Silenes, Germenes

UNIT 2 CHEMISTRY OF SI, S, P COMPOUNDS 9 Hrs. Silanes, silicon halides, silicates, silicones, silanols – Silenes, germenes and stannenes – Phosphorous halides, acids and oxyacids of phosphorous, phosphazenes – Sulphur halides, oxo acids of sulpur – Structural features and reactivity of reactivity of S-N heterocycles. Silanes Introduction Silane is an inorganic compound with chemical formula, SiH4. It is a colourless, flammable gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Silane may also refer to many compounds containing silicon, such as trichlorosilane (SiHCl3), tetramethylsilane (Si(CH3)4) and silane methacrylate (H2C=C(CH3)CO2(CH2)3Si(OCH3)3). Silane is the silicon analogue of methane. Preparation Methylsilanes are the raw materials for manufacturing silicones. They are produced by direct reaction between silicon and methyl chloride in the Mueller Rochow synthesis. They are extremely mobile, colourless, liquids that are soluble in organic solvents and, in some cases, in anhydrous alcohol. Silanes have low molecular weights and are thus highly volatile. Industrial silicone production has its commercial basis in the direct synthesis of methylchlorosilanes from silicon and methylchloride via a process called the Mueller-Rochow synthesis. This technique, developed independently in the presence of a copper catalyst at approximately 280°C. Finely ground and well mixed Si and Cu are brought together in a fluid bed reactor with methyl chloride in gaseous form. This produces a silane mixture from which the most important organochlorosilanes are derived. Structure and bonding Silane has sp3-Hybridization.The molecular structure of the silanes directly affects their physical and chemical characteristics. It is derived from the electron configuration of silicon, which has four valence electrons. The silicon atoms in silanes are always sp3 hybridised, that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 3s orbital and the three 3p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a tetrahedron, the angle of between them. A silane molecule has only Si – H and Si – Si single bonds. The former result from the overlap of a sp³-orbital of silicon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp³- orbitals on different silicon atoms. The bond lengths amount to 146.0 pm for a Si – H bond and 233 pm for a Si – Si bond. The spatial arrangement of the bonds is similar to that of the four sp³- orbitals—they are tetrahedrally arranged, with an angle of 109.47° between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality. Uses 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 fibersto 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, applying polycrystalline silicon layers on silicon wafers when manufacturing semiconductors, and sealants. Silicon halides Silicon tetrabromide is the inorganic compound with the formula SiBr4. This colorless liquid has a suffocating odor due to its tendency to hydrolyze with release of hydrogen bromide. The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride. Synthesis Silicon tetrabromide is synthesized by the reaction of silicon with hydrogen bromide at 600 °C. Si + 4 HBr → SiBr4 + 2 H2 Side products include dibromosilane (SiH2Br2) and tribromosilane (SiHBr3). Si + 2 HBr → SiH2Br2 Si + 3 HBr → SiHBr3 + H2 Reactivity Reduction Silicon tetrabromide can be readily reduced by hydrides or complex hydrides. 4 R2AlH + SiBr4 → SiH4 + 4 R2AlBr Reactions with alcohols and amines SiBr4 + 4 ROH → Si(OR)4 + 4 HBr SiBr4 + 8 HNR2 → Si(NR2)4 + 4 HNR2HBr Reactions with Grignard reactions Grignard reactions with metal alkyl halides are particularly important reactions due to their production of organosilicon compounds which can be converted to silicones. SiBr4 + n RMgX → RnSiBr4-n + n MgXBr Uses The pyrolysis of SiBr4 does have the advantage of depositing silicon at faster rates. Pyrolysis of SiBr4 followed by treatment with ammonia yields silicon nitride (Si3N4) coatings, a hard compound used for ceramics, sealants and the production of many cutting tools. Silicates The silicates are the largest, the most interesting and the most complicated class of minerals than any other minerals. Approximately 30% of all minerals are silicates and some geologists estimate 4- that 90% of the Earth's crust is made up of silicates, SiO4 based material. Silicates is based on 4- the basic chemical unit SiO4 , tetrahedron shaped anionic group. The central silicon ion has a charge of positive four while each oxygen has a charge of negative two (-2) and thus each silicon-oxygen bond is equal to one half (½) the total bond energy of oxygen. This condition leaves the oxygens with the option of bonding to another silicon ion and therefore linking one 4- SiO4 tetrahedron to another. Classification is orthosilicates, pyrosilicates, ring and chain silicates, double chain silicates, silicates with sheet structures and silicates with 3-dimensional framework. Structure and bonding.

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