Aluminium chloride, commonly written as AlCl3, is a well known chemical used in both labs and industries. It’s mainly used as a catalyst in making perfumes, dyes, medicines, and many organic compounds. AlCl3 plays an important role in various chemical reactions, especially in organic chemistry. It exists in two forms anhydrous and hydrated and both are useful in different ways. In this article, you’ll learn about the structure, preparation methods, properties, chemical reactions, and uses of aluminium chloride. Scroll down to explore everything you need to know.

Define Aluminum Chloride?

Aluminium chloride, also known as aluminium trichloride or aluminium (III) chloride, is a chemical compound formed when aluminium and chlorine react. Its chemical formula is AlCl3. Aluminium chloride generally appears as a white substance, however, it can take on a yellowish hue due to the presence of contaminants, such as iron(III) chloride.

Aluminium chloride finds widespread use in industrial settings, especially in the production of aluminium metal and as a Lewis acid in the chemical industry. Solid aluminium chloride (AlCl3) forms a covalent bond and has low melting and boiling points.

The compound was first discovered in 1825 by a Danish physicist and chemist named Hans Christian Oersted. Aluminium chloride has since become one of the most commonly used chemicals, particularly in the field of organic chemistry. We will delve into the details of this compound in the following sections.

Here are some basic details about Aluminium Chloride that you should know, including its formula, weight, melting point, and more. 

How is Aluminium Chloride Prepared?

Aluminium chloride can be produced through an exothermic reaction of aluminium and chlorine. However, it can also be obtained through other methods such as reacting aluminium metal with hydrogen chloride or through a single displacement reaction between copper chloride and aluminium metal. Below are the reactions for each method:

• 2Al + 3Cl2 → 2AlCl3
• 2Al + 6HCl → 2AlCl3 + H2
• 2Al + 3CuCl2 → 2AlCl3 + 3Cu

What is the Structure of Aluminium Chloride? 

Aluminum chloride changes its structure depending on its state and temperature. In the solid state, it has a layered cubic structure where each aluminum atom is surrounded by six chloride ions in an octahedral shape. When melted, it forms a dimer (Al2Cl6) with tetrahedral geometry. At high temperatures, the dimer breaks into single AlCl3 units with a trigonal planar shape.

Aluminium Chloride Properties

Let's explore the physical and chemical properties of aluminium chloride.

Physical Properties of AlCl3

• Aluminium chloride has a very low melting and boiling point.
• It sublimes at a temperature of 180°C.
• AlCl3 is a poor conductor of electricity in its molten state.
• Although its natural color is white, it can take on a yellowish hue when contaminated with iron trichloride.
• Aluminium chloride is in a liquid state only at pressures above 2.5 atm and temperatures above 190°C.

Chemical Properties of AlCl3

• Aluminium chloride is a powerful Lewis acid.
• It serves as a major industrial catalyst.
• AlCl3 is an anhydrous, non-explosive, non-flammable, but corrosive solid.
• It reacts violently when it comes into contact with water or bases.

Aluminium Chloride Reactions

AlCl3 exhibits a variety of reactions with different compounds. Anhydrous aluminium chloride is a powerful Lewis acid, meaning it can form Lewis acid-base adducts with even weak bases, such as mesitylene and benzophenone. Some common reactions include:

• When chloride ions are present, AlCl3 can form tetrachloroaluminate (AlCl4–).
• Aluminium chloride can react with magnesium and calcium hydride in tetrahydrofuran to form tetrahydridoaluminate.

Aluminium Chloride Reaction with Water

Aluminium chloride is hygroscopic, meaning it can absorb moisture from the air. It often fumes in moist air and hisses when it comes into contact with water. During the reaction, the Cl– ions are replaced with H2O molecules to form the hexahydrate [Al(H2O)6]Cl3. The anhydrous state of AlCl3 is lost, and when heat is applied, HCl also dissipates, leaving aluminium hydroxide as the final product.

Al(H2O)6Cl3 → Al(OH)3 + 3HCl + 3H2O

\(Al(H2O)6Cl3 → Al(OH)3 + 3HCl + 3H2O\)

When the temperature is further increased to about 400°C, aluminium oxide is formed from the hydroxide.

\(2Al(OH)3 → Al2O3 + 3H2O\)

One distinct characteristic of AlCl3 aqueous solutions is that they are ionic. Due to this, they are good conductors of electricity. They are also acidic, which can lead to partial hydrolysis in Al3+ ions. The reaction can be written as:

[Al(H2O)6]3+(aq) ⇌ [Al(OH)(H2O)5]2+(aq) + H+(aq)

Aluminium salts that contain hydrated Al3+ ions are similar to aqueous solutions of aluminium chloride. They also behave similarly. For example, it gives a thick precipitate of Al(OH)3 when it is reacted with dilute sodium hydroxide.

AlCl3 + 3NaOH → Al(OH)3 + 3NaCl

Uses of Aluminium Chloride (AlCl3)

Aluminium Chloride is a versatile chemical compound and finds use in a variety of areas, especially in chemical reactions and synthesis. Below are some notable uses of aluminium chloride.

1. AlCl3 is primarily used as a catalyst for various chemical reactions. It is extensively used in Friedel-Crafts reactions, including both acylations and alkylations. It is used in the preparation of anthraquinone from phosgene and benzene.
2. Aluminium chloride can be used to introduce or attach aldehyde groups onto aromatic series or rings. For instance, in the Gatterman-Koch reaction, the Lewis acid (aluminium chloride) is used to remove chloride ions from the species.
3. It is also used in polymerization and isomerization reactions of light molecular weight hydrocarbons. Some common examples include the production of dodecylbenzene for detergents.
4. Aluminium chloride can be mixed with aluminium along with arene to synthesise bis(arene) metal complexes.
5. Aluminium chloride also has a variety of other applications, especially in organic chemistry. For example, it is used to catalyse the “ene reaction”. For example, the addition of 3-buten-2-one to carvone.
6. Aluminium chloride is used to induce a variety of hydrocarbon couplings and rearrangements.

Industrial Uses of Aluminium Chloride (AlCl3)

• Aluminium chloride is widely used in the manufacturing of rubber, lubricants, wood preservatives, and paints.
• It is used in pesticides and pharmaceuticals.
• It is used as a flux in aluminium melting.
• It is used as an antiperspirant.
• It’s also used in manufacturing petrochemicals like ethylbenzene and alkylbenzene.

Is Aluminium Chloride (AlCl3) Hazardous?

Care should be taken to keep anhydrous aluminium chloride away from water or bases. Aluminium chloride can explode if it comes into contact with water due to the high heat of hydration. It also fumes in the air. Safety items like glasses, gloves, face guards, etc., should be worn during chemical reactions. This chemical compound should be stored in a tightly sealed container and protected from moisture.

• When it comes into contact with moist air, AlCl3 can absorb the moisture present in the air to become highly acidic, and it turns into a sticky substance.
• It can furiously corrode materials such as stainless steel and rubber.
• Prolonged exposure to this chemical can cause irritation to the skin, eyes and respiratory tract.
• In some studies, aluminium chloride has been found to be a neurotoxin that can be destructive to nerve tissues and can cause permanent damage.