Aluminium chloride

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Aluminium chloride
General
Systematic name	Aluminium(III) chloride
Other names	Aluminium trichloride
Molecular formula	AlCl3
Molar mass	133.34 g mol−1 (anhydrous) 241.432 g mol−1 (hexahydrate)
Appearance	Pale yellow solid, hygroscopic.
CAS number	[7446-70-0] (anhydrous) [10124-27-3] (hexahydrate)
Properties
Density (solid)	2.44 g cm−3 (anhydrous) 2.40 g cm³ (hexahydrate)
Solubility in water	43.9 g/100 ml (0°C) 44.9 g/100 ml (10°C) 45.8 g/100 ml (20°C) 46.6 g/100 ml (30°C) 47.3 g/100 ml (40°C) 48.1 g/100 ml (60°C) 48.6 g/100 ml (80°C) 49 g/100 ml (100°C)
In ethanol In chloroform In diethyl ether In CCl4	100 g/100 ml (12.5°C) 0.072 g/100 ml (20°C) Soluble Soluble
Melting point	190 ° C (463 K) under 2.5 atm pressure
Boiling point	178 ° C (351 K) ( subl)
Acidity (pKa)	?
Structure
Molecular shape	Trigonal planar ( monomeric vapour)
Coordination geometry	Octahedral (solid) Tetrahedral (liquid)
Crystal structure	6-coordinate layer lattice
Dipole moment	? D
Hazards
MSDS	External MSDS
EU classification	Corrosive (C)
NFPA 704	anhydrous
R-phrases	R34
S-phrases	S1/2, S7/8, S28, S45
Supplementary data page
Structure & properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Aluminium fluoride Aluminium bromide Aluminium iodide
Other cations	Boron trichloride Gallium(III) chloride Indium(III) chloride Thallium(III) chloride Magnesium chloride
Related Lewis acids	Iron(III) chloride Boron trifluoride
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

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 in contact with water because of the high heat of hydration. It partially hydrolyses with H₂O, forming some hydrogen chloride and/or hydrochloric acid. Aqueous solutions of AlCl₃ are fully ionized, and thus conduct electricity well. Such solutions are found to be acidic, indicating that partial hydrolysis of the Al³⁺ ion is occurring. This can be described (simplified) as:

AlCl₃ is probably the most commonly used non-Bronsted Lewis 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 chloride, like similar compounds such as Aluminium chlorohydrate, is also commonly used as an antiperspirant.

Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapour phase.

Chemical Properties

Aluminium chloride is a powerful Lewis acid, capable of forming stable Lewis acid-base adducts with even weak Lewis bases such as benzophenone or mesitylene. Not surprisingly it forms AlCl₄⁻ in the presence of chloride ion.

In water, partial hydrolysis forms HCl gas or H₃O⁺, as described in the overview above. Aqueous solutions behave similarly to other aluminium salts containing hydrated Al³⁺ ions - for example giving a gelatinous precipitate of aluminium hydroxide upon reaction with the correct quantity of aqueous sodium hydroxide:

AlCl₃( aq) + 3NaOH( aq) → Al(OH)₃( s) + 3 NaCl( aq)

Preparation

Aluminium chloride is manufactured by the exothermic reaction of the elements, aluminium and chlorine. It is commercially available in large quantities.

Uses

The Friedel-Crafts reaction is the major use for aluminium chloride, for example in the preparation of anthraquinone (for the dyestuffs industry) from benzene and phosgene. In the general Friedel-Crafts reaction an acyl chloride or alkyl halide reacts with an aromatic system as shown:

With benzene derivatives, the major product is the para isomer. The alkylation reaction has many associated problems, such as in Friedel-Crafts, so it is less widely used than the acylation reaction. For both reactions the aluminium chloride (and other materials and the equipment) must be moderately dry, although a trace of moisture is necessary for the reaction to proceed. A general problem with the Friedel-Crafts reaction is that the aluminium chloride " catalyst" needs to be present in full stoichiometric quantities in order for the reaction to go to completion, because it complexes strongly with the products (see chemical properties above). This makes it very difficult to recycle, so it must be destroyed after use, generating a large amount of corrosive waste. For this reason chemists are examining the use of more environmentally benign catalysts such as ytterbium(III) triflate or dysprosium(III) triflate, which can be recycled.

Aluminium chloride can also be used to introduce aldehyde groups onto aromatic rings, for example via the Gatterman-Koch reaction which uses carbon monoxide, hydrogen chloride and a copper(I) chloride co- catalyst):

Aluminium chloride finds a wide variety of other applications in organic chemistry. For example, it can catalyse the " ene reaction", such as the addition of 3-buten-2-one (methyl vinyl ketone) to carvone:

AlCl₃ is also widely used for polymerization and isomerization reactions of hydrocarbons. Important examples include the manufacture of ethylbenzene, which used to make styrene and thus polystyrene, and also production of dodecylbenzene, which is used for making detergents.

Aluminum 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 so-called Fischer-Hafner synthesis.

Precautions

Avoid bringing anhydrous AlCl₃ in contact with water or bases, or an explosive reaction may result. Gloves and safety goggles should be worn, along with a face shield for larger amounts. The material should be handled in a fume cupboard or chemical hood. When handled in moist air, AlCl₃ rapidly absorbs moisture to become a highly acidic and sticky "goo", and it rapidly attacks many materials such as stainless steel and rubber.