Question

Why is `AlCl_3` an electrophile?

Answer 1

Because the aluminum centre is electron deficient and tends to accept electron density.

Explanation:

We write `AlCl_3`, but its actual structure is `{Cl_2Al}_2(mu-Cl)_2`, i.e. dimeric `Al_2Cl_6`; the aluminum centre is electron deficient and drafts in available electron density to satisfy its Lewis-acidity. Much of the chemistry of aluminum may be rationalized on the basis of completing its octet.

Of course aluminum forms such ions as `AlCl_4^-`, tetrachloroaluminate, and `AlH_4^-`, aluminum hydride.