Aluminum chloride

At first glance, aluminum chloride appears to be an inorganic salt with the empirical formula AlCl₃; however, aluminum, like other group III elements, tends to form covalent bonds with halides. The energetically preferred coordination number of four causes AlCl₃ to form the dimer Al₂Cl₆ shown, even in the vapor phase. AlCl₃ is perhaps the quintessential Lewis acid; it reacts violently with water to form strongly acidic solutions. It has been used for decades in industry, most notably as a catalyst in Friedel–Crafts alkylation and acylation reactions, as well as in dozens of other processes. Its hexahydrate is the active ingredient in many antiperspirants.