Aluminium bromide

Aluminium bromide is any chemical compound with the empirical formula AlBr_x. 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 disproportionates near room temperature:

6/n "[AlBr]_n" → Al₂Br₆ + 4 Al

This reaction is reversed at temperatures > 1000 °C.

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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 HBr → Al₂Br₆ + 3 H₂

Alternatively, the direct bromination occurs also:

2 Al + 3 Br₂ → Al₂Br₆

Structure

"Aluminium tribromide" is really dialuminium hexabromide with the molecular formula of Al₂Br₆ in the solid state, solutions in noncoordinating solvents (e.g. CS₂), 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

Representative reactions of Al₂Br₆

Al₂Br₆ dissociates readily to give the strong Lewis acid, 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 empircal formulae. Lighter main group halides 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 adducts, such as AlBr₃L.

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.

Structures of the aluminium bromides

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 C_2v.

The monomer AlBr₃, observed only in the vapor, can be described as trigonal planar, D_3h point group. The atomic hybridization of aluminium is often described as sp². The Br-Al-Br bond angles are 120 °.

Safety precautions

Al₂Br₆ is corrosive and can cause skin burns if not washed off immediately after contact. It should be stored in a dry area.

References

1. “Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions” Dohmeier, C.; Loos, D.; Schnöckel, H. Angewandte Chemie International Edition in English (1996) Volume 35, Pages 129 - 149.
2. Gugelchuk, M. "Aluminum Bromide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
3. Armour, M.A., Hazardous laboratory chemicals : disposal guide. 2ème. Boca Raton, FL. : CRC Press. (1996). [RT-435017]