Silanes

This article is about a class of chemical compounds. For the compound with chemical formula SiH₄, see Silane.

Silanes are chemical compounds of silicon and hydrogen, which are analogues of alkane hydrocarbons. Silanes consist of a chain of silicon atoms covalently bonded to each other and to hydrogen atoms. The general formula of a silane is Si_nH_2n+2. Much of the early work establishing that silicon does indeed form an homologous series of hydrides analogous to the alkanes, albeit to a much smaller extent, was conducted by Alfred Stock and Carl Somiesky.^[1] Although monosilane and disilane were already known, Stock and Somiesky discovered, beginning in 1916, the next four members of the Si_nH_2n+2 series, up to n = 6, and they also documented the formation of solid phase polymeric silicon hydrides (vide infra).^[2]

Nomenclature

There exists a regular nomenclature for silanes. For linear silanes, each silane's name is the word -silane preceded by a numerical prefix (mono-, 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 called either monosilane or 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.

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

Circular structures, called cyclosilanes, also exist. These are analogous to the cycloalkanes. Solid phase polymeric silicon hydrides called polysilicon hydrides are also known. When hydrogen in a linear polysilene polysilicon hydride is replaced with alkyl or aryl side-groups, the term polysilane is used.

Unsaturated silicon hydride molecules called silenes contain the trivalent (doubly-bonded) silicon atom, >Si= (e.g. methylene silane or silaethene, H₂Si(CH₂). They cannot be isolated under normal conditions. The silylenes contain the divalent Si atom (e.g. SiH₂, SiF₂). Most silylenes are unstable at normal temperatures and can only be synthesized in the gas phase at elevated temperatures or isolated in a low-temperature matrix.

Synthesis

The reactions of metal silicides with dilute acids produce mixtures of monosilane, along with higher member of the homologous series. If the gaseous products are collected and condensed (by cooling) into a solid, and then liquified, the components can be separated by fractionation. These reactions may be classified as heterogeneous acid-base chemical reactions since the silicon ion can serve as a Brønsted–Lowry base capable of accepting protons. An example is the reaction between magnesium silicide, Mg₂Si, and hydrochloric acid.

In general, the alkaline-earth metals form silicides with various stoichiometry. However, in all cases these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include silane and/or higher molecules in the homologous series, a polymeric silicon hydride, or a silicic acid. Hence, MSi with their zigzag chains of Si anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH₂)_x.

Properties

The silanes (Si_nH_2n+2) are much less thermally stable than alkanes (C_nH_2n+2) and they are kinetically labile, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of Si_nH_2n+2 molecules with n greater than about 8 difficult.^[3] Greater catenation of the Si atoms can be obtained with the halides (Si_nX_2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen that occurs through the sigma bonding.^[4]

Silanes can also incorporate the same functional groups as alkanes, e.g. –OH, to make a silanol (an analogue of alcohol) or a halogen to make a silicon halide (an analogue of alkyl halide). There is (in principle) a silicon analogue for all carbon alkanes derivatives.

Applications

Main article: Silane#Applications

Several industrial and medical applications exist for the simplest silane (SiH₄) and functionalized 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,^[5] applying polycrystalline silicon layers on silicon wafers when manufacturing semiconductors, and sealants.

Silane (SiH₄) and similar compounds containing Si—H bonds are used as reducing agents in organic and organometallic chemistry.^[6]

“Magic sand” exposes regular sand to trimethylhydroxysilane vapors to make the sand waterproof.

See also

• Silane, the simplest silicon-hydrogen compound with chemical formula SiH₄
• Silanols

References

1. ^ E. Wiber, Alfred Stock and the Renaissance of Inorganic Chemistry," Pure & Appl. Chem., Vol. 49 (1977) pp. 691-700.
2. ^ J. W. Mellor, "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VI, Longman, Green and Co. (1947) pp. 223 - 227.
3. ^ W. W. Porterfield "Inorganic Chemistry: A Unified Approach," Academic Press (1993) p. 219.
4. ^ A. Earnshaw, N. Greenwood, "Chemistry of the Elements," Butterworth-Heinemnann (1997) p. 341.
5. ^ Graffiti protection systems
6. ^ Reductions of organic compounds using silanes