Aluminium chloride
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Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula Template:Chem2. It forms a hexahydrate with the formula Template:Chem2, containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are colourless crystals, but samples are often contaminated with iron(III) chloride, giving them a yellow colour.
The anhydrous form is commercially important. It has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium, but large amounts are also used in other areas of the chemical industry.<ref name="Ull">Template:Ullmann</ref> The compound is often cited as a Lewis acid. It is an inorganic compound that reversibly changes from a polymer to a monomer at mild temperature.
Structure
Anhydrous
Template:Chem2 adopts three structures, depending on the temperature and the state (solid, liquid, gas). Solid Template:Chem2 has a sheet-like layered structure with cubic close-packed chloride ions. In this framework, the Al centres exhibit octahedral coordination geometry.<ref name="Wells">Template:Cite book</ref> Yttrium(III) chloride adopts the same structure, as do a range of other compounds. When aluminium trichloride is in its melted state, it exists as the dimer (Template:Chem2 point group D2h), with tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm3) versus solid aluminium trichloride (2.48 g/cm3). Template:Chem2 dimers are also found in the vapour phase. At higher temperatures, the Template:Chem2 dimers dissociate into trigonal planar Template:Chem2 monomer (point group D3h), which is structurally analogous to [[boron trifluoride|Template:Chem2]]. The melt conducts electricity poorly,<ref name="Greenwood">Template:Greenwood&Earnshaw1st</ref> unlike more ionic halides such as sodium chloride.
Hexahydrate
The hexahydrate consists of octahedral Template:Chem2 cation centers and chloride anions (Template:Chem2) as counterions. Hydrogen bonds link the cation and anions.<ref>Template:Cite journal </ref> The hydrated form of aluminium chloride has an octahedral molecular geometry, with the central aluminium ion surrounded by six water ligand molecules. Being coordinatively saturated, the hydrate is of little value as a catalyst in Friedel-Crafts alkylation and related reactions.
Uses
Alkylation and acylation of arenes
Template:Chem2 is a common Lewis-acid catalyst for Friedel-Crafts reactions, both acylations and alkylations.<ref name="Friedel"/> These types of reactions are the major use for aluminium chloride, for example, in the preparation of anthraquinone (used in the dyestuffs industry) from benzene and phosgene.<ref name="Greenwood"/> In the general Friedel-Crafts reaction, an acyl chloride or alkyl halide reacts with an aromatic system as shown:<ref name="Friedel"/>
The alkylation reaction is more widely used than the acylation reaction, although its practice is more technically demanding. For both reactions, the aluminium chloride, as well as other materials and the equipment, should be dry, although a trace of moisture is necessary for the reaction to proceed.<ref>Template:Cite journal</ref> Detailed procedures are available for alkylation<ref>Template:Cite journal</ref> and acylation<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> of arenes.
A general problem with the Friedel-Crafts reaction is that the aluminium chloride catalyst sometimes is required in full stoichiometric quantities, because it complexes strongly with the products. This complication sometimes generates a large amount of corrosive waste. For these and similar reasons, the use of aluminium chloride has often been displaced by zeolites.<ref name="Ull" />
Aluminium chloride can also be used to introduce aldehyde groups onto aromatic rings, for example via the Gattermann-Koch reaction which uses carbon monoxide, hydrogen chloride and a copper(I) chloride co-catalyst.<ref>Template:Cite book</ref>
Other applications in organic and organometallic synthesis
Aluminium chloride finds a wide variety of other applications in organic chemistry.<ref>Template:Cite book</ref> For example, it can catalyse the ene reaction, such as the addition of 3-buten-2-one (methyl vinyl ketone) to carvone:<ref>Template:Cite journal</ref>
It is used to induce a variety of hydrocarbon couplings and rearrangements.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Aluminium chloride combined with aluminium in the presence of an arene can be used to synthesize bis(arene) metal complexes, e.g. bis(benzene)chromium, from certain metal halides via the Fischer–Hafner synthesis. Dichlorophenylphosphine is prepared by reaction of benzene and phosphorus trichloride catalyzed by aluminium chloride.<ref>Template:Cite journal</ref>
Medical
Topical aluminum chloride hexahydrate is used for the treatment of hyperhidrosis (excessive sweating).<ref name="pmid30215934">Template:Cite journal</ref><ref name="pmid30710603">Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Reactions
Anhydrous aluminium chloride is a powerful Lewis acid, capable of forming Lewis acid-base adducts with even weak Lewis bases such as benzophenone and mesitylene.<ref name="Friedel">Template:Cite book</ref> It forms tetrachloroaluminate (Template:Chem2) in the presence of chloride ions.
Aluminium chloride reacts with calcium and magnesium hydrides in tetrahydrofuran forming tetrahydroaluminates.Template:Citation needed
Reactions with water
Anhydrous aluminium chloride is hygroscopic, having a very pronounced affinity for water. It fumes in moist air and hisses when mixed with liquid water as the Template:Chem2 ligands are displaced with Template:Chem2 molecules to form the hexahydrate Template:Chem2. The anhydrous phase cannot be regained on heating the hexahydrate. Instead HCl is lost leaving aluminium hydroxide or alumina (aluminium oxide):
Like metal aquo complexes, aqueous Template:Chem2 is acidic owing to the ionization of the aquo ligands:
Aqueous solutions behave similarly to other aluminium salts containing hydrated Template:Chem2 ions, giving a gelatinous precipitate of aluminium hydroxide upon reaction with dilute sodium hydroxide:
Synthesis
Aluminium chloride is manufactured on a large scale by the exothermic reaction of aluminium metal with chlorine or hydrogen chloride at temperatures between Template:Convert.<ref name="Greenwood"/>
Aluminium chloride may be formed via a single displacement reaction between copper(II) chloride and aluminium.
In the US in 1993, approximately 21,000 tons were produced, not counting the amounts consumed in the production of aluminium.<ref name="Ull" />
Hydrated aluminium trichloride is prepared by dissolving aluminium oxides in hydrochloric acid. Metallic aluminium also readily dissolves in hydrochloric acid ─ releasing hydrogen gas and generating considerable heat. Heating this solid does not produce anhydrous aluminium trichloride, the hexahydrate decomposes to aluminium hydroxide when heated:
Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapour phase.<ref name="Greenwood"/>
Natural occurrence
Anhydrous aluminium chloride is not found as a mineral. The hexahydrate, however, is known as the rare mineral chloraluminite.<ref name="ima-mineralogy.org">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A more complex, basic and hydrated aluminium chloride mineral is cadwaladerite.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="ima-mineralogy.org" />
History
Aluminium chlorides were known in the 18th century as muriate of alumina, marine alum, argillaceous marine salt,<ref>Template:Cite book</ref> muriated clay.<ref>Template:Cite book</ref> It was first chemically studied in the 1830s.<ref>Template:Cite book</ref>
Safety
Anhydrous Template:Chem2 is strongly corrosive and releases hydrochloric acid in contact with water.<ref name="Ull"/>
See also
References
External links
- International Chemical Safety Card 1125
- Index of Organic Synthesis procedures that utilize AlCl3
- The period 3 chlorides
- MSDS Template:Webarchive
- Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts
Template:Aluminium compounds Template:Chlorides Template:Authority control