15 julio, 2024

Aluminum chloride (AlCl3): structure, properties, uses, risks

What is aluminum chloride?

He aluminum chloride It is an inorganic chemical compound whose formula is AlCl3. It occurs as a white solid or powder, although due to contamination with ferric chloride and moisture, it may 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, a covalent bond exists 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.

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 is also used as the main catalyst in chemical reactions in industry.

Aluminum Chloride Structure

In the image above we have different representations for each of the crystalline or molecular structures exhibited by anhydrous aluminum chloride, AlCl3, in various of its physical states.

Solid AlCl3 consists of a monoclinic crystal structure made up of layers of coordinated octahedrons. In these octahedrons (brown color), the Al3+ ion is coordinated with six Cl– ions, the chlorides acting as bridges between the two octahedrons.

When the temperature is increased and the AlCl3 melts, the layers separate into Al2Cl6 units (center of image). Now, the aluminum atom forms bonds with three chlorine atoms and one bond with the other aluminum atom: Cl3Al-AlCl3. These dimers explain why molten AlCl3 is less dense than its crystals.

In the vapor phase, the Al2Cl6 dimers drift apart from each other. But if this vapor is subjected to high temperatures, the dimer dissociates into AlCl3 molecular units, in which the aluminum atom forms three bonds with the chlorine atoms. AlCl3 presents a trigonal planar geometry.

Aluminum Chloride Properties

formulas

AlCl3 (anhydrous)

AlCl3 6H2O (hexahydrate)

molar mass

133.34 g/mol (anhydrous)

241.43 g/mol (hexahydrate)

Physical appearance

White crystalline solid or powder. May turn yellowish due to ferric chloride contamination. It can also change to a grayish color from moisture.

Melting point

192.4ºC

Boiling point

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

Density

2.48 g/cm3 (anhydrous)

2.398 g/cm3 (hexahydrate)

Water solubility

439 g/L at 0ºC

458 g/L at 20ºC

466 g/L at 30ºC

Aluminum chloride is a hygroscopic compound that can absorb water from humid air, emitting smoke in 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 151ºC (0.131 atm).

Goo

0.35 cP at 197ºC

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 is a catalyst in many chemical reactions; among them, the Friedel-Crafts reaction.

Aluminum chloride is even classified as an acid, instead of a salt.

Anhydrous aluminum chloride in aqueous solution is transformed into the hexahydrate salt (AlCl3 6H2O), which does not return to the anhydrous form on heating.

The aluminum atoms of two AlCl3 molecules interact with each other to complete 8 electrons in their valence shell, resulting in the formation of the Al2Cl6 dimer. This dimer is present in the molten aluminum chloride and in the steam.

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

AlCl3 + NaOH → Al(OH)3 + 3 NaCl

Aluminum Chloride Applications

Antiperspirant and antihemorrhagic

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 bleeding, such as those that occur in dental treatment.

Sewage treatment

Aluminum chloride in the form of PAC (polyaluminum 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 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 production 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 injuries to the skin and eyes, even causing eye damage.

When dissolved in water, it can give off poisonous gases, such as hydrogen chloride; Gases capable of producing irritation to the nose and throat when inhaled, and may cause coughing and lung irritation. This irritation can lead to fluid buildup in the lungs (pulmonary edema), resulting in a medical emergency.

Attempts have been made to establish a connection between the generation of Alzheimer’s disease and aluminum chloride; although the results of the investigation are not conclusive.

Likewise, the connection between the use of deodorants that use AlCl3 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 organism and the emission of heat, such as sweat.

References

Wikipedia. (2021). Aluminum chloride. Retrieved from: en.wikipedia.org
National Center for Biotechnology Information (2021). Aluminum chloride, PubChem Compound Summary for CID 24012. Retrieved from: pubchem.ncbi.nlm.nih.gov
NJ Health. (2008). Hazardous Substance Fact Sheet: Aluminum Chloride. [PDF]. Retrieved from: nj.gov
Encyclopaedia Britannica. (2021). Aluminum chloride. Recovered from: britannica.com
Elsevier BV (2021). Anthraquinones. Science Direct. Retrieved from: sciencedirect.com
John P. Cunha. (02 May 2020). Aluminum Chloride Topical. Retrieved from: rxlist.com

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