Aluminium chloride, also known as aluminium trichloride, is an
inorganic compound
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''.
Inorgan ...
with the formula . It forms a
hexahydrate with the formula , 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.
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 ...
. It is an
inorganic compound
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''.
Inorgan ...
that
reversibly changes from a
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
to a
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
at mild temperature.
Structure

Anhydrous
adopts three structures, depending on the
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
and the
state
State most commonly refers to:
* State (polity), a centralized political organization that regulates law and society within a territory
**Sovereign state, a sovereign polity in international law, commonly referred to as a country
**Nation state, a ...
(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.
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 (
point group
In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
D
2h), 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
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an aci ...
into
trigonal planar monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
(
point group
In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
D
3h), which is structurally analogous to
. The melt
conducts electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
poorly,
unlike more
ionic halides such as
sodium chloride
Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...
.
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
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 ...
() as
counterions.
Hydrogen bond
In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
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
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
in
Friedel-Crafts alkylation and related reactions.
Uses
Alkylation and acylation of arenes
is a common Lewis-acid
catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
for
Friedel-Crafts reactions, both acylations and alkylations.
These types of reactions are the major use for aluminium chloride, for example, in the preparation of
anthraquinone
Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic hydrocarbon, aromatic organic compound with formula . Several isomers exist but these terms usually refer to 9,10-anthraquinone (IUPAC: 9,10-dioxoanthracene) wherein th ...
(used in the
dye
Juan de Guillebon, better known by his stage name DyE, is a French musician. He is known for the music video of the single "Fantasy
Fantasy is a genre of speculative fiction that involves supernatural or Magic (supernatural), magical ele ...
stuffs industry) from
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
and
phosgene
Phosgene is an organic chemical compound with the formula . It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. It can be thought of chemically as the double acyl chloride analog of ...
.
In the general Friedel-Crafts reaction, an
acyl chloride or
alkyl halide reacts with an
aromatic
In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
system as shown:
:

The
alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...
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
Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions.
Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...
quantities, because it
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
es strongly with the products. This complication sometimes generates a large amount of
corrosive
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
waste. For these and similar reasons, the use of aluminium chloride has often been displaced by
zeolite
Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
s.
Aluminium chloride can also be used to introduce
aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
groups onto
aromatic
In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
rings, for example via the
Gattermann-Koch reaction which uses
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
,
hydrogen chloride
The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...
and a
copper(I) chloride
Copper(I) chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid. Impure samples appear gr ...
co-catalyst.
:
Other applications in organic and organometallic synthesis
Aluminium chloride finds a wide variety of other applications in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
. For example, it can catalyse the
ene reaction, such as the addition of
3-buten-2-one (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
Fischer–Hafner synthesis.
Dichlorophenylphosphine is prepared by reaction of
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
and
phosphorus trichloride catalyzed by aluminium chloride.
Medical
Topical aluminum chloride hexahydrate is used for the treatment of
hyperhidrosis
Hyperhidrosis is a medical condition in which a person exhibits excessive perspiration, sweating, more than is required for the Thermoregulation, regulation of body temperature. Although it is primarily a physical burden, hyperhidrosis can deterio ...
(excessive sweating).
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 ...
, capable of forming Lewis acid-base
adduct
In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...
s with even weak
Lewis bases such as
benzophenone
Benzophenone is a naturally occurring organic compound with the formula (C6H5)2CO, generally abbreviated Ph2CO. Benzophenone has been found in some fungi, fruits and plants, including grapes. It is a white solid with a low melting point and ros ...
and
mesitylene.
It forms
tetrachloroaluminate () in the presence of
chloride
The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
ions.
Aluminium chloride reacts with
calcium
Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
and
magnesium hydrides in
tetrahydrofuran
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
forming tetrahydroaluminates.
Reactions with water
Anhydrous aluminium chloride is
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...
, having a very pronounced affinity for water. It fumes in moist air and hisses when mixed with liquid water as the ligands are displaced with 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
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...
is acidic owing to the ionization of the
aquo ligands:
:
Aqueous solutions behave similarly to other
aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
salts
In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions ( cations) and negatively charged ions (anions), which results in a compound with no net electric charge (electrically neutral). ...
containing hydrated
ions, giving a gelatinous
precipitate
In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...
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 . It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly corrosive base (chemistry), ...
:
:
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
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
metal with chlorine or
hydrogen chloride
The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...
at temperatures between .
:
:
Aluminium chloride may be formed via a
single displacement reaction between
copper(II) chloride
Copper(II) chloride, also known as cupric chloride, is an inorganic compound with the chemical formula . The monoclinic crystal system, monoclinic yellowish-brown anhydrous form slowly absorbs moisture to form the orthorhombic blue-green hydrate, ...
and aluminium.
:
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
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
. 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
The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
,
aluminium(I) chloride (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.
History
Aluminium chlorides were known in the 18th century as muriate of alumina, marine alum, argillaceous marine salt, muriated clay. It was first chemically studied in the 1830s.
Safety
Anhydrous is strongly corrosive and releases hydrochloric acid in contact with water.
See also
*
Aluminium monochloride
References
External links
International Chemical Safety Card 1125Index of Organic Synthesis procedures that utilize AlCl3MSDS
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 materials
Acid catalysts