Silane

Silane is a chemical compound with chemical formula SiH₄. It is the silicon analogue of methane. At room temperature, silane is a gas, and is pyrophoric — it undergoes spontaneous combustion in air, without the need for external ignition. However, one school of thought holds that silane itself is stable and that the natural formation of larger silanes during production causes its pyrophoricity.^{[citation needed]} Above 420°C, silane decomposes into silicon and hydrogen; it can therefore be used in the chemical vapor deposition of silicon.

More generally, a silane is any silicon analogue of an alkane hydrocarbon. Silanes consist of a chain of silicon atoms covalently bound to hydrogen atoms. The general formula of a silane is Si_nH_2n+2. Silanes tend to be less stable than their carbon analogues because the Si–Si bond has a strength slightly lower than the C–C bond. Oxygen decomposes silanes easily, because the silicon-oxygen bond is quite stable.

There exists a regular nomenclature for silanes. Each silane's name is the word silane preceded by a numerical prefix (di, tri, tetra, etc.) for the number of silicon atoms in the molecule. Thus Si₂H₆ is disilane, Si₃H₈ is trisilane, and so forth. There is no need for a prefix for one; SiH₄ is simply silane. Silanes can also be named like any other inorganic compound; in this naming system, silane is named silicon tetrahydride. However, with longer silanes, this becomes cumbersome.

A cyclosilane is a silane in a ring, just as a cycloalkane is an alkane in a ring.

Branched silanes are possible. The radical ·SiH₃ is termed silyl, ·Si₂H₅ is disilanyl, and so on. Trisilane with a silyl group attached to the middle silicon is named silyltrisilane. The nomenclature parallels that of alkyl radicals.

Silanes can also incorporate the same functional groups as alkanes, e.g. –OH to make a silanol. There is (at least in principle) a silicon analogue for all carbon alkanes.

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Production

Industrially, silane is produced from metallurgical grade silicon in a two-step process. In the first step, powdered silicon is reacted with hydrochloric acid at about 300 °C to produce trichlorosilane, HSiCl₃, along with hydrogen gas, according to the chemical equation:

Si + 3HCl → HSiCl₃ + H₂

The trichlorosilane is then boiled on a resinous bed containing a catalyst which promotes its disproportionation to silane and silicon tetrachloride according to the chemical equation:

4HSiCl₃ → SiH₄ + 3SiCl₄

The most commonly used catalysts for this process are metal halides, particularly aluminium chloride.

Applications

Several industrial and medical applications exist for silanes. For instance, silanes are used as coupling agents to adhere glass fibers to a polymer matrix, stabilizing the composite material. They can also be used to couple a bio-inert layer on a titanium implant. Other applications include water repellents, masonry protection, control of graffiti,^[1] applying polycrystalline silicon layers on silicon wafers when manufacturing semiconductors, and sealants.

Silane is also used in Supersonic combustion ramjets to initiate combustion in the compressed air stream.

Silane and similar compounds containing Si-H-bonds are used as reducing agents in organic and organometallic chemistry.^[2]

"Mars sand" exposes regular sand to trimethylhyroxysilane vapors to make the sand waterproof.

References

1. ^ Graffiti protection systems
2. ^ Reductions of organic compounds using silanes

See also

• silanization