Aluminium chloride

IUPAC name	aluminium trichloride trichloroalumane trichloridoaluminium
Other names	aluminum trichloride
Identifiers
CAS number	[7446-70-0], 10124-27-3 (hexahydrate)
RTECS	BD0530000
PubChem	24012
Properties
Chemical formula	AlCl3
Molar mass	133.34 g/mol (anhydrous) 241.43 g/mol (hexahydrate)
Appearance	white or pale yellow solid, hygroscopic
Density	2.48 g/cm3 (anhydrous) 1.3 g/cm3 (hexahydrate)
Melting point	192.4 °C *anhydrous) 0 °C (hexahydrate)
Boiling point	120 °C (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)
Solubility	soluble in hydrogen chloride, ethanol, chloroform, carbon tetrachloride slightly soluble in benzene
Structure
Crystal structure	Monoclinic, mS16
Space group	C12/m1, No. 12
Coordination geometry	Octahedral (solid) Tetrahedral (liquid)
Molecular geometry	Trigonal planar (monomeric vapour)
Hazards
EU classification	Corrosive (C)
R-phrases	Template:R34
S-phrases	Template:S1/2, Template:S7/8, Template:S28, Template:S45
LD50	anhydrous: 380 mg/kg, rat (oral) hexahydrate: 3311 mg/kg, rat (oral)
Related compounds
Other anions	Aluminium fluoride Aluminium bromide Aluminium iodide
Other cations	Boron trichloride Gallium trichloride Indium(III) chloride Thallium(III) chloride Magnesium chloride
Other 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)

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 can explode upon abrupt contact with water because of the high heat of hydration. Aqueous solutions of AlCl₃ are ionic and thus conduct electricity well. Such solutions are found to be acidic, indicative of partial hydrolysis of the Al³⁺ ion. The reactions can be described (simplified) as:

[Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(OH)(H₂O)₅]²⁺ + H₃O⁺

AlCl₃ is probably the most commonly used Lewis acid and also one of the most powerful. It finds widespread application in the chemical industry as the classic catalyst for Friedel-Crafts reactions, both acylations and alkylations. It also finds use in polymerization and isomerization reactions of hydrocarbons. Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapor 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 ions.

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, giving a gelatinous precipitate of aluminium hydroxide upon reaction with the correct quantity of aqueous sodium hydroxide:

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

Preparation

Aluminium chloride is manufactured on a large scale by the exothermic reaction of aluminium metal with chlorine or hydrogen chloride at temperatures between 650 to 750 °C.

2 Al + 3 Cl₂ → 2 AlCl₃

2 Al + 6 HCl → 2 AlCl₃ + 3 H₂

Hydrated forms are prepared by dissolving aluminium oxides with dry hydrochloric acid at 150°C.

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, as well as 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 Gattermann-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.

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 so-called Fischer-Hafner synthesis.

Aluminium chloride, often in the form of derivatives such as aluminium chlorohydrate, is a common component in antiperspirants at low concentrations. Hyperhidrosis sufferers need a much higher concentration (15% or higher), sold under such brand names as Drysol, Maxim, Odaban, CertainDri, B+Drier, Anhydrol Forte and Driclor.

Chemical Reactions

Aluminium chloride reacts with calcium and magnesium hydrides in tetrahydrofuran forming tetrahydro aluminates:

AlCl₃ + 3 LiH → AlH₃ + 3 LiCl

The hexahydrate decomposes to aluminum oxide when heated at 300 °C:

2 AlCl₃·6H₂O → 2 Al₂O₃ + 6 HCl + 9 H₂O

Precautions

Anhydrous AlCl₃ reacts vigorously with water and bases, so suitable precautions are required. Hydrated salts are less problematic.

External links

• Aluminum Chloride Information
• International Chemical Safety Card 1125
• Index of Organic Synthesis procedures that utilize AlCl₃
• The period 3 chlorides
• MSDS