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What is aluminum chloride?

The aluminum chloride is a chemical compound inorganic whose formula is AlCl 3 . It appears as a white solid or powder, although due to contamination with ferric chloride and humidity, it can turn yellow. It consists of a Lewis acid, but under certain circumstances it also behaves like a Lewis base.

Despite aluminum being classified as a metal on the periodic table, there is a covalent bond between aluminum and chlorine to form aluminum chloride.

This covalent bond could explain why molten aluminum chloride is a poor conductor of electricity; while molten salts of other compounds, such as sodium chloride, are good conductors of electricity.

Aluminum chloride is produced by an exothermic reaction between aluminum and chlorine, or hydrogen chloride, at a temperature between 650 and 750 ºC.

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Aluminum chloride is used as an antiperspirant and to combat excessive sweating. One of its derivatives is used as a coagulating agent in the treatment of industrial wastewater, and it is also used as the main catalyst in chemical reactions in industry.

Structure of aluminum chloride

In the upper image we have different representations for each of the crystalline or molecular structures that anhydrous aluminum chloride, AlCl 3 , exhibits in several of its physical states.

Solid AlCl 3 consists of a monoclinic crystalline structure formed by layers of coordinated octahedra. In these octahedra (brown color), the Al 3+ ion is coordinated with six Cl  ions , the chlorides acting as bridges between two octahedra.

As the temperature increases and AlCl 3 melts , the layers separate into Al 2 Cl 6 units (center of image). Now, the aluminum atom forms bonds with three chlorine atoms and a bond with the other aluminum atom: Cl 3 Al-AlCl 3 . These dimers explain why molten AlCl 3 is less dense than its crystals.

In the vapor phase, the Al 2 Cl 6 dimers distance themselves from each other. But if this vapor is subjected to high temperatures, the dimer dissociates into AlCl 3 molecular units , in which the aluminum atom forms three bonds with the chlorine atoms. AlCl 3 has a trigonal plane geometry.

Solid sample of aluminum chloride with yellowish hues. Source: Xlollitox, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Formulas

AlCl 3 (anhydrous)

AlCl 3 · 6H 2 O (hexahydrate)

Molar mass

133.34 g / mol (anhydrous)

241.43 g / mol (hexahydrate)

Physical appearance

White crystalline solid or powder. May turn yellowish from ferric chloride contamination. It can also turn gray from moisture.

Melting point

192.4 ºC

Boiling point

It sublimates at 178 ºC, so it is not easy to obtain the boiling point of aluminum chloride.

Density

2.48 g / cm 3 (anhydrous)

2.398 g / cm 3 (hexahydrate)

Water solubility

439 g / L at 0 ºC

458 g / L at 20 ºC

466 g / L at 30 ºC

Aluminum chloride is a hygroscopic compound that can absorb water from humid air, emitting smoke during the process.

Solubility in other solvents

Soluble in ethanol, chloroform, carbon tetrachloride, benzophenone and nitrobenzene. Slightly soluble in benzene.

Vapor pressure

13.3 kPa at 151 ° C (0.131 atm).

Viscosity

0.35 cP at 197 ºC

Reactivity

Aluminum chloride is an amphoteric substance, that is, it behaves like an acid or a base. However, its action as an acid is predominant, since aluminum chloride is used as a Lewis acid and at the same time it is a catalyst in many chemical reactions; among them, the Friedel-Crafts reaction.

Aluminum chloride is even classified as an acid, rather than a salt.

The anhydrous aluminum chloride in aqueous solution is transformed into the hexahydrate salt (AlCl 3 · 6H 2 O), which does not return to the anhydrous form on heating.

The aluminum atoms of two AlCl 3 molecules interact with each other to complete 8 electrons in their valence shell, which results in the formation of the Al 2 Cl 6 dimer . This dimer is present in molten aluminum chloride and in vapor.

Aluminum chloride reacts with sodium hydroxide to form aluminum hydroxide, a gelatinous precipitate:

AlCl 3    + NaOH → Al (OH) 3     + 3 NaCl

Aluminum Chloride Applications

Antiperspirant and anti-hemorrhagic

Aluminum chloride is one of the ingredients used in the formulations of various deodorants. Source: Th145, CC BY-SA 2.0 DE <https://creativecommons.org/licenses/by-sa/2.0/de/deed.en>, via Wikimedia Commons

Aluminum chloride is used as an antiperspirant in personal deodorants, and is also used to control excessive sweating (hyperhidrosis). It is also used in formulations to control small bleeds, such as those that occur in dental treatments.

Sewage treatment

Aluminum chloride in the form of PAC (polyaluminium chloride) is used as a coagulant in the treatment of industrial wastewater, due to its ability to clarify water and dehydrate sludge.

Catalyst

Aluminum chloride is one of the main catalysts used in industry, acting as a catalyst in numerous reactions, including that of Friedel-Crafts.

Aluminum chloride catalyzes the reactions for the preparation of anthraquinones, some of which are used in the textile industry. There are also anthraquinones that have antibacterial, antiparasitic, fungicidal, and antiviral properties.

Aluminum chloride catalyzes the reactions that produce dodecylbenzene, used in the production of detergents, and ethylbenzene, a compound used in the manufacture of polystyrene, a polymer used in the manufacture of plastics, resins and rubber.

Aluminum chloride is also used as a catalyst in a reaction that is part of the production of high-octane gasoline. It also catalyzes the oil cracking process .

Risks

Aluminum chloride is a corrosive substance and its contact can cause irritation and injury to the skin and eyes, leading to eye damage.

When dissolved in water, it can release poisonous gases, such as hydrogen chloride; gases capable of producing by inhalation irritation of the nose and throat, which may cause coughing and lung irritation. This irritation can lead to fluid build-up in the lungs (pulmonary edema), leading to a medical emergency.

An attempt has been made to establish a connection between the generation of Alzheimer’s disease and aluminum chloride; although the research results are not conclusive.

Likewise, the connection between the use of deodorants that use AlCl 3 as an antiperspirant and breast cancer has been investigated . Research in this regard has not been able to demonstrate a conclusive relationship. In any case, aluminum chloride blocks the functioning of a natural detoxification system of the body and the emission of heat, such as sweat.

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