Lewis Concept of Acids and Bases

In 1938 G.N. Lewis put forward a more generalized acid-base concept.

According to this concept, an acid is a species (charged or uncharged), which can accept a pair of electrons and a base is a species (charged and uncharged), which can donate a pair of electrons towards the formation of a covalent bond. Thus, a Lewis acid is an electrophile, and a Lewis base is a nucleophile. For example, in the reaction

:NH₃ is a donor of electron pair and BF₃ accepts a pair of electrons. So, according to the Lewis concept, :NH₃ is a Lewis base, and BF₃ is a Lewis acid.

Lewis acids

The following types of compounds or species are Lewis acids:

• All molecules possessing an atom with incomplete octet of electrons or Lewis acids.

Examples: BF₃, AlCl₃

• All cations are Lewis acids,

Examples: Ag⁺, Cu²⁺, Fe²⁺, Fe³⁺ etc. are Lewis acids.

• All molecules in which the central atom can acquire more than eight valence electrons.

Examples: SiCl4, SiF4, etc. are Lewis acids.

 

 

• Molecules with multiple bonds between atoms of different electronegativities e.g. CO₂, SO₂ etc. are Lewis acids.

 

 

Lewis bases

The following types of compounds or species are Lewis bases.

• All anions are Lewis bases e.g. OH^-, CN^-, CH₃COO^- etc.

• All molecules having lone-pairs of electrons viz. :NH₃, H₂O:, CO:, and amines etc. are Lewis bases.

It may be noted that all Bