Lewis Acids and Bases

In the periodic table, all elements in group 13 have three valence electrons. Consequently, they form trivalent compounds in which the central atom has a sextet of electrons and a vacant orbital.

These electron-deficient compounds achieve their octets by receiving electrons from other species in a chemical reaction.

For example, aluminum chloride reacts with ammonia by accepting the lone pair from the nitrogen atom.

This makes aluminum chloride — the electron-pair acceptor — a Lewis acid, and ammonia — the electron-pair donor — a Lewis base.

As the two oppositely charged species attract each other, the electron pair is transferred from the base to the acid. This completes the octet of the Lewis acid, resulting in a Lewis acid—base adduct.

Now, consider another Lewis acid—base reaction between the electron-deficient boron trifluoride and ammonia.

The electrostatic potential map of boron trifluoride indicates a significant positive charge centered on the boron atom.

In the case of ammonia, the nitrogen atom carries a negative charge on account of its nonbonding electron pair.

The opposite charges cause boron trifluoride and ammonia to react. During the reaction, the lone pair of ammo