Aluminium bromide

Structure

The dimeric form of aluminium tribromide (Al₂Br₆) predominates in the solid state, solutions in noncoordinating solvents (e.g. CS₂), in the melt, and 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 disproportionates near room temperature:

6/n "[AlBr]_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 Al₄Br₄, akin to the analogous boron compound.

Al₂Br₆ consists of two AlBr₄ tetrahedra that share a common edge. The molecular symmetry is D_2h.

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 °.

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

Reactions

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 empirical 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, water hydrolizes Al₂Br₆ 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.

Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

and with phosgene yields carbonyl bromide and aluminium chlorobromide:

AlBr₃ + COCl₂ → COBr₂ + AlCl₂Br

Al₂Br₆ is used as a catalyst 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.