Silicon tetrachloride

Silicon tetrachloride

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Names
IUPAC name Silicon (IV) chloride
Other names Silicon tetrachloride Tetrachlorosilane
Identifiers
CAS Number	10026-04-7 Y
ChemSpider	23201 Y
EC Number	233-054-0
Jmol 3D model	Interactive image
PubChem	24816
RTECS number	VW0525000
UN number	1818
InChI InChI=1S/Cl4Si/c1-5(2,3)4 Y Key: FDNAPBUWERUEDA-UHFFFAOYSA-N Y InChI=1/Cl4Si/c1-5(2,3)4
SMILES [Si](Cl)(Cl)(Cl)Cl
Properties
Chemical formula	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)
Solubility in water	Reaction
Solubility	soluble in benzene, toluene, chloroform, ether, hydrochloric acid
Vapor pressure	25.9 kPa at 20 °C
Structure
Crystal structure	Tetrahedral
Coordination geometry	4
Thermochemistry
Std molar entropy (So298)	240 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−687 kJ·mol−1
Vapor pressure	{{{value}}}
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
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
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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

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.

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

Si + 2 Cl₂ → SiCl₄

It was first prepared by Jöns Jakob Berzelius in 1823.

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 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. 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. With methanol 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.

Reactions with other nucleophiles

Silicon tetrachloride is a classic electrophile in its reactivity. 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

Silicon tetrachloride is used as an intermediate in the manufacture of polysilicon, a hyper pure form of silicon, 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

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.

See also

• Silicon tetrachloride (data page)