Question

Why is anhydrous $AlC{l_3}$ used as a catalyst?

Answer 1

Aluminium trichloride $\left( {AlC{l_3}} \right)$ is a chemical compound used as a catalyst in many organic reactions. It is a white solid but $AlC{l_3}$ also sometimes appears yellow due to the contamination with Iron.

Complete answer:
Aluminium trichloride ($AlC{l_3}$ ) is electron deficient as a result it acts as Lewis acid (accepts pair of electrons). It is a powerful Lewis acid.
It is used as a catalyst for halogenation of aromatic hydrocarbons and Friedel crafts alkylation and acylation reactions.
Let’s see how it works;
In the halogenation (chlorination) reaction. $AlC{l_3}$ being a Lewis acid will coordinate to halogens. As a result of which the bond within the halogen group will break, it will increase the electrophilicity of its binding partner making it more reactive. Thus, aluminium trichloride acts as a catalyst.
For example in Chlorination;
Aluminium trichloride will form $AlC{l_4}^ -$ along with the generation of electrophile $C{l^ + }$ which can further attack at aromatic groups and constitute electrophilic substitution. Aluminium trichloride $\left( {AlC{l_3}} \right)$ is regenerated at the end of the reaction.

Additional Information: Anhydrous aluminium chloride has a very profound affinity for water. $AlC{l_3}$ is also used in the Gattermann-koch reaction in which an aldehyde group is introduced in an aromatic ring. $AlC{l_3}$ finds a wide variety of applications in many chemical reactions. It can cause irritation to eyes, skin if contacted. Thus, it must be handled with care.

Note:
We have seen that Aluminium trichloride ($AlC{l_3}$ ) increases the reactivity in aromatic electrophilic substitution reactions. It acts as a Lewis acid catalyst which helps in the generation of electrophile which further reacts. It is a major industrial catalyst.