Aluminium bromide

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.

Theory suggests that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster Al4Br4, akin to the analogous boron compound.

It is prepared by the reaction of HBr with Al: Alternatively, the direct bromination occurs also: 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.

Ball and stick model of dimeric aluminium bromide
Ball and stick model of dimeric aluminium bromide
Aluminium bromide from 1967 In large ampoules
Aluminium bromide from 1967 In large ampoules
NFPA 704 four-colored diamond Health 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gas Flammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oil Instability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calcium Special hazards (white): no code
Experiment showing synthesis of aluminium bromide from the elements.
A demonstration of the reaction of the exothermic reaction of the strong Lewis acid (Al 2 Br 6 ) and strong Lewis base (H 2 O).