Silicon tetrabromide
Encyclopedia
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
. The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride
.
. These trends apply also to the mixed halides. Melting points, boiling points
, and bond length
s increase with the atomic mass
of the halide. The opposite trend is observed for the Si-X bond energies
.
(Ge) and tin
(Sn), are Lewis acid
s. Although silicon tetrahalides obey the octet rule
, they add Lewis basic ligands to give adduct
s with the formula SiBr4L and SiBr4L2 (where L= Lewis base). The Lewis acid
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.
at 600 °C.
Side products include dibromosilane (SiH2Br2) and tribromosilane
(SiHBr3).
s, amide
s, 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 hydride
s or complex hydrides.
Reactions with alcohol
s and amine
s proceeed as follows:
Grignard reaction
s with metal alkyl halides are particularly important reactions due to their production of organosilicon
compounds which can be converted to silicone
s.
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. (Can occur with X= H, F, Cl, Br, and I)
Silicon tetrabromide hydrolyzes
readily when exposed to air causing it to fume:
Silicon tetrabromide is stable in the presence of oxygen
at room temperature, but bromosiloxanes form at 670-695 ˚C .
of SiBr4 does have the advantage of depositing silicon at faster rates than does SiCl4, however SiCl4 is usually preferred due to its availability in high purity. 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.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula SiBr4. This colorless liquid has a suffocating odor due to its tendency to hydrolyze
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
with release of hydrogen bromide
Hydrogen bromide
Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and...
. The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride
Silicon tetrachloride
Silicon tetrachloride 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:...
.
Comparison of SiX4
The properties of the tetrahalosilanes, all of which are tetrahedral, are significantly effected by nature of the halideHalide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
. These trends apply also to the mixed halides. Melting points, boiling points
Boiling Points
Boiling Points is a prank reality television show, much like the format used on Candid Camera. It is broadcast on MTV in the United States. In each half-hour episode, annoying situations are set up and deliberately inflicted on one or more young adults who are unaware that they are being tested...
, and bond length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
s increase with the atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
of the halide. The opposite trend is observed for the Si-X bond energies
Bond energy
In chemistry, bond energy is the measure of bond strength in a chemical bond. It is the heat required to break one Mole of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E is the enthalpy change involved with breaking up one molecule of methane into...
.
| SiF4 | SiCl4 | SiBr4 | SiI4 | |
|---|---|---|---|---|
| b.p. (˚C) | -90.3 | 56.8 | 155.0 | 290.0 |
| m.p. (˚C) | -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) | 582 | 391 | 310 | 234 |
Lewis acidity
Covalently saturated silicon complexes like SiBr4, along with tetrahalides of germaniumGermanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
(Ge) and tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
(Sn), are Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s. Although silicon tetrahalides obey the octet rule
Octet rule
The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
, they add Lewis basic ligands to give adduct
Adduct
An adduct 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 considered a distinct molecular species...
s with the formula SiBr4L and SiBr4L2 (where L= Lewis base). The Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
ic properties of the tetrahalides tend to increase as follows: SiI4 < SiBr4 < SiCl4 < SiF4
Silicon tetrafluoride
Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...
. This trend is attributed to the relative electronegativities
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
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 bromideHydrogen bromide
Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and...
at 600 °C.
- Si + 4 HBr → SiBr4 + 2 H2
Side products include dibromosilane (SiH2Br2) and tribromosilane
Tribromosilane
Tribromosilane is a chemical compound containing silicon, hydrogen, and bromine. At high temperatures, it decomposes to produce silicon, and is an alternative to purified trichlorosilane of ultrapure silicon in the semiconductor industry....
(SiHBr3).
- Si + 2 HBr → SiH2Br2
- Si + 3 HBr → SiHBr3 + H2
Reactivity
Like other halosilanes, SiBr4 can be converted to hydrides, alkoxideAlkoxide
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...
s, amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
s, 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 hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
s or complex hydrides.
- 4 R2AlH + SiBr4 → SiH4 + 4 R2AlBr
Reactions with alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s and amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s proceeed as follows:
- SiBr4 + 4 ROH → Si(OR)4 + 4 HBr
- SiBr4 + 8 HNR2 → Si(NR2)4 + 4 HNR2HBr
Grignard reaction
Grignard reaction
The Grignard reaction is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon–carbon bonds...
s with metal alkyl halides are particularly important reactions due to their production of organosilicon
Organosilicon
Organosilicon compounds are organic compounds containing carbon silicon bonds. Organosilicon chemistry is the corresponding science exploring their properties and reactivity.Like carbon, the organically bound silicon is tetravalent and tetrahedral...
compounds which can be converted to silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
s.
- 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
Molecular mass
The molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
increase. (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
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
readily when exposed to air causing it to fume:
- SiBr4 + 2 H2O → SiO2 + 4 HBr
Silicon tetrabromide is stable in the presence of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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 pyrolysisPyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of SiBr4 does have the advantage of depositing silicon at faster rates than does SiCl4, however SiCl4 is usually preferred due to its availability in high purity. Pyrolysis of SiBr4 followed by treatment with ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
yields silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
(Si3N4) coatings, a hard compound used for ceramics, sealants, and the production of many cutting tools.