Aluminum chloride (AlCl₃) is a compound of aluminium and chlorine. Pure samples are white, but it is often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point, and is ionicly bonded with covalent character. It sublimes at 178 °C. Molten AlCl₃ conducts electricity poorly, unlike more ionic halides such as sodium chloride. It exists in the solid state as a six-coordinate layer lattice. AlCl₃ adopts the "YCl₃" structure, featuring Al³⁺ cubic close packed layered structure. In contrast, AlBr₃ has a more molecular structure, with the Al³⁺ centers occupying adjacent tetrahedral holes of the close-packed framework of Br⁻ ions. In the melt, aluminium trichloride exists as the dimer Al₂Cl₆, with tetracoordinate aluminium. This is why the density of the liquid phase is so much less than the solid (1.78 g/cm³ vs 2.48 g/cm³. Al₂Cl₆ dimers are also found in the vapour phase. At higher temperatures this Al₂Cl₆ dimer dissociates into trigonal planar AlCl₃, which is structurally analogous to BF₃. Aluminum chloride is highly deliquescent, and can explode upon abrupt contact with water because of the high heat of hydration. Aqueous solutions of AlCl₃ are ionic and thus conduct electricity well. Such solutions are found to be acidic, indicative of partial hydrolysis of the Al³⁺ ion. The reactions can be described (simplified) as: Aluminum chloride crystallizes from water as the hexahydrate AlCl₃·6H₂O, which has been used as a topical antiperspirant. AlCl₃ is probably the most commonly used Lewis acid and also one of the most powerful. It finds widespread application in the chemical industry as the classic catalyst for Friedel-Crafts reactions, both acylations and alkylations. It also finds use in polymerization and isomerization reactions of hydrocarbons. Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapor phase.

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