Tetraethyl orthosilicate

September 14, 2020
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Tetraethyl orthosilicate (TEOS) is an ester of orthosilicic acid, which exists in small amounts in nature wherever silica is in contact with water. The ester is also known by several other names, including ethyl silicate (which is somewhat ambiguous), silicon tetraethoxide, and tetraethoxysilane.

In a landmark paper published in 1928, A. W. Dearing and E. Emmet Reid* at Johns Hopkins University (Baltimore) reported improved syntheses of various alkyl orthosilicates. TEOS was obtained in 70% yield by slowly adding silicon tetrachloride (SiCl4) to cold anhydrous ethanol, followed by removing byproduct hydrogen chloride with a stream of dry air. The reaction system must be rigorously free of water because even though TEOS is only slightly soluble in water, it hydrolyzes to form silica and ethanol. The reaction is the basis of modern TEOS production.

TEOS has multiple specialty uses, including stone hardening (which arrests the decay of structures and art objects), mortar and cement manufacture, and cross-linking silicone polymers. Its 2020 global market is valued at US$245 million for an estimated production of ≈120,000 t.

Tetraethyl orthosilicate hazard information

Hazard class*Hazard statement
Flammable liquids, category 3H226—Flammable liquid and vaporChemical Safety Warning
Serious eye damage/eye irritation, category 2AH319—Causes serious eye irritationChemical Safety Warning
Acute toxicity, inhalation, category 3H331—Toxic if inhaledChemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3H335—May cause respiratory irritationChemical Safety Warning

 *Globally Harmonized System of Classification and Labeling of Chemicals.
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MOTW update

Scopolamine and its biochemical precursor hyoscyamine were the Molecules for the Week of October 21, 2013. Both are highly toxic and hallucinogenic. They are found in plants of the Solanaceae family such as deadly nightshade, mandrake, jimsonweed, and—surprisingly—tomato. Despite their hazardous nature, both alkaloids are used in medicine. Recently, Christina D. Smolke* and Prashanth Srinivasan at Stanford University (CA) engineered Saccharomyces cerevisiae (brewer’s yeast) to produce hyoscyamine and scopolamine. The biosynthetic pathway is considered to be the longest and most complex to date for engineered yeast.

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Tetraethyl orthosilicate fast facts

CAS Reg. No.78-10-4
Silicic acid (H4SiO4), tetraethyl ester
Empirical formulaC8H20O4Si
Molar mass208.33 g/mol
AppearanceColorless liquid
Boiling point169 ºC
Water solubility 1.5 g/L (dec.)
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