Introduction:
Anhydrous aluminum chloride (AlCl3) is a commonly used reagent in many organic reactions. It is used as a Lewis acid catalyst and has a wide range of applications in the chemical industry. In this answer, we will discuss why anhydrous aluminum chloride is used in reactions.

What is anhydrous aluminum chloride?
Anhydrous aluminum chloride is a white or yellowish powder that is highly hygroscopic. It is used in its anhydrous form because the presence of water can alter its reactivity and lead to side reactions.

Why is anhydrous aluminum chloride used as a catalyst?
Anhydrous aluminum chloride is a Lewis acid catalyst, which means that it can accept a pair of electrons from a Lewis base. This makes it an excellent catalyst for a wide range of reactions. It is commonly used in reactions involving carbonyl compounds, such as Friedel-Crafts reactions, where it acts as a catalyst to activate the carbonyl group.

How does anhydrous aluminum chloride work?
Anhydrous aluminum chloride works by coordinating with the reactants and stabilizing the transition state of the reaction. It can also form complexes with the intermediate species of the reaction, which helps to prevent unwanted side reactions.

What are the advantages of using anhydrous aluminum chloride?
There are several advantages to using anhydrous aluminum chloride as a catalyst:

- It is a highly effective catalyst for a wide range of reactions.
- It is relatively inexpensive and readily available.
- It can be easily removed from the reaction mixture by filtration, which makes it easy to recover and reuse.
- It can be used in small quantities, which reduces the cost of the reaction.

Conclusion:
Anhydrous aluminum chloride is a versatile and effective catalyst for many organic reactions. Its ability to coordinate with reactants and stabilize transition states makes it a valuable tool for synthetic chemists. Its low cost and easy recovery make it a popular choice for many industrial applications.

Halogenation of benzene is a electophilic substitution reaction and AlCl3 is a strong electrophile as it is electron deficient species.