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Part XXV | ||||||||||||||||||||||||||||||||||||
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The Organic Chemistry of Silicon The element silicon, atomic number 14, is more abundant on the earth than any element except oxygen. However, it has never been found in the elemental form, but instead occurs mainly as its oxide silica, SiO2, which makes up about 60% of the earth's crust. Silica is best known as quartz, and is the major component of beach sand and of glass. Silicon itself, first reported by Berzelius in 1817, is readily obtained by reducing silica with charcoal: SiO2 + 2 C The element is a hard, solid material with a gray metallic sheen. Its properties as a semiconductor led to the development of the transistor in 1950, to integrated circuits in 1958, and of course to the ubiquitous computers of today. The carbon atom, as we have seen, has two electron shells, with 2 electrons in the inner (complete) shell and 4 electrons in the outer (valence) shell. Silicon, on the other hand, has three shells containing 2 + 8 + 4 electrons. Since both carbon and silicon have four valence electrons and are thus tetravalent, they might be expected to form analogous compounds. This is partly the case.
Silicon forms a series of compounds with hydrogen that are analogous to the alkane series formed by carbon. They are named as follows:
The radical (substituent) -SiH3 is named the silyl radical, while the higher radicals are called the disilanyl, tetrasilanyl, and dodecasilanyl radicals, as examples. The compound
In most cases, the hydrogen atoms of silanes can be replaced by the organic substituents we have discussed previously:
The simple silanols SiH3OH (silanol), SiH2(OH)2 (silanediol), SiH(OH)3, and Si(OH)4 have never been isolated. They spontaneously lose water and condense to form ethers. For example silyl chloride SiH3Cl reacts with water to form disilyl ether (disiloxane) (SiH3)2O instead of silanol SiH3OH. However, phenylsilanetriol C6H5Si(OH)3 is a stable compound. Silyl ethers are named either as ethers or as siloxanes: (SiH3)2O Disilyl ether or disiloxane (SiH3SiH2)2O Bisdisilanyl ether or 1,2-disilyldisiloxane SiH3OSiH2SiH3 Silyl disilanyl ether or 1-silyldisiloxane (CH3)3SiOSiH3 Silyl trimethylsilyl ether or 1,1,1-trimethyldsiloxane Alkoxysilanes SiH3OR, SiH2(OR)2, SiH(OR)3, and Si(OR)4 are generally stable substances: CH3OSiH3 Methoxysilane (C2H5O)4Si Tetraethoxysilane or tetraethyl orthosilicate Tetra(2-ethylbutoxy)silane, (C6H13O)4Si and its disiloxane derivative [(C6H13O)3Si]2O are important high-temperature lubricants and hydraulic fluids for military applications. Tetraethoxysilane (tetraethyl orthosilicate) is a relatively inexpensive additive used in the coating industry.
CAUTION! German "Silizium" and French "silicium" are silicon in English, while German "Silikon" and French "silicone" are silicone in English. It's important to distinguish between the element and the polymer. Silicones are polymeric ethers of the form [RnSiO(4-n)]m, where n is between 0 and 3, and m is larger than 2. Linear dimethyl silicones (polydimethylsiloxanes), as an example, have the formula X[-Si(CH3)2O-]nX and may contain thousands of units (n > 1000). Such polymers may be oils, gums, resins, or rubbers, and can be modified in many ways, or crosslinked, using comonomers or additives. Their uses are too numerous to be listed here. Their medical use as implants has been in the news in recent years. This very brief introduction to silicon chemistry is intended only as a reference point for English nomenclature. Kirk-Othmer's Encyclopedia of Chemical Technology devotes hundreds of pages to the subject, which obviously defies complete coverage here. The author asks readers to suggest topics for future installments. Kindly email your suggestions and/or questions to cclaff@cs.com. |
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Si + 2 CO
