Silicon tetrachloride
Silicon tetrachloride or tetrachlorosilane is the inorganic compound with the formula SiCl4. It is a colourless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications.
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| Names | |||
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| IUPAC name
Silicon (IV) chloride | |||
| Other names
Silicon tetrachloride Tetrachlorosilane | |||
| Identifiers | |||
3D model (JSmol) |
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| ChemSpider | |||
| ECHA InfoCard | 100.030.037 | ||
| EC Number |
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PubChem CID |
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| RTECS number |
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| UNII | |||
| UN number | 1818 | ||
CompTox Dashboard (EPA) |
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| Properties | |||
| SiCl4 | |||
| Molar mass | 169.90 g/mol | ||
| Appearance | Colourless liquid | ||
| Density | 1.483 g/cm3 | ||
| Melting point | −68.74 °C (−91.73 °F; 204.41 K) | ||
| Boiling point | 57.65 °C (135.77 °F; 330.80 K) | ||
| Reaction | |||
| Solubility | soluble in benzene, toluene, chloroform, ether[1] | ||
| Vapor pressure | 25.9 kPa at 20 °C | ||
| −88.3·10−6 cm3/mol | |||
| Structure | |||
| Tetrahedral | |||
| 4 | |||
| Thermochemistry | |||
Std molar entropy (S |
240 J·mol−1·K−1[2] | ||
Std enthalpy of formation (ΔfH⦵298) |
−687 kJ·mol−1[2] | ||
| Hazards | |||
| Safety data sheet | See: data page MSDS | ||
EU classification (DSD) (outdated) |
Irritant (Xi) | ||
| R-phrases (outdated) | R14, R36/37/38 | ||
| S-phrases (outdated) | (S2), S7/8, S26 | ||
| NFPA 704 (fire diamond) | |||
| Related compounds | |||
Other anions |
Silicon tetrafluoride Silicon tetrabromide Silicon tetraiodide | ||
Other cations |
Carbon tetrachloride Germanium tetrachloride Tin(IV) chloride Titanium tetrachloride | ||
Related chlorosilanes |
Chlorosilane Dichlorosilane Trichlorosilane | ||
| Supplementary data page | |||
| Refractive index (n), Dielectric constant (εr), etc. | |||
Thermodynamic data |
Phase behaviour solid–liquid–gas | ||
| UV, IR, NMR, MS | |||
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 | |||
Preparation
Silicon tetrachloride is prepared by the chlorination of various silicon compounds such as ferrosilicon, silicon carbide, or mixtures of silicon dioxide and carbon. The ferrosilicon route is most common.[3]
In the laboratory, SiCl4 can be prepared by treating silicon with chlorine:[1]
- Si + 2 Cl2 → SiCl4
It was first prepared by Jöns Jakob Berzelius in 1823.
Brine can be contaminated with silica when the production of chlorine is a byproduct of a metal refining process from metal chloride ore. In rare occurrences, the silicon dioxide in silica is converted to silicon tetrachloride when the contaminated brine is electrolyzed.[4]
Reactions
Hydrolysis and related reactions
Like other chlorosilanes, silicon tetrachloride reacts readily with water:
- SiCl4 + 2 H2O → SiO2 + 4 HCl
In contrast, carbon tetrachloride does not hydrolyze readily. The reaction can be noticed on exposure of the liquid to air, the vapour produces fumes as it reacts with moisture to give a cloud-like aerosol of hydrochloric acid.[5]
With alcohols and ethanol it reacts to give tetramethyl orthosilicate and tetraethyl orthosilicate:
- SiCl4 + 4 ROH → Si(OR)4 + 4 HCl
Polysilicon chlorides
At higher temperatures homologues of silicon tetrachloride can be prepared by the reaction:
- Si + 2 SiCl4 → Si3Cl8
In fact, the chlorination of silicon is accompanied by the formation of Si2Cl6. A series of compounds containing up to six silicon atoms in the chain can be separated from the mixture using fractional distillation.[1]
Reactions with other nucleophiles
Silicon tetrachloride is a classic electrophile in its reactivity.[6] It forms a variety of organosilicon compounds upon treatment with Grignard reagents and organolithium compounds:
- 4 RLi + SiCl4 → R4Si + 4 LiCl
Reduction with hydride reagents afford silane.
