Silicon tetrachloride

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Silicon tetrachloride
Silicon tetrachloride.svg
Silicon-tetrachloride-3D-vdW.png
Names
IUPAC name
Silicon (IV) chloride
Other names
Silicon tetrachloride
Tetrachlorosilane
Identifiers
10026-04-7 YesY
ChemSpider 23201 YesY
EC number 233-054-0
Jmol-3D images Image
PubChem 24816
RTECS number VW0525000
UN number 1818
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, hydrochloric acid
Vapor pressure 25.9 kPa at 20 °C
Structure
Crystal structure Tetrahedral
4
Thermochemistry
240 J·mol−1·K−1[1]
−687 kJ·mol−1[1]
Hazards
MSDS External MSDS
EU Index 014-002-00-4
EU classification Irritant (Xi)
R-phrases R14, R36/37/38
S-phrases (S2), S7/8, S26
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored 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 constantr), etc.
Thermodynamic
data
Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Silicon tetrachloride 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.

Preparation[edit]

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.[2]

In the laboratory, SiCl4 can prepared by treating silicon with chlorine:

Si + 2 Cl2 → SiCl4

It was first prepared by Jöns Jakob Berzelius in 1823.

Reactions[edit]

Hydrolysis and related reactions[edit]

Like other chlorosilanes, silicon tetrachloride reacts readily with water:

SiCl4 + 2 H2O → SiO2 + 4 HCl

In contrast, carbon tetrachloride does not hydrolyze readily. The differing rates of hydrolysis are attributed to the greater atomic radius of the silicon atom allowing attack at silicon, and to the polar nature of the Si-Cl bonds favoring nucleophilic attack.[citation needed] 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.[3] With methanol and ethanol it reacts to give tetramethyl orthosilicate and tetraethyl orthosilicate:

SiCl4 + 4 ROH → Si(OR)4 + 4 HCl

Polysilicon chlorides[edit]

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.

Reactions with other nucleophiles[edit]

Silicon tetrachloride is a classic electrophile in its reactivity.[4] 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.

Uses[edit]

Silicon tetrachloride is used as an intermediate in the manufacture of polysilicon, a hyper pure form of silicon,[2] 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 reapears in both these two processes as a by-product and is recycled in the hydrogenation reactor. 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[edit]

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.[5]

See also[edit]

References[edit]

  1. ^ a b Zumdahl, S. S. (2009). Chemical Principles (6th ed.). Houghton Mifflin. p. A22. ISBN 0-618-94690-X. 
  2. ^ a b Simmler, W. (2005), "Silicon Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a24_001 
  3. ^ Clugston, M.; Flemming, R. (2000). Advanced Chemistry. Oxford University Press. p. 342. ISBN 9780199146338. 
  4. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  5. ^ "Solar Energy Firms Leave Waste Behind in China". The Washington Post. 9 March 2008.