Aluminium tribromide, AlBr₃

Aluminium tribromide, AlBr₃, is most readily prepared by adding aluminium carefully, in small quantities at a time, to bromine, the reaction being extremely vigorous. Other methods of preparation are to pass bromine vapour over heated aluminium or an intimate mixture of alumina and carbon heated to redness. The product should be rectified over metallic aluminium.

Aluminium tribromide forms colourless, transparent rhombohedra of density 2.54. It melts at 93° and boils at 263.3° under 747 mm. pressure (Mallet); its critical temperature is 772° C. At 444° and 1 atmosphere the vapour density is 18.62, the formula Al₂Br₆ corresponding to the value 18.42. The specific heat of aluminium bromide (22° to 76°) is 0.08912, and the latent heat of fusion 10.47 cals. per gram.; as a cryoscopic solvent the molecular depression of the freezing-point is 262. The bromide is a non-electrolyte, but dissolves numerous halogen salts with the production of conducting solutions. Aluminium bromide dissolves in alcohol and many organic liquids. The molecular weight corresponds to the double formula Al₂Br₆ in carbon disulphide and in bromine, but in nitrobenzene it corresponds to the single formula AlBr₃. From a solution of aluminium bromide in ethyl bromide, aluminium can be deposited by electrolysis, but the deposited metal readily attacks the solvent, butane being produced.

Aluminium bromide is decomposed when heated in oxygen. It is extremely deliquescent and dissolves in water with the evolution of much heat. The aqueous solution, which may be prepared from aluminium or its hydroxide and hydrobromic acid, reacts acid and becomes basic when boiled. The hexahydrate AlBr₃.6H₂O can be prepared by methods corresponding to those given for the hexahydrate of aluminium chloride; a dilute aqueous solution when cooled is said to deposit the hydrate 2AlBr₃.15H₂O.

Aluminium bromide forms double salts with the alkali bromides, of which AlBr₃.KBr and AlBr₃.NaBr have been described. With the tribromides of antimony and bismuth it forms solid solutions. It combines with hydrogen sulphide, forming a white, crystalline compound, AlBr₃.H₂S, which melts at 84°; and with phosphorus oxychloride, forming a compound of the molecular formula Al₂Br₆.2POCl₃ in carbon disulphide solution. The compounds AlBr₃.2Br₂.CS₂ and 2AlBr₃.2Br₂.CS₂ have also been described. Like the chloride, it combines with many organic compounds. The following are the molecular formula in carbon disulphide solution, of a few such compounds: - Al₂Br₆.2C₆H₅SO₂Cl, Al₂Br₆.2(C₆H₅)₂CO, Al₂Br₆.2C₆H₅COCl, Al₂Br₆.2C₆H₅NO₂.