si₂O₃ + 2O = H₂O + 2SiO₂
H₂Si₂O₄ + O = H₂O + 2SiO₂.

The name silico-oxalic acid is hardly justifiable since this substance forms no salts.

As regards the constitution of this compound, the formula (SiOOH)₂ postulates reducing properties allied to those of (COOH)₂; nevertheless it has been proposed to formulate this substance thus:



Now the formula is derivable directly from , whilst the above formula involves an intramolecular change, for which some analogy appears to be provided, for instance by phosphorous acid:

PCl₃ → P(OH)₃ → O=PH(OH)₂

The cases, however, are not parallel, for in the latter example the valency of the phosphorus atom is increased. Moreover, there is serious objection to assuming an oxygen chain to be present in the molecule of silico-oxalic acid, and no objection to retaining the formula analogous to that of oxalic acid.

Further, it has been shown by Martin that silico-oxalic acid is produced, in a yield of 99-100 per cent., by the reaction:

Si₂(OEt)₆ + 4H₂O = Si₂O₄H₂ + 6EtOH;

and since the series of reactions:

Cl₃Si.SiCl₃ -(EtOH)→ (EtO)₃Si.Si(OEt)₃ -(H₂))→ (HO)₃Si.Si(OH)₃ -(-2H₂O)→ HO.SiO.SiO.OH,

proceeds quantitatively, this fact affords additional evidence that the constitution of silico-oxalic acid is analogous to that of oxalic acid. The compound is, however, probably a polymer with a cyclic constitution, as appears below.