Organosilicon Compounds

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, R_nSiX₄-n; organohydrohalosilanes R_nSiH_mX_{4-(n + m)}, where X is Cl in most cases; organoalkoxysilanes R_nSi(OR’)_4-n; organoacyloxysilanes R_nSi(OCOR’)_4-n; and organoaminosilanes R_nSi(NH₂)_4-n.

(2) The organosilanes (substituted silanes) R_nSiH_4-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 SiH_mX_4-n, silanes Si_xH_y, 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 (RSiO_1.5)_n and (SiO₂)_n, respectively.

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.