a class of chemical com-pounds that contain silicon-carbon bonds in their molecules. The organosilicon compounds are divided into the following groups:
(1) The organohalosilanes, or alkyl (aryl)-halosilanes, RnSiX4-n; organohydrohalosilanes RnSiHmX4-(n + m), where X is Cl in most cases; organoalkoxysilanes RnSi(OR’)4-n; organoacyloxysilanes RnSi(OCOR’)4-n; and organoaminosilanes RnSi(NH2)4-n.
(2) The organosilanes (substituted silanes) RnSiH4-n.
(3) The organosiloxanes, including disiloxanes R3SiOSiR3, trisiloxanes R3SiOSi(R2)OSiR3, and higher siloxanes; cyclosiloxanes (R2SiO)n, where n = 3-10 (most often 3-4).
(4) Heterocyclic compounds, such as dimethylsilacyclobutane:
Compounds of the first two groups resemble the corresponding chemically analogous inorganic compounds, such as the halosilanes SiHmX4-n, silanes SixHy, and alkoxysilanes Si(OR)4.
Preparation. The most important organosilicon compounds are produced on an industrial scale mainly by direct reaction of alkyl (aryl) chlorides with silicon in the presence of copper. In addition to the principal product of the reaction 2RCl + Si → R2SiCl2, a mixture of various by-products is also formed (RSiCl3, R3SiCl, RHSiCl2, and so on); these by-products also have industrial uses, such as in the synthesis of organosilicon compounds with more complicated structures, or polymers. Organosilicon compounds are also produced by reactions with organometallic compounds, CH3SiCl3 + C6H5MgCl → CH3(C6H5)SiCl2 + MgCl2; by thermal condensation, CH3SiHCl2 + CH2 = CHCl → CH3(CH2 = CH)SiCl2 + HCl; by liquid-phase dehydrocondensation, CH3SiHCl2 + C6H6 → CH3(C6H5)SiCl2 + H2; and by the addition of organohydrochlorosilanes to unsaturated compounds, CH3SiHCl2 + RCH=CH2 → CH3(RCH2CH2)SiCl2. Alkoxysilanes and acyloxysilanes are most frequently prepared by the reaction of organochlorosilanes with alcohols, acids, and other reagents.
Properties and use. Almost all organosilicon compounds are colorless liquids (see Table 1); only some of them, such as the cyclosiloxanes (R2SiO)3, are solid crystalline substances. Organosilicon compounds are usually thermally stable (~600°C), mabe distilled at atmospheric pressure and under vacuum, are highly soluble in hydrocarbons, chlorinated hydrocarbons, ethers, and other organic solvents, and are immiscible with water. Organosilicon compounds are readily hydrolyzed, especially the organochlorosilanes, which fume in air:
The organohydroxysilanes that form during hydrolysis undergo intermolecular condensations leading to the formation of organosiloxanes:
The evolved or added acid causes polycondensation, with the formation of silicones. Depending on the number of hydrolyzable groups (usually chlorine; sometimes alkoxy, acyloxy, or amino groups), a distinction is made among mono-, di-, tri-, and tetrafunctional organosilicon compounds, which hydrolyze to disiloxanes, mixtures of linear polymers HO(R2SiO)nH and low-molecular-weight cyclic diorganosiloxanes (R2SiO)n (usually,n = 4), and polymers of cyclolinear and crosslinked structure (RSiO1.5)n and (SiO2)n, respectively.
|Table 1. Properties of some organosilicon compounds|
|Boiling point (°C)||Density at 20°C (g/cm3)|
Cyclic diorganosiloxanes (mainly trimers and tetramers), which are also formed during the thermocatalytic degradation (alkali, 150°-400°C) of polymers containing terminal hydroxyl groups, are used in the production of silicone rubbers and sili-cone oils. A mixture of tetraethoxysilane with the products of its partial hydrolysis is used under the name “ethyl silicate” for the preparation of molds for precision casting.
S. A. GOLUBTSOV