Aluminium Chloride (AlCl3)

What Is Aluminium Chloride (AlCl3)?

Aluminium chloride is also referred to as aluminium trichloride or aluminium (III) chloride. The compound is formed when aluminium and chlorine are reacted together. Its chemical formula is written as AlCl₃. As for physical appearance, aluminium chloride is usually white in colour. However, due to the presence of contaminants (iron(III) chloride), it acquires a yellowish colour.

Industrially, aluminium chloride is used in the production of aluminium metal, but it also has a wide range of uses in the chemical industry, particularly as a Lewis acid. Solid aluminium chloride (AlCl₃) is covalently bonded with low melting as well as boiling points.

A Danish physicist and chemist named Hans Christian Oersted discovered aluminium chloride for the first time in the year 1825. This chemical compound is one of the oldest chemicals used, especially in the branch of organic chemistry. We will learn about this compound in detail below.

Table of Contents

• Preparation of Aluminium Chloride
• AlCl₃ Structure
• Properties
• Reactions
• Uses of AlCl3

Overview

Preparation of Aluminium Chloride

Aluminium chloride is mainly produced using an exothermic reaction of two elements, namely aluminium and chlorine. There are several other ways in which aluminium chloride can be obtained.

Some common ways are by reacting the aluminium metal with hydrogen chloride or by conducting a single displacement reaction between copper chloride and aluminium metal. The reactions for the same are given below:

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

Aluminium Chloride Structure

When we talk about the structure of AlCl3, it is sometimes confusing. This chemical compound tends to form different types of structures when it is exposed to varying temperatures. It also depends on the state of the compound, whether it is in a solid, liquid or gaseous state.

Structure of Aluminium Chloride (AlCl3)

When AlCl3 is in the solid state, it features a cubic close-packed layered structure. In this case, its coordination geometry will be octahedral. When aluminium chloride is in a liquid or molten state, it exists as a dimer. Here, its coordination geometry will be tetrahedral. At higher temperatures, the dimers dissociate into trigonal planar.

Aluminium Chloride Properties

We will discuss the chemical and physical properties of aluminium chloride below.

Physical Properties of AlCl₃

• Aluminium chloride has a very low melting and boiling point.
• It sublimes at a temperature of 180°C.
• AlCl3, in a molten state, is a poor conductor of electricity.
• The colour of aluminium chloride is white, but it is often contaminated with iron trichloride, which makes it yellow.
• It 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 is a major industrial catalyst.
• AlCl₃ is an anhydrous, non-explosive, non-flammable, but corrosive solid.
• It reacts violently when it comes in contact with water or bases.

Aluminium Chloride Reactions

Here, we will understand how AlCl3 reacts with other compounds. If we take anhydrous aluminium chloride, it is a powerful Lewis acid, which means that it is capable of forming Lewis acid-base adducts even with bases that are weak in nature. For example, mesitylene and benzophenone. Some common reactions are given below:

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

Aluminium Chloride Reaction with Water

Aluminium chloride is deemed to be hygroscopic, where it can absorb moisture from the air. Usually, this chemical compound fumes in the air containing moisture. It creates a hissing sound when it comes in contact with water. When the reaction occurs, the Cl^– ions are replaced with H₂O molecules and form hexahydrate [Al(H₂O)₆]Cl₃. The anhydrous state of AlCl₃ is lost, and when the heat is applied, HCl also dissipates, and the final product that is obtained is aluminium hydroxide.

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, and this can lead to partial hydrolysis in Al³⁺ ions. The reaction can be written as:

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

Aluminium Chloride (AlCl3) Uses

Aluminium Chloride is often regarded as a versatile chemical compound, and therefore finds application in many areas, especially in chemical reactions and synthesis. We will learn about the uses of aluminium chloride below.

1. AlCl₃ is used mainly as a catalyst for different chemical reactions. It is used extensively in Friedel-Crafts reaction, including both acylations and alkylations. It is used for the preparation of anthraquinone from phosgene and benzene.
2. Aluminium chloride can be used to bring in or attach aldehyde groups on aromatic series or rings. For instance, if we look at 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”.We can take the case of the addition of (methyl vinyl ketone) 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 used widely in manufacturing 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?

Precaution should be taken to keep anhydrous aluminium chloride away from water or bases. Aluminium chloride can explode if it comes in contact with water because of 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 in 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.