The Brønsted-Lowry definition works well for the reactions we learned so far, however it also limits the scope of acid-base reactions in a way where the proton H⁺ must be involved. Lewis acids and Lewis bases are defined in a more inclusive way that was first introduced by G. N. Lewis in 1923.

Lewis Acid: a species that can accept an electron pair;

Lewis Base: a species that can donate an electron pair.

All Brønsted-Lowry acids and bases fit into the Lewis definition, because the proton transfer process is essentially the reaction where the base uses its electron pair to accept a proton, as indicated by the mechanism arrow that we learned earlier. Therefore in the following reaction, the BL acid, H⁺, is also the Lewis acid, and BL base, NH₃, also fits to the definition of the Lewis base.

However, the Lewis definition is broader and covers more situations. For the following reaction, B(CH₃)₃ is the Lewis acid because boron has an incomplete octet, and the empty 2p orbital on boron is able to accept electrons. (CH₃)₃N behaves as the Lewis base with the lone pair electron on N that is able to be donated.

The product between Lewis acids and Lewis bases is usually a species that has the acid and base joined together, and the product is called the “LA-LB adduct”.

Other examples of Lewis acids include electron-deficient species, such as H⁺, M⁺, M²⁺, BH₃, BF₃, AlCl₃ etc. Lewis bases can be: amine, ether or other species that have lone pair electrons to donate.

Exercises 3.3

Show the product of the following LA-LB reaction: