Silicon tetrachloride
Encyclopedia
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.
In the laboratory, SiCl4 can prepared by treating silicon
with chlorine
:
It was first prepared by Jöns Jakob Berzelius
in 1823.
In contrast, carbon tetrachloride
does not hydrolyze readily. The differing rates of hydrolysis are attributed to the greater atomic radius of the silicon atom, which allows attack at silicon. 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
:
of silicon tetrachloride can be prepared by the reaction:
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
.
Reduction with hydride reagents afford silane
.
, since it has a boiling point convenient for purification by repeated fractional distillation
. It can be reduced to silicon by hydrogen gas. Very pure silicon derived from silicon tetrachloride is used in large amounts in the semiconductor industry, and also in the production of photovoltaic cells. It can also be hydrolysed to high purity fused 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.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
SiCl4. It is a colourless volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
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 prepared by treating silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
with chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
:
- Si + 2 Cl2 → SiCl4
It was first prepared by Jöns Jakob Berzelius
Jöns Jakob Berzelius
Jöns Jacob Berzelius was a Swedish chemist. He worked out the modern technique of chemical formula notation, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry...
in 1823.
Hydrolysis and related reactions
Like other chlorosilanes, silicon tetrachloride reacts readily with water:- SiCl4 + 2 H2O → SiO2 + 4 HCl
In contrast, carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
does not hydrolyze readily. The differing rates of hydrolysis are attributed to the greater atomic radius of the silicon atom, which allows attack at silicon. 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
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
.
With methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
and ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
it reacts to give tetramethyl orthosilicate
Tetramethyl orthosilicate
Tetramethyl orthosilicate is the chemical compound with the formula Si4. This molecule consists of four methyl groups attached to the hypothetical anion SiO44-...
and tetraethyl orthosilicate
Tetraethyl orthosilicate
Tetraethyl orthosilicate is the chemical compound with the formula Si4. Often abbreviated TEOS, this molecule consists of four ethyl groups attached to SiO44- ion, which is called orthosilicate. As an ion in solution, orthosilicate does not exist. Alternatively TEOS can be considered to be the...
:
- SiCl4 + 4 ROH → Si(OR)4 + 4 HCl
Polysilicon chlorides
At higher temperatures homologuesHomologous series
In chemistry, a homologous series is a series of compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and showing a gradation in physical properties as a result of increase in molecular size and mass...
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
Fractional distillation
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of 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
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
.
Uses
Silicon tetrachloride is used as an intermediate in the manufacture of high purity siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, since it has a boiling point convenient for purification by repeated fractional distillation
Fractional distillation
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation...
. It can be reduced to silicon by hydrogen gas. Very pure silicon derived from silicon tetrachloride is used in large amounts in the semiconductor industry, and also in the production of photovoltaic cells. It can also be hydrolysed to high purity fused 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.