Silicon tetrabromide, also known as tetrabromosilane, 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.Encyclopedia of Inorganic Chemistry; King, B. R.; John Wiley & Sons Ltd.: New York, NY, 1994; Vol 7, pp 3779–3782. The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride.
Comparison of SiX4 The properties of the tetrasilanes, all of which are tetrahedral, are significantly affected by nature of the halide. These trends apply also to the mixed halides. Melting points, boiling points, and bond lengths increase with the atomic mass of the halide. The opposite trend is observed for the Si-X bond energies. SiH4 SiF4 SiCl4 SiBr4 SiI4 b.p. (˚C)Silicon Compounds, Silicon Halides. Collins, W.: Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001. −111.9 −90.3 56.8 155.0 290.0 m.p. (˚C) −185 −95.0 −68.8 5.0 155.0 Si-X bond length (Å) 1.55 2.02 2.20 2.43 Si-X bond energy (kJ/mol)Ebsworth, E. A. V. In Volatile Silicon Compounds; Taube, H.; Maddock, A. G.; Inorganic Chemistry; Pergamon Press Book: New York, NY, 1963; Vol. 4. 384 582 391 310 234
Lewis acidity Covalently saturated silicon complexes like SiBr4, along with tetrahalides of germanium (Ge) and tin (Sn), are Lewis acids.Davydova, E. I.; Timoshkin, A. Y.; Sevastianova, T. N.; Suvorov, A. V.; Frenking, G. J. Mol. Struct. 2006, vol, 767-1-3. Although silicon tetrahalides obey the octet rule, they add Lewis basic ligands to give adducts with the formula SiBr4L and SiBr4L2 (where L is a Lewis base).Beattie, I. R.; Gilson, T.; Webster, M.; (in part) McQuillan, G. P. J. Chem. Soc. 1964, 238-244. Mironov, S. L.; Gorlov, Y. I.; Chuiko, A. A. Theor. Exp. Chem. 1979, vol, 14–16. Beattie, I. R.; Ozin, G. A. J. Chem. Soc., Inorg. Phys. Theor. 1969, 2267–2269 The Lewis acidic properties of the tetrahalides tend to increase as follows: SiI4 < SiBr4 < SiCl4 < SiF4. This trend is attributed to the relative electronegativities of the halogens.
The strength of the Si-X bonds decrease in the order: Si-F > Si-Cl > Si-Br > Si-I.
Synthesis Silicon tetrabromide is synthesized by the reaction of silicon with hydrogen bromide at 600 °C.Schumb, W. B. Silicobromoform Inorganic Syntheses 1939, volume 1, pp 38-42. . Si + 4 HBr → SiBr4 + 2 H2 Side products include dibromosilane (SiH2Br2) and tribromosilane (SiHBr3). Si + 2 HBr → SiH2Br2 Si + 3 HBr → SiHBr3 + H2
It can also be produced by treating silicon-copper mixture with bromine:
Reactivity Like other halosilanes, SiBr4 can be converted to hydrides, alkoxides, amides, and alkyls, i.e., products with the following functional groups: Si-H, Si-OR, Si-NR2, Si-R, and Si-X bonds respectively.
Silicon tetrabromide can be readily reduced by hydrides or complex hydrides. 4 R2AlH + SiBr4 → SiH4 + 4 R2AlBr
Reactions with alcohols and amines proceed as follows: SiBr4 + 4 ROH → Si(OR)4 + 4 HBr
SiBr4 + 8 HNR2 → Si(NR2)4 + 4 HNR2HBr 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
Redistribution reactions occur between two different silicon tetrahalides (as well as halogenated polysilanes) when heated to 100 ˚C, resulting in various mixed halosilanes. The melting points and boiling points of these mixed halosilanes generally increase as their molecular weights increase.Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press Inc.: New York, NY, 1984; pp 391-393. (Can occur with X= H, F, Cl, Br, and I) 2 SiBr4 + 2 SiCl4 → SiBr3Cl + 2 SiBr2Cl2 + SiBrCl3 Si2Cl6 + Si2Br6 → Si2ClnBr6−n
Silicon tetrabromide hydrolyzes readily when exposed to air causing it to fume:Silicon Compounds, Silanes. Arkles, B.; Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001. SiBr4 + 2 H2O → SiO2 + 4 HBr
Silicon tetrabromide is stable in the presence of oxygen at room temperature, but bromosiloxanes form at 670–695 ˚C . 2 SiBr4 + 1⁄2 O2 → Br3SiOSiBr3 + Br2
Uses Due to its close similarity to silicon tetrachloride, there are few applications unique to SiBr4. The pyrolysis of SiBr4 does have the advantage of depositing silicon at faster rates than that of SiCl4, however SiCl4 is usually preferred due to its availability in high purity.Silicon Compounds, Inorganic. Simmler W.; Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH, 2002. 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.
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