Lewis acids and bases

Lewis Acids and Bases

Basic Overview

According to Lewis theory, an acid is a substance that can accept an electron pair, while a base is a substance that can donate an electron pair.
Lewis acids and bases expand the definitions beyond hydrogen ion and hydroxide ion transfer that are described in the Bronsted-Lowry acid-base theory.

Lewis Acids

Lewis acids are electron pair acceptors. They have vacant orbitals or positively polarised atoms that can accept an electron pair from a base.
Common examples of Lewis acids include BF3, AlCl3, and FeBr3. Each of these have central atoms that can accept an electron pair.
H+ ion itself is a Lewis acid as it can accept an electron pair to form a co-ordinate bond.

Lewis Bases

Lewis bases are electron pair donors. They have a lone pair of electrons which can be used to form a co-ordinate bond with an acid.
Water (H2O), ammonia (NH3), and fluoride ion (F-), are examples of Lewis bases because they can donate a lone pair of electrons to a Lewis acid.
All Bronsted-Lowry bases are also Lewis bases but not all Lewis bases are Bronsted-Lowry bases. For example, NH3 is a base under both definitions, but BF3 is only a base under the Lewis description.

Lewis Acid-Base Reactions

A Lewis acid-base reaction results in the formation of a co-ordinate covalent bond, where both electrons come from the base.
The species which forms from the reaction, containing the co-ordinate bond, is called an adduct.
An example of a Lewis acid-base reaction is BF3 + F- → BF4-. In this reaction, the Lewis acid BF3 accepts a pair of electrons from the Lewis base F- to form a co-ordinate bond, and the adduct BF4- is produced.