Iron(III) chloride is the chemical compound with formula FeCl3. It is produced industrially both from iron and from ore in a closed-loop process. It exists in two forms: the hydrated hexahydrate and anhydrous iron(III) chloride. Both are brown crystalline solids. The hexahydrate salt is very soluble in water, alcohol and ethanol; however, strong heating converts it to the anhydrous form, which decomposes to iron oxychloride gas and hydrogen chloride. Both forms are toxic and corrosive. They are used extensively in chemistry as coagulants for sewage and industrial wastes, for the production of dyes, pigments, and inks; as a chlorinating agent; and as a catalyst for a number of reactions like Friedel-Crafts acylation or alkylation of aromatic rings.
The reactivity of iron(III) chloride reflects its description as a moderately strong Lewis acid, with an electron configuration characterized by the fact that it is oxophilic. Hence, it readily binds oxygen ligands to give oxalate salts. It also is a mild oxidizing agent; for example, it reacts with copper to give copper(II) chloride and iron(III) chloride.
Its reactivity with phenols is exploited in a traditional colorimetric test for phenols, the ferric chloride test. It is also a popular printed circuit board etchant. The hexahydrate crystal has a semicovalent layer structure with each iron atom surrounded octahedrally by six chlorines in a biI3 type structure. The anhydrous form of the compound has a molecular moment at room temperature of 5-73 B.M. The hydrate has a slightly higher moment.