Tetramethyl orthosilicate (TMOS) is the chemical compound with the formula Si(OCH3)4. This molecule consists of four methoxy groups bonded to a silicon atom. The basic properties are similar to the more popular tetraethyl orthosilicate, which is usually preferred because the product of hydrolysis, ethanol, is less toxic than methanol.
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Names | |||
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IUPAC name
Tetramethyl orthosilicate
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Other names
tetramethyl orthosilicate; methyl silicate; silicic acid, tetramethyl ester; silicon methoxide; TMOS
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Identifiers | |||
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3D model (JSmol)
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ChemSpider |
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ECHA InfoCard | 100.010.598 | ||
PubChem CID
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UNII |
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CompTox Dashboard (EPA)
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Properties | |||
SiC4H12O4 | |||
Molar mass | 152.25 | ||
Appearance | colourless liquid | ||
Density | 1.032 | ||
Melting point | 4 to 5 °C (39 to 41 °F; 277 to 278 K) | ||
Boiling point | 121 to 122 °C (250 to 252 °F; 394 to 395 K) | ||
organic solvents | |||
Vapor pressure | 12 mmHg (25°C)[1] | ||
Hazards | |||
Occupational safety and health (OHS/OSH): | |||
Main hazards
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toxic | ||
Flash point | 96 °C; 205 °F; 369 K[1] | ||
NIOSH (US health exposure limits): | |||
PEL (Permissible)
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none[1] | ||
REL (Recommended)
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TWA 1 ppm (6 mg/m3)[1] | ||
IDLH (Immediate danger)
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N.D.[1] | ||
Related compounds | |||
Other cations
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Trimethyl borate Trimethyl phosphite | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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Tetramethyl orthosilicate hydrolyzes to SiO2:
In organic synthesis, Si(OCH3)4 has been used to convert ketones and aldehydes to the corresponding ketals and acetals, respectively.[2]
The hydrolysis of Si(OCH3)4 produces insoluble SiO2 and CH3OH (methanol). Even at low concentrations inhalation causes lung lesions, and at slightly higher concentrations eye contact with the vapor causes blindness[citation needed]. Worse, at low concentrations (200 ppm/15 min) the damage is often insidious, with onset of symptoms hours after exposure.[3] The mode of action is the precipitation of silica in the eyes and/or lungs[citation needed]. Contrary to common information, including several erroneous MSDS sheets, the methanol produced is only a risk through chronic exposure and is a comparatively small concern. The mechanisms of methanol toxicity are well established, methanol causes blindness via conversion to formaldehyde, then to toxic formic acid in the liver; methanol splashes to the eye cause only moderate and reversible eye irritation.[4]