Anhydrous aluminium chloride is used as a catalyst and fumes in moist air. Explain these observations.

Explanation

The Al-atom in `AlCl_3` has only six electrons in its valence shell and it requires two more electrons to complete its octet. Therefore, it can act as a Lewis acid catalyst. For example, in Friedel-Crafts alkylation and acylation reactions, `AlCl_3` acts as a Lewis acid catalyst to generate the electrophile `(R^(o+) or R overset(o+)CO)`.

In moist air, `AlCl_3` undergoes partial hydrolysis to form HCI gas and because of this, it fumes in moist air.
`AlCl_3 (s) + 3H_3O(l) to Al(OH)_3+3HCl(g)`