Comparison with other SiX4 compounds
| SiH4 | SiF4 | SiCl4 | SiBr4 | SiI4 | |
|---|---|---|---|---|---|
| b.p. (˚C)[7] | -111.9 | -90.3 | 56.8 | 155.0 | 290.0 |
| m.p. (˚C)[7] | -185 | -95.0 | -68.8 | 5.0 | 155.0 |
| Si-X bond length (Å) | >0.74 [8] | 1.55 | 2.02 | 2.20 | 2.43 |
| Si-X bond energy (kJ/mol)[9] | 384 | 582 | 391 | 310 | 234 |
Uses
Silicon tetrachloride is used as an intermediate in the manufacture of polysilicon, a hyper pure form of silicon,[3] since it has a boiling point convenient for purification by repeated fractional distillation. It is reduced to trichlorosilane (HSiCl3) by hydrogen gas in a hydrogenation reactor, and either directly used in the Siemens process or further reduced to silane (SiH4) and injected into a fluidized bed reactor. Silicon tetrachloride reappears in both these two processes as a by-product and is recycled in the hydrogenation reactor. Vapor phase epitaxy of reducing silicon tetrachloride with hydrogen at approximately 1250oC was done:
- SiCl
4(g) + 2 H
2(g) → Si(s) + 4 HCl(g) at 1250oC[10]
The produced polysilicon is used as wafers in large amounts by the photovoltaic industry for conventional solar cells made of crystalline silicon and also by the semiconductor industry.
Silicon tetrachloride can also be hydrolysed to fumed silica. High purity silicon tetrachloride is used in the manufacture of optical fibres. This grade should be free of hydrogen containing impurities like trichlorosilane. Optical fibres are made using processes like MCVD and OFD where silicon tetrachloride is oxidized to pure silica in the presence of oxygen.
Safety and environmental issues
Pollution from the production of silicon tetrachloride has been reported in China associated with the increased demand for photovoltaic cells that has been stimulated by subsidy programs.[11] The MSDS notes that one should "avoid all contact! In all cases consult a doctor! ... inhalation causes sore throat and Burning sensation".[12]
See also
References
- P. W. Schenk (1963). "Phosphorus(V) fluoride". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. 1. NY,NY: Academic Press. pp. 282–683.
- Zumdahl, S. S. (2009). Chemical Principles (6th ed.). Houghton Mifflin. p. A22. ISBN 978-0-618-94690-7.
- Simmler, W. "Silicon Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_001.
- White, George Clifford (1986). The handbook of chlorination (2nd ed.). New York: Van Nostrand Reinhold. pp. 33–34. ISBN 0-442-29285-6.
- Clugston, M.; Flemming, R. (2000). Advanced Chemistry. Oxford University Press. p. 342. ISBN 978-0199146338.
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- Silicon Compounds, Silicon Halides. Collins, W.: Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001.
- https://www.answers.com/Q/What_is_the_bond_length_of_the_H-H_bond
- Ebsworth, E. A. V. In Volatile Silicon Compounds; Taube, H.; Maddock, A. G.; Inorganic Chemistry; Pergamon Press Book: New York, NY, 1963; Vol. 4.
- Morgan, D. V.; Board, K. (1991). An Introduction To Semiconductor Microtechnology (2nd ed.). Chichester, West Sussex, England: John Wiley & Sons. p. 23. ISBN 0471924784.
- "Solar Energy Firms Leave Waste Behind in China". The Washington Post. 9 March 2008.
- "International Chemical Safety Cards Tetrachlorosilane".
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