Aluminium bromide is any chemical compound with the empirical formula AlBr_x. Aluminium tribromide is the most common form of aluminium bromide. It is a colorless, sublimable

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...

solid; hence old samples tend to be hydrated, mostly as aluminium tribromide hexahydrate (AlBr₃·6H₂O).

Structure

The dimeric form of aluminium tribromide (Al₂Br₆) predominates in the solid state, in solutions in noncoordinating solvents (e.g. CS₂), in the melt, and in the gas phase. Only at high temperatures do these dimers break up into monomers: : Al₂Br₆ → 2 AlBr₃ ΔH°_diss = 59 kJ/mol The species aluminium monobromide forms from the reaction of HBr with Al metal at high temperature. It

disproportionate In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...

s near room temperature: :6/n " lBrsub>n" → Al₂Br₆ + 4 Al This reaction is reversed at temperatures higher than 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 may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...

Al₄Br₄, akin to the analogous boron compound. Al₂Br₆ consists of two AlBr₄ tetrahedra that share a common edge. The

molecular symmetry Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain m ...

is D_2h. The monomer AlBr₃, observed only in the vapor, can be described as

trigonal planar In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands ...

, D_3h point group. The atomic hybridization of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

is often described as ''sp²''. The Br- Al- Br bond angles are 120 °.

Synthesis

By far the most common form of aluminium bromide is Al₂Br₆. This species exists as

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 HBr → Al₂Br₆ + 3 H₂ Alternatively, the direct bromination occurs also: :2 Al + 3 Br₂ → Al₂Br₆

Reactions

Al₂Br₆ dissociates readily to give the strong

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 ...

, AlBr₃. Regarding the tendency of Al₂Br₆ to

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

ize, it is common for heavier main group halides to exist as aggregates larger than implied by their empirical formulae. Lighter

main group In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as ...

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 flu ...

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 is

hydrolyzed 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 ...

by 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 base 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 (L), Al₂Br₆ forms

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, such as AlBr₃L. Aluminium tribromide reacts with

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...

at 100 °C to form carbon tetrabromide: : 4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄ and with

phosgene Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, esp ...

yields carbonyl bromide and aluminium chlorobromide: : AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br Al₂Br₆ is used as a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

for the Friedel-Crafts alkylation reaction. Related Lewis acid-promoted reactions include as epoxide ring openings and decomplexation of dienes from iron carbonyls. It is a stronger Lewis acid than the more common Al₂Cl₆.

Safety

Aluminium tribromide is a highly reactive material.

References

{{Authority control Bromides Aluminium compounds Metal halides Acid catalysts