Why ${\mathrm{AlCl}}_3$ is a Lewis acid?

Lewis theory of acid and base

• Any species (molecule or ion) that can accept a pair of electrons is referred to as a Lewis acid.
• Any species (molecule or ion) that can donate a pair of electrons is referred to as a Lewis base.
• When a base provides a pair of electrons to an acid, a Lewis acid-base reaction occurs.

Aluminum chloride

• Aluminum chloride $\left({\mathrm{AlCl}}_3\right)$ is a Lewis acid because it can receive electrons.
• Structure of Aluminum chloride:

• The presence of high electron negative chlorine $\left(\mathrm{Cl}\right)$ atoms decreases the electron density from the core atom.
• Aluminum $\left(\mathrm{Al}\right)$ possesses unoccupied orbitals and is a powerful electrophile (electron deficient species).