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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

with release of

hydrogen bromide Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room tempe ...

.''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 Silicon tetrachloride or tetrachlorosilane is the inorganic compound with the formula SiCl4. It is a colourless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications. Preparation Silic ...

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 The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...

boiling points The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...

, and

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...

s increase with the

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nb ...

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 (Ge) and

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

(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 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 rul ...

, they add Lewis basic ligands to give

adduct An adduct (from the Latin ''adductus'', "drawn toward" 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 co ...

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 Lewis acidic properties of the tetrahalides tend to increase as follows: SiI₄ < SiBr₄ < SiCl₄ < SiF₄. This trend is attributed to the relative

electronegativities Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

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

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

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 compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...

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 is derived from a cycloa ...

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−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...

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

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen Hydrogen is the chemical element wi ...

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

Grignard reaction The Grignard reaction () is an organometallic chemical reaction in which alkyl, allyl, vinyl, or aryl-magnesium halides (Grignard reagent) is added to a carbonyl group in an aldehyde or ketone. This reaction is important for the formation of ...

s with metal

halides are particularly important reactions due to their production of organosilicon compounds which can be converted to silicones. :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 The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

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 the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

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 The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

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 compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

yields

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...

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

References