Aluminium chloride

This article is about the trichloride, AlCl₃, and its hexahydrate. For the unstable monochloride, AlCl, see Aluminium monochloride.

Aluminium trichloride (anhydrous) IUPAC name aluminium trichloride trichloroalumane trichloridoaluminium Other names aluminum trichloride Identifiers InChI InChI=1/Al.3ClH/h;3*1H/q+3;;;/p-3 InChIKey VSCWAEJMTAWNJL-DFZHHIFOAR Standard InChI InChI=1S/Al.3ClH/h;3*1H/q+3;;;/p-3 Standard InChIKey VSCWAEJMTAWNJL-UHFFFAOYSA-K CAS number [7446-70-0] EC number 231-208-1 RTECS BD0530000 ChemSpider 22445 PubChem 24012 Properties Chemical formula AlCl3 Molar mass 133.34 g/mol (anhydrous) Appearance white or pale yellow solid, very hygroscopic Density 2.44 g/cm3 (25 ºC), solid 1.31 g/cm3 (200 ºC), liquid Melting point 190 °C at 2.5 atm Boiling point 177.8 °C subl. Solubility in water reacts violently Solubility in ethanol 100 g/100 ml Solubility in chloroform 0.072 g/100 ml Solubility in benzene slightly soluble Solubility soluble in carbon tetrachloride, diethyl ether Structure Crystal structure Monoclinic, mS16 Space group C12/m1, No. 12 Coordination geometry Octahedral (Al3+) Hazards Material safety data sheet (MSDS) ICSC 1125 EU index number 013-003-00-7 GHS pictograms GHS signal word DANGER GHS hazard statements H314 Related compounds Other anions Aluminium fluoride Aluminium bromide Aluminium iodide Other cations Boron trichloride Gallium(III) chloride Indium(III) chloride Thallium(III) 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 trichloride hexahydrate IUPAC name aluminium trichloride—water (1/6) Identifiers InChI InChI=1/Al.3ClH.6H2O/h;3*1H;6*1H2/q+3;;;;;;;;;/p-3 InChIKey JGDITNMASUZKPW-DFZHHIFOAF Standard InChI InChI=1S/Al.3ClH.6H2O/h;3*1H;6*1H2/q+3;;;;;;;;;/p-3 Standard InChIKey JGDITNMASUZKPW-UHFFFAOYSA-K CAS number [7784-13-6] EC number 231-208-1 RTECS BD0530000 ATC code D10AX01 ChemSpider 22970 PubChem 24012 Properties Chemical formula AlCl3·6H2O Molar mass 241.43 g/mol Appearance colourless or white solid, hygroscopic Density 2.398 g/cm3 Melting point 100 °C decomp. Solubility in water 45.8 g/100 ml (20 °C) 48.6 g/100 ml (80 °C) Solubility in ethanol 50 g/100 ml Solubility in diethyl ether soluble Solubility in hydrochloric acid slightly soluble Refractive index (nD) 1.6 Hazards GHS pictograms GHS signal word DANGER GHS hazard statements H280, H290, H302, H314 GHS precautionary statements P234, P260, P264, P270, P280, P301+312, P301+330+331, P303+361+353, P304+340, P305+351+338, P310, P321, P363, P405 Flash point non-flammable 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.

AlCl₃ adopts the YCl₃ structure in the solid, featuring a cubic close packed array of chloride ions with Al³⁺ ions occupying one-third of the octahedral holes, forming a layered structure. 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 hydrogen chloride 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 + 3NaOH → Al(OH)₃ + 3NaCl

Preparation

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

2Al + 3Cl₂ → 2AlCl₃

2Al + 6HCl → 2AlCl₃ + 3H₂

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, 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 tetrahydroaluminates:

AlCl₃ + 3LiH → AlH₃ + 3LiCl

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

2AlCl₃·6H₂O → Al₂O₃ + 6HCl + 3H₂O

Precautions

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

External links

• International Chemical Safety Card 1125
• NIOSH Pocket Guide to Chemical Hazards 0024
• IPCS Environmental Health Criteria 194: Aluminium
• NLM Hazardous Substances Data Bank entry for Aluminum chloride
• Preparations from Organic Syntheses in which aluminium chloride appears
• Aluminum chloride Information
• The period 3 chlorides