Anhydrous aluminium chloride is used as a catalyst.

Answer

The `Al` atoms in `AlCl_(3)`has only `6` electrons in its valence shell. It needs two more electrons to complete its octet.It therefore, acts as a Lewis acid catalyst in Friedel-Crafts alkylation and acylation reactions of benzene and other aromotic compounds.In these reactions, it helps to generate the electrophile `(R^(+)` or `RCO^(+)`) which attacks the benzene / aromatic ring.