Silicon tetrabromide, also known as tetrabromosilane, is the

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

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 Sili ...

Comparison of SiX₄

The properties of the tetrasilanes, all of which are tetrahedral, are significantly affected by nature of the

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a f ...

. 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 depe ...

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 ...

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&nbs ...

of the halide. The opposite trend is observed for the Si-X

bond energies In chemistry, bond energy (''BE''), also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond. IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually a ...

Lewis acidity

Covalently saturated silicon complexes like SiBr₄, along with tetrahalides of

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbo ...

(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, ...

(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 sp ...

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 ...

, they add

Lewis basic 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 spec ...

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

ic 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 Tribromosilane is the chemical compound with the formula Br3Si. At high temperatures, it decomposes to produce silicon, and is an alternative to purified trichlorosilane of ultrapure silicon in the semiconductor industry. The Schumacher Proces ...

(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 is ...

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 cycloal ...

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 of ...

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

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

s 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 atoms have been replaced by a substituent su ...

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 Organosilicon compounds are organometallic compounds containing carbon–silicon bonds. Organosilicon chemistry is the corresponding science of their preparation and properties. Most organosilicon compounds are similar to the ordinary organic c ...

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 hydrolysis ...

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 we ...

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 ''p ...

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 nitrogeno ...

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