Tetraethyl orthosilicate

Tetraethyl orthosilicate is the major chemical compound with the formula Si(OC2H5)4. Often abbreviated TEOS, this molecule consists of four ethyl groups attached to SiO44- ion, which is called orthosilicate. As an ion in solution, orthosilicate does not exist. Alternatively TEOS can be considered to be the ethyl ester of orthosilicic acid, Si(OH)4. It is a prototypical alkoxide.

TEOS is a tetrahedral molecule. Many analogues exist, and most are prepared by alcoholysis of silicon tetrachloride:
 * SiCl4 +  4 ROH   →  Si(OR)4  +  4  HCl

where R = alkyl such as methyl, ethyl, propyl, etc.

Applications
TEOS has many remarkable properties, but perhaps the most useful is its easy conversion into silicon dioxide. This reaction occurs upon the addition of water:
 * Si(OC2H5)4 + 2 H2O  →  SiO2  +  4  C2H5OH

This hydrolysis reaction is an example of a sol-gel process. The side product is ethanol. The reaction proceeds via a series of condensation reactions that convert the TEOS molecule into a mineral-like solid via the formation of Si-O-Si linkages. Rates of this conversion are sensitive to the presence of acids and bases, both of which serve as catalysts.

At elevated temperatures (>600 °C), TEOS converts to silicon dioxide:
 * Si(OC2H5)4 →  SiO2  +  O(C2H5)2

The volatile coproduct is diethylether.