Aluminium chloride (AlCl3) is a compound of aluminium and chlorine. The solid has a low melting and boiling point, and is covalently bonded. 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. Upon melting AlCl3 gives the dimer Al2Cl6, which can vaporise. At higher temperatures this Al2Cl6 dimer dissociates into trigonal planar AlCl3, which is structurally analogous to BF3.
Aluminium chloride is highly deliquescent, and it 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 is occurring. The reactions can be described (simplified) as:
AlCl3 is probably the most commonly used non-Brønsted-Lowry acid and also one of the most powerful. It finds widespread application in the chemical industry as a 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, aluminum(I) chloride (AlCl), but this is very unstable and only known in the vapour phase.