Everything about Aluminium Chloride totally explained
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Aluminium chloride (AlCl
3) 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 AlCl
3 conducts electricity poorly, unlike more
ionic halides such as
sodium chloride. It exists in the solid state as a six-coordinate layer lattice.
AlCl
3 adopts the "YCl
3" structure, featuring Al
3+ cubic close packed layered structure. In contrast, AlBr
3 has a more molecular structure, with the Al
3+ centers occupying adjacent tetrahedral holes of the close-packed framework of Br
− ions. Upon melting AlCl
3 gives the
dimer Al
2Cl
6, which can
vaporise. At higher
temperatures this Al
2Cl
6 dimer
dissociates into trigonal planar AlCl
3, 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 AlCl
3 are ionic and thus
conduct electricity well. Such solutions are found to be
acidic, indicative of partial
hydrolysis of the Al
3+ ion. The reactions can be described (simplified) as:
» [Al(H
2O)
6]
3+ + H
2O ⇌ [Al(OH)(H
2O)
5]
2+ + H
3O
+
AlCl
3 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. Not surprisingly it forms AlCl
4− in the presence of
chloride ions.
In water, partial hydrolysis forms
HCl gas or H
3O
+, as described in the overview above.
Aqueous solutions behave similarly to other
aluminium salts containing hydrated Al
3+ ions, giving a gelatinous
precipitate of
aluminium hydroxide upon reaction with the correct quantity of aqueous
sodium hydroxide:
» AlCl
3(
aq) + 3 NaOH(aq) → Al(OH)
3(
s) + 3
NaCl(aq)
Preparation
Aluminium chloride is manufactured on a large scale by the
exothermic reaction of aluminium metal with chlorine or hydrogen chloride.
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:
AlCl
3 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, for example
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 and Driclor.
Precautions
Anhydrous AlCl
3 reacts vigorously with
water and
bases, so suitable precautions are required. Hydrated salts are less problematic.
Further Information
Get more info on 'Aluminium Chloride'.
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