Aluminium bromide
Encyclopedia
Aluminium bromide is any chemical compound with the empirical formula AlBrx. The species called "aluminium tribromide," is the most common aluminium bromide. The species aluminium monobromide forms from the reaction of HBr with Al metal at high temperature. It disproportionate
s near room temperature:
This reaction is reversed at temperatures > 1000 °C.
Aluminium monobromide has been crystallographically characterized in the form the tetrameric adduct Al4Br4(NEt3)4 (Et = C2H5). This species is electronically related to cyclobutane. Theory suggest that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster
Al4Br4, akin to the analogous boron compound.
Al2Br6 consists of two AlBr4 tetrahedra that share a common edge. The molecular symmetry
is D2h.
The monomer AlBr3, observed only in the vapor, can be described as trigonal planar
, D3h point group. The atomic hybridization of aluminium
is often described as sp2. The Br
-Al
-Br
bond angles are 120 °.
Alternatively, the direct bromination occurs also:
, AlBr3. Regarding the tendency of Al2Br6 to dimerize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter main group halide
s such as boron tribromide
do not show this tendency, in part due to the smaller size of the central atom.
Consistent with its Lewis acidic character, Al2Br6 reacts vigorously with water with evolution of HBr and formation of Al-OH-Br species. Similarly, it also reacts quickly with alcohols and carboxylic acids, although less vigorously than with water. With simple Lewis bases (L), Al2Br6 forms adduct
s, such as AlBr3L.
Aluminium tribromide reacts with carbon tetrachloride
at 100 °C to form carbon tetrabromide:
and with phosgene
yields carbonyl bromide
and aluminum chlorobromide:
Al2Br6 is an excellent reagent for Friedel-Crafts
and related Lewis acid-promoted reactions such as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acidic than the more common Al2Cl6
. Because it is hygroscopic, older samples tend to be hydrated and less useful. Impure samples can be purified by vacuum sublimation.
. Its catalytic activity is similar to anhydrous aluminum chloride. However, commercial applications are rare.
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
s near room temperature:
- 6/n "[AlBr]n" → Al2Br6 + 4 Al
This reaction is reversed at temperatures > 1000 °C.
Structure
"Aluminium tribromide" is really dialuminium hexabromide with the molecular formula of Al2Br6 in the solid state, solutions in noncoordinating solvents (e.g. CS2), and in the melt. Even upon evaporation, Al2Br6 exists in the gas phase. At high temperatures, the gaseous molecules break up into monomers:- Al2Br6 → 2 AlBr3 ΔH°diss = 59 kJ/mol
Aluminium monobromide has been crystallographically characterized in the form the tetrameric adduct Al4Br4(NEt3)4 (Et = C2H5). This species is electronically related to cyclobutane. Theory suggest that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster
Cluster (physics)
In physics, the term clusters denotes small, multiatom particles. As a rule of thumb, any particle of somewhere between 3 and 3x107 atoms is considered a cluster. Two-atom particles are sometimes considered clusters as well....
Al4Br4, akin to the analogous boron compound.
Al2Br6 consists of two AlBr4 tetrahedra that share a common edge. The molecular symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
is D2h.
The monomer AlBr3, observed only in the vapor, can be described as trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...
, D3h point group. The atomic hybridization of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
is often described as sp2. The Br
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
-Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
-Br
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
bond angles are 120 °.
Synthesis
By far the most common form of aluminium bromide is Al2Br6. This species exists as hygroscopic colorless solid at standard conditions. Typical impure samples are yellowish or even red-brown due to the presence of iron-containing impurities. It is prepared by the reaction of HBr with Al:- 2 Al + 6 HBrHydrogen bromideHydrogen 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...
→ Al2Br6 + 3 H2
Alternatively, the direct bromination occurs also:
- 2 Al + 3 Br2BromineBromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
→ Al2Br6
Reactions
Al2Br6 dissociates readily to give the strong Lewis acidLewis 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,...
, AlBr3. Regarding the tendency of Al2Br6 to dimerize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter main group halide
Halide
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...
s such as boron tribromide
Boron tribromide
Boron tribromide, BBr3, is a colorless, fuming liquid compound containing boron and bromine. It is usually made by heating boron trioxide with carbon in the presence of bromine: this generates free boron which reacts vigorously with the bromine...
do not show this tendency, in part due to the smaller size of the central atom.
Consistent with its Lewis acidic character, Al2Br6 reacts vigorously with water with evolution of HBr and formation of Al-OH-Br species. Similarly, it also reacts quickly with alcohols and carboxylic acids, although less vigorously than with water. With simple Lewis bases (L), Al2Br6 forms 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, such as AlBr3L.
Aluminium tribromide reacts with carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
at 100 °C to form carbon tetrabromide:
- 4 AlBr3 + 3 CCl4 → 4 AlCl3 + 3 CBr4
and with phosgene
Phosgene
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles...
yields carbonyl bromide
Carbonyl bromide
Carbonyl bromide, also known as bromophosgene by analogy to phosgene, is an organic chemical compound. It is a decomposition product of halon compounds used in fire extinguishers.-Reactions:...
and aluminum chlorobromide:
- AlBr3 + COCl2 → COBr2 + AlCl2Br
Al2Br6 is an excellent reagent for Friedel-Crafts
Friedel-Crafts reaction
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...
and related Lewis acid-promoted reactions such as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acidic than the more common Al2Cl6
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
. Because it is hygroscopic, older samples tend to be hydrated and less useful. Impure samples can be purified by vacuum sublimation.
Uses
The anhydrous form is used as a catalyst for the Friedel-Crafts alkylation reactionFriedel-Crafts reaction
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...
. Its catalytic activity is similar to anhydrous aluminum chloride. However, commercial applications are rare.