Aluminium perchlorate

Aluminium perchlorate is an inorganic chemical compound of aluminium with the formula Al₃·nH₂O, where n can range from 0 to 9. It is most commonly found as the nonahydrate Al₃·9H₂O, but other hydrates, such as the trihydrate, have also been reported. All forms of aluminium perchlorate are hygroscopic white solids that are soluble in water.

Preparation

The nonahydrate is produced by the dissolution of aluminium hydroxide in aqueous perchloric acid, followed by the evaporation of the solution:
The dehydration of the nonahydrate by heating at 82 °C produces a basic perchlorate. However, heating at 80 °C under vacuum produces the trihydrate. Further heating of the trihydrate does not form the anhydrous form; instead, it decomposes to aluminium oxide.
Anhydrous aluminium perchlorate is produced by the reaction of aluminium chloride and dichlorine hexoxide at –20 °C:
The resulting ClO₂Al₄ is then heated to 90 °C under vacuum to yield anhydrous aluminium perchlorate.

Structure

The nonahydrate, structurally ₃₃, has a trigonal crystal structure and consists of octahederal ³⁺ centers as well as perchlorate and three molecules of water of crystallization.
The anhydrous form, on the other hand, consists of bidentate perchlorate ligands with a hexagonal crystal lattice.

Reactions and complexes

Aluminium perchlorate dissolves in water to form discrete ³⁺ and ClO₄^– ions. The ³⁺ partially hydrolyses, similar to the hexaaquoiron(III) ion:
This can be suppressed by the addition of perchloric acid. Like other aqueous Al³⁺ solutions, the addition of base, such as ammonium hydroxide or sodium hydroxide results in the precipitation of aluminium hydroxide; when base is further added, the precipitate redissolves to form ^–.
The hydrates, the tri- and nonahydrate, are soluble in various organic solvents and are used to prepare multiple complexes with organic ligands, such as pyridine, dimethylacetamide, dimethyl sulfoxide, and acetonitrile. Specifically, the dimethyl sulfoxide complex, ₃ is explosive. Other aluminium perchlorate complexes tetraperchloratoaluminate and hexaperchloratoaluminate are also known.
Anhydrous aluminium perchlorate decomposes when heated to 160 °C to Al₂O₄:
The oxyperchlorate decomposes to aluminium oxide if further heated to 450 °C.