Silicon tetrabromide

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Silicon tetrabromide
Stereo structural formula of silicon tetrabromide Space fill model of silicon tetrabromide
Ball and stick model of silicon tetrabromide
Identifiers
CAS number 7789-66-4 YesY
PubChem 82247
ChemSpider 74225 YesY
EC number 232-182-4
UN number 3264
Jmol-3D images Image 1
Properties
Molecular formula Br4Si
Molar mass 347.70 g mol−1
Appearance Colorless liquid
Density 2.79 g·cm−3
Melting point 5 °C (41 °F; 278 K)
Boiling point 153 °C (307 °F; 426 K)
Refractive index (nD) 1.5685
Hazards
EU classification Corrosive C
R-phrases R34
S-phrases S26, S27, S28, S36/37/39
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Related compounds
Related tetrahalosilanes Silicon tetrachloride
Silicon tetrafluoride
Silicon tetraiodide
Related compounds Platinum(IV) bromide
Tellurium tetrabromide
Tetrabromomethane
Tin(IV) bromide
Titanium tetrabromide
Zirconium(IV) bromide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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.[1] The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride.[1]

Comparison of SiX4[edit]

The properties of the tetrahalosilanes, 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.

SiF4 SiCl4 SiBr4 SiI4
b.p. (˚C)[2] -90.3 56.8 155.0 290.0
m.p. (˚C)[2] -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)[3] 582 391 310 234

Lewis acidity[edit]

Covalently saturated silicon complexes like SiBr4, along with tetrahalides of germanium (Ge) and tin (Sn), are Lewis acids.[4] 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).[5][6][7] 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.[6][3]

The strength of the Si-X bonds decrease in the order: Si-F > Si-Cl > Si-Br > Si-I.[3][2]

Synthesis[edit]

Silicon tetrabromide is synthesized by the reaction of silicon with hydrogen bromide at 600 °C.[8]

Si + 4 HBr → SiBr4 + 2 H2

Side products include dibromosilane (SiH2Br2) and tribromosilane (SiHBr3).[8]

Si + 2 HBr → SiH2Br2
Si + 3 HBr → SiHBr3 + H2

Reactivity[edit]

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.[1]

Silicon tetrabromide can be readily reduced by hydrides or complex hydrides.[2]

4 R2AlH + SiBr4 → SiH4 + 4 R2AlBr

Reactions with alcohols and amines proceeed as follows:[2]

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.[2]

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.[1][3] The melting points and boiling points of these mixed halosilanes generally increase as their molecular weights increase.[9] (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:[10]

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 .[10]

2 SiBr4 + 1⁄2 O2 → Br3SiOSiBr3 + Br2

Uses[edit]

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.[11] 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.[11]

References[edit]

  1. ^ a b c d Encyclopedia of Inorganic Chemistry; King, B. R.; John Wiley & Sons Ltd.: New York, NY, 1994; Vol 7, pp 3779–3782.
  2. ^ a b c d e f Silicon Compounds, Silicon Halides. Collins, W.: Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001.
  3. ^ a b c d Ebsworth, E. A. V. In Volatile Silicon Compounds; Taube, H.; Maddock, A. G.; Inorganic Chemistry; Pergamon Press Book: New York, NY, 1963; Vol. 4.
  4. ^ Davydova, E. I.; Timoshkin, A. Y.; Sevastianova, T. N.; Suvorov, A. V.; Frenking, G. J. Mol. Struct. 2006, vol, 767-1-3. doi:10.1016/j.theochem.2006.05.011
  5. ^ Beattie, I. R.; Gilson, T.; Webster, M.; (in part) McQuillan, G. P. J. Chem. Soc. 1964, 238-244. doi:10.1039/JR9640000238
  6. ^ a b Mironov, S. L.; Gorlov, Y. I.; Chuiko, A. A. Theor. Exp. Chem. 1979, vol, 14–16. doi:10.1007/BF00519073
  7. ^ Beattie, I. R.; Ozin, G. A. J. Chem. Soc., Inorg. Phys. Theor. 1969, 2267–2269
  8. ^ a b Schumb, W. B. Silicobromoform" Inorganic Syntheses 1939, volume 1, pp 38-42. doi:10.1002/9780470132326.
  9. ^ Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press Inc.: New York, NY, 1984; pp 391-393.
  10. ^ a b Silicon Compounds, Silanes. Arkles, B.; Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc, 2001.
  11. ^ a b Silicon Compounds, Inorganic. Simmler W.; Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH, 2002. doi:10.1002/14356007.a24_001