Silicon tetrabromide, also known as tetrabromosilane, is the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

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 length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...

s increase with the atomic mass of the halide. The opposite trend is observed for the Si-X bond energies.

Lewis acidity

Covalently saturated silicon complexes like SiBr₄, along with tetrahalides of

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

(Ge) and tin (Sn), are

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

s.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 The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ru ...

, they add Lewis basic ligands to give

adduct In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...

s 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

ic 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,

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

s, and

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

s, 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

hydride In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all che ...

s or complex hydrides. :4 R₂AlH + SiBr₄ → SiH₄ + 4 R₂AlBr Reactions with

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

s and

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s proceed as follows: :SiBr₄ + 4 ROH → Si(OR)₄ + 4 HBr :SiBr₄ + 8 HNR₂ → Si(NR₂)₄ + 4 HNR₂HBr Grignard reactions with metal

halides are particularly important reactions due to their production of organosilicon compounds which can be converted to

silicone In Organosilicon chemistry, organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = Organyl group, organic group). They are typically colorless oils or elastomer, rubber ...

s. :SiBr₄ + ''n'' RMgX → R_''n''SiBr_4−''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 → Si₂Cl_''n''Br_6−''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 Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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 Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...

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 Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

yields silicon nitride (Si₃N₄) coatings, a hard compound used for ceramics, sealants, and the production of many cutting tools.

References