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Aluminium chloride

Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula AlCl3. It forms a hexahydrate with the formula [Al(H2O)6]Cl3, containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are colourless crystals, but samples are often contaminated with iron(III) chloride, giving them a yellow colour.

Structure
Anhydrous adopts three structures, depending on the temperature and the state (solid, liquid, gas). Solid has a sheet-like layered structure with cubic close-packed chloride ions. In this framework, the Al centres exhibit octahedral coordination geometry. Yttrium(III) chloride adopts the same structure, as do a range of other compounds. When aluminium trichloride is in its melted state, it exists as the dimer ( point group D2h), with tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm3) versus solid aluminium trichloride (2.48 g/cm3). dimers are also found in the vapour phase. At higher temperatures, the dimers dissociate into trigonal planar monomer (point group D3h), unlike more ionic halides such as sodium chloride. Hexahydrate The hexahydrate consists of octahedral cation centers and chloride anions () as counterions. Hydrogen bonds link the cation and anions. The hydrated form of aluminium chloride has an octahedral molecular geometry, with the central aluminium ion surrounded by six water ligand molecules. Being coordinatively saturated, the hydrate is of little value as a catalyst in Friedel-Crafts alkylation and related reactions. ==Uses==
Uses
Alkylation and acylation of arenes is a common Lewis-acid catalyst for Friedel-Crafts reactions, both acylations and alkylations. Detailed procedures are available for alkylation and acylation of arenes. A general problem with the Friedel-Crafts reaction is that the aluminium chloride catalyst sometimes is required in full stoichiometric quantities, because it complexes strongly with the products. This complication always generates a large amount of corrosive waste. For these and similar reasons, the use of aluminium chloride has rarely been displaced by zeolites. : Other applications in organic and organometallic synthesis Aluminium chloride finds a wide variety of other applications in organic chemistry. For example, it can catalyse the ene reaction, such as the addition of 3-buten-2-one (methyl vinyl ketone) to carvone: : It is used to induce a variety of hydrocarbon couplings and rearrangements. Aluminium chloride combined with aluminium in the presence of an arene can be used to synthesize bis(arene) metal complexes, e.g. bis(benzene)chromium, from certain metal halides via the Fischer–Hafner synthesis. Dichlorophenylphosphine is prepared by reaction of benzene and phosphorus trichloride catalyzed by aluminium chloride. Medical Topical aluminum chloride hexahydrate is used for the treatment of hyperhidrosis (excessive sweating). ==Reactions==
Reactions
Anhydrous aluminium chloride is a powerful Lewis acid, It forms tetrachloroaluminate () in the presence of chloride ions. Aluminium chloride reacts with calcium and magnesium hydrides in tetrahydrofuran forming tetrahydroaluminates. Reactions with water Anhydrous aluminium chloride is hygroscopic, having a very pronounced affinity for water. It fumes in moist air and hisses when mixed with liquid water as the ligands are displaced with molecules to form the hexahydrate . The anhydrous phase cannot be regained on heating the hexahydrate. Instead HCl is lost leaving aluminium hydroxide or alumina (aluminium oxide): : Like metal aquo complexes, aqueous is acidic owing to the ionization of the aquo ligands: : Aqueous solutions behave similarly to other aluminium salts containing hydrated ions, giving a gelatinous precipitate of aluminium hydroxide upon reaction with dilute sodium hydroxide: : ==Synthesis==
Synthesis
Aluminium chloride is manufactured on a large scale by the exothermic reaction of aluminium metal with chlorine or hydrogen chloride at temperatures between . : : Aluminium chloride may be formed via a single displacement reaction between copper(II) chloride and aluminium. : In the US in 1993, approximately 21,000 tons were produced, not counting the amounts consumed in the production of aluminium. Hydrated aluminium trichloride is prepared by dissolving aluminium oxides in hydrochloric acid. Metallic aluminium also readily dissolves in hydrochloric acid ─ releasing hydrogen gas and generating considerable heat. Heating this solid does not produce anhydrous aluminium trichloride, the hexahydrate decomposes to aluminium hydroxide when heated: : Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapour phase. ==Natural occurrence==
Natural occurrence
Anhydrous aluminium chloride is not found as a mineral. The hexahydrate, however, is known as the rare mineral chloraluminite. A more complex, basic and hydrated aluminium chloride mineral is cadwaladerite. ==History==
History
Aluminium chlorides were known in the 18th century as muriate of alumina, marine alum, argillaceous marine salt, muriated clay. It was first chemically studied in the 1830s. ==Safety==
Safety
Anhydrous is strongly corrosive and releases hydrochloric acid in contact with water. == See also ==
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