Aluminium chloride, also known as aluminium trichloride, is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with the formula . It forms hexahydrate with the formula , containing six
water molecules of hydration. Both are colourless crystals, but samples are often contaminated with
iron(III) chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
, giving a yellow color.
The anhydrous material is important commercially. It has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large amounts are also used in other areas of the chemical industry.
The compound is often cited as a
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. It is an example of an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
that reversibly changes from a
polymer to a
monomer at mild temperature.
Structure
Anhydrous
adopts three structures, depending on the
temperature and the
state (solid, liquid, gas). Solid has a sheet-like layered structure with cubic close-packed chloride ions. In this framework, the Al centres exhibit
octahedral coordination geometry.
[ In contrast, has a more molecular structure, with the centers occupying adjacent tetrahedral holes of the close-packed framework of ions. Wells, A. F. (1984) ''Structural Inorganic Chemistry'', Oxford Press, Oxford, United Kingdom. .] 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 , with
tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm
3) versus solid aluminium trichloride (2.48 g/cm
3). dimers are also found in the
vapour phase. At higher temperatures, the dimers
dissociate into
trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...
monomer, which is structurally analogous to
. The melt
conducts electricity poorly,
unlike more
ionic halide
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
s such as
sodium chloride
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
.
Aluminium chloride monomer belongs to the
point group
In geometry, a point group is a mathematical group of symmetry operations (isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every p ...
D
3h in its monomeric form and D
2h in its dimeric form.
Hexahydrate

The hexahydrate consists of
octahedral cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
centers and chloride
anions () as
counterion
160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...
s.
Hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s link the cation and anions.
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
is a common Lewis-acid
catalyst for
Friedel-Crafts reactions, both acylations and alkylations.
Important products are
detergent
A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...
s and
ethylbenzene. These types of reactions are the major use for aluminium chloride, for example, in the preparation of
anthraquinone (used in the
dye
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...
stuffs industry) from
benzene and
phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, espe ...
.
In the general Friedel-Crafts reaction, an
acyl chloride
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...
or
alkyl halide reacts with an
aromatic system as shown:
:

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. Detailed procedures are available for alkylation and acylation 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.
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.
:
Other applications in organic and organometallic synthesis
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 (MVK, IUPAC name: butenone) is the organic compound with the formula CH3C(O)CH=CH2. It is a reactive compound classified as an enone, in fact the simplest example thereof. It is a colorless, flammable, highly toxic liquid wi ...
(methyl vinyl ketone) to
carvone:
:

It is used to induce a variety of hydrocarbon couplings and rearrangements.
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.
Dichlorophenylphosphine is prepared by reaction of
benzene and
phosphorus trichloride catalyzed by aluminium chloride.
Reactions
Anhydrous aluminium chloride is a powerful
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
, capable of forming Lewis acid-base
adducts with even weak
Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
s such as
benzophenone and
mesitylene.
It forms
tetrachloroaluminate () in the presence of
chloride ions.
Aluminium chloride reacts with
calcium and
magnesium hydride
Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.
Preparation
In 1951 preparation from the elements was first reporte ...
s in
tetrahydrofuran forming tetrahydroaluminates.
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 Cl
− ligands are displaced with H
2O molecules to form the hexahydrate . 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 is acidic owing to the ionization of the
aquo ligands:
:
Aqueous solutions behave similarly to other
aluminium salts containing hydrated
ions, giving a gelatinous
precipitate of
aluminium hydroxide upon reaction with dilute
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
:
:
Synthesis
Aluminium chloride is manufactured on a large scale by the
exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
reaction of
aluminium metal with chlorine or
hydrogen chloride at temperatures between .
:
:
Aluminium chloride may be formed via a
single displacement reaction between
copper(II) chloride and aluminium metal.
:
In the US in 1993, approximately 21,000 tons were produced, not counting the amounts consumed in the production of aluminium.
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
Aluminium monochloride, or chloridoaluminium is the metal halide with the formula AlCl. Aluminum monochloride as a molecule is thermodynamically stable at high temperature and low pressure only. This compound is produced as a step in the Alcan pr ...
(AlCl), but this is very unstable and only known in the vapour phase.
Natural occurrence
Anhydrous aluminium chloride is not found as a mineral. The hexahydrate, however, is known as the rare mineral chloraluminite.
A more complex, basic and hydrated aluminium chloride mineral is
cadwaladerite.
Safety
Anhydrous reacts vigorously with
bases, so suitable precautions are required.
It can cause irritation to the eyes, skin, and the respiratory system if inhaled or on contact.
Aluminum Chloride
solvaychemicals.us
See also
* Aluminium monochloride
References
External links
International Chemical Safety Card 1125
Index of Organic Synthesis procedures that utilize AlCl3
MSDS
Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts
{{Authority control
Chlorides
Metal halides
Aluminium compounds
Inorganic compounds
Reagents for organic chemistry
Deliquescent substances
Acid catalysts