Orthosilicic acid
chemical compound
Orthosilicic acid () is an inorganic compound with the formula Si(OH)4. Although rarely observed, it is the key compound of silica and silicates and the precursor to other silicic acids [H2xSiOx+2]n. Silicic acids play important roles in biomineralization and technology. It is the parent acid of the orthosilicate anion SiO4−4.
Isolation
Typically orthosilicic acid is assumed to be a product of the hydrolysis of its esters, Si(OR)4, where R stands for organyl group, as is practiced in sol-gel syntheses. These conditions are however too vigorous to allow isolation of the parent acid.
Orthosilicic acid can be produced by Pd-catalyzed hydrogenolysis of tetrabenzoxysilicon:
Si(OCH2Ph)4 + 4 H2 → Si(OH)4 + 4 PhCH3
The acid was crystallized from a solution of dimethylacetamide and tetrabutylammonium chloride. As established by X-ray crystallography, the chloride anions interact with the acid via hydrogen bonds. Otherwise, the structure consists of the expected tetrahedral silicon center.
Reactions
Silicic acid readily condenses to give "higher" silicic acids including disilicic (pyrosilicic) and cyclo-tetrasilicic acid, (−O−Si(OH)2−)4:
2 Si(OH)4 → O(Si(OH)3)2 + H2O
4 Si(OH)4 → (−O−Si(OH)2−)4 + 4 H2O
These derivatives have also been characterized crystallographically.
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