Aluminium trichloride, AlCl₃

Aluminium trichloride, AlCl₃, was originally made by heating an intimate mixture of alumina and carbon to redness in a stream of chlorine (Oersted's method). It may be more readily prepared by heating aluminium in a wide glass tube in a rapid current of dry hydrogen chloride, or in a stream of chlorine. If it is required to prepare the chloride from the oxide, a neater method than Oersted's is to heat the oxide in a current of chlorine and sulphur chloride: -

4Al₂O₃ + 3S₂Cl₂ + 9Cl₂ = 8AlCl₃ + 6SO₂.

Instead of chlorine and sulphur chloride, carbon tetrachloride vapour or carbonyl chloride may be used. A simple method of preparation is said to consist in heating crude alumina or clay to redness in a current of hydrogen chloride and carbon disulphide vapour, and purifying the aluminium chloride so obtained by sublimation over iron filings.

Vapour density of aluminium chloride. Results by Dumas' method shown x, results by Victor Meyer's method shown.

Aluminium chloride, purified by sublimation over aluminium, forms white, lustrous, six-sided plates which are said by Seubert and Pollard to possess rhombic symmetry. The slightly impure chloride is usually yellow owing to the presence of a little ferric chloride. When slowly heated, aluminium chloride sublimes completely, but when a mass of the chloride is rapidly heated it melts. The sublimation pressure of the chloride reaches one atmosphere at 179° to 183°; the melting-point at 2.5 atmospheres pressure is 190° to 194°. The critical temperature is 629.5° C. The vapour density of aluminium chloride has been the subject of numerous researches. The results that have been obtained at atmospheric pressure and different temperatures (method of Dumas) are shown by crosses in fig. The asterisks in the same figure represent the values obtained by the method of Victor Meyer, in which the partial pressure of the aluminium chloride was less than one atmosphere, but had no definite value throughout a series of experiments. The theoretical value for Al₂Cl₆ is 9.2 (air = l), and for AlCl₃, 4.6. It will be seen that from 200° to 400° the molecular formula is Al₂Cl₆; above 400° the dissociation Al₂Cl₆ ⇔ 2AlCl₃ becomes marked; and at and above 800° the dissociation is complete. The specific heat of aluminium chloride is 0.188.

Aluminium chloride dissolves in alcohol and many organic liquids. In pyridine, ether, and in nitrobenzene the molecular weight corresponds with the simple formula AlCl₃.

Aluminium chloride is extremely deliquescent and fumes in the air. It dissolves readily in water with the evolution of much heat. An aqueous solution of aluminium chloride is readily obtained by dissolving aluminium or its hydroxide in aqueous hydrochloric acid. A solution containing 40 parts of aluminium chloride to 100 of water has a density of 1.3415 at 15°. The aqueous solution has an acid reaction and on prolonged boiling loses hydrochloric acid and becomes turbid. According to Baud, a number of definite basic chlorides exist.

The hexahydrate, AlCl₃.6H₂O, separates out when an aqueous solution of aluminium chloride is either slowly evaporated or saturated with hydrogen chloride. The hydrate forms deliquescent rhombohedral crystals (a:c = 1:0.5356) of prismatic habit and does not lose water in a sulphuric acid desiccator. Dilute solutions, cooled to -8°, are said to deposit the ennea-hydrate AlCl₃.9H₂O.

Aluminium chloride enters into the composition of a large number of double compounds, many of which have been studied by Baud. The following compounds with metallic chlorides are known: 2AlCl₃.2NH₄Cl; 2AlCl₃.6KCl; 2AlCl₃.2KCl; 2AlCl₃.2AgCl; 2AlCl₃.2NaCl; 2AlCl₃.BaCl₂; 2AlCl₃.3NaCl; 2AlCl₃.1½SrCl₂; 2AlCl₃.3KCl; 2AlCl₃.l½CaCl₂; 2 AlCl₃.6NaCl; 2AlCl₃.1½ZnCl₂.

The compound AlCl₃.NaCl was formerly manufactured for use in the preparation of aluminium. It is not so hygroscopic as aluminium chloride, melts at 185°, and volatilises at a red heat. The compounds AlCl₃.KCl and AlCl₃.NH₄Cl are similar.

The compound AlCl₃.SCl₄ has been prepared by Ruff and Plato, double compounds with selenium and tellurium tetrachlorides by Weber, and with phosphorus pentachloride and oxychloride by Casselmann.

Anhydrous aluminium chloride rapidly absorbs dry ammonia. At low temperatures the compound AlCl₃.9NH₃ is formed, the dissociation-point of which is - 14'6°. It passes into the compound AlCl₃.6NH₃, which is the product formed at the ordinary temperature and pressure. At 150° this passes into AlCl₃.5NH₃; at 275° this becomes AlCl₃.3NH₃; and when this last compound is distilled in hydrogen, a compound of the remarkable composition 6AlCl₃.7NH₃ is produced which may be repeatedly distilled in hydrogen without decomposition. The compounds AlCl₃.XCl.6NH₃ (where X = NH₄, Na, or K) are also known. Aluminium chloride also combines with phosphine.

The compound AlCl₃.H₂S can only be obtained by the use of liquid hydrogen sulphide. It dissociates at -45° into hydrogen sulphide and the compound 2AlCl₃.H₂S, which is stable at the ordinary temperature and pressure. The compound AlCl₃.SO₂, prepared by subliming aluminium chloride in a current of sulphur dioxide, dissociates at 80°, giving rise to the compound 2AlCl₃.SO₂, which can be distilled at 200°.

With carbonyl chloride three compounds are formed, 2AlCl₃.5COCl₂, melting at - 2°, 2AlCl₃.3COCl₂, and 4AlCl₃.COCl₂. The last two compounds are said to occur in commercial aluminium chloride.

The behaviour of the preceding double compounds towards water is such as would be expected from the behaviour of the constituent compounds.

Aluminium chloride combines with many organic compounds, such as acid chlorides, ketones, esters, nitro-compounds, and tertiary amines. Examples of such compounds are AlCl₃.(C₂H₅)₂O, AlCl₃.C₆H₅.CO₂.C₂H₅, AlCl₃.NO₂.C₆H₄.CH₃, Al₂Cl₆.2C₆H₅NO₂, the last compound having a molecular weight in carbon disulphide solution corresponding to the formula here given. In organic chemistry, anhydrous aluminium chloride is a very valuable catalytic agent, as, for example, in the well-known Friedel and Crafts' syntheses.