# Aluminium bromide

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 solid; hence old samples tend to be hydrated, mostly as aluminium tribromide hexahydrate (AlBr$_3$·6H$_2$O).

## Structure

The dimeric form of aluminium tribromide (Al$_2$Br$_6$) predominates in the solid state, in solutions in noncoordinating solvents (e.g. CS$_2$), in the melt, and in the gas phase. Only at high temperatures do these dimers break up into monomers:
    Al$_2$Br$_6$ → 2 AlBr$_3$ 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]$_n$" → Al$_2$Br$_6$ + 4 Al
This reaction is reversed at temperatures higher than 1000 °C. Aluminium monobromide has been crystallographically characterized in the form the tetrameric adduct Al$_4$Br$_4$(NEt$_3$)$_4$ (Et = C$_2$H$_5$). This species is electronically related to cyclobutane. Theory suggests that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster Al$_4$Br$_4$, akin to the analogous boron compound.
Al$_2$Br$_6$ consists of two AlBr$_4$ tetrahedra that share a common edge. The molecular symmetry is D$_{2h}$.
The monomer AlBr$_3$, observed only in the vapor, can be described as trigonal planar, D$_{3h}$ point group. The atomic hybridization of aluminium is often described as sp$^2$. The Br-Al-Br bond angles are 120°.

## Synthesis

By far the most common form of aluminium bromide is Al$_2$Br$_6$. 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 → Al$_2$Br$_6$ + 3 H$_2$
Alternatively, the direct bromination occurs also:
    2 Al + 3 Br$_2$ → Al$_2$Br$_6$

## Reactions

Al$_2$Br$_6$ dissociates readily to give the strong Lewis acid, AlBr$_3$. Regarding the tendency of Al$_2$Br$_6$ 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, Al$_2$Br$_6$ is hydrolyzed 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 bases (L), Al$_2$Br$_6$ forms adducts, such as AlBr$_3$L.
Aluminium tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide:
    4 AlBr$_3$ + 3 CCl$_4$ → 4 AlCl$_3$ + 3 CBr$_4$
and with phosgene yields carbonyl bromide and aluminium chlorobromide:
    AlBr$_3$ + COCl$_2$ → COBr$_2$ + AlCl$_2$Br
Al$_2$Br$_6$ 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 Al$_2$Cl$_6$.

## Safety

Aluminium tribromide is a highly reactive material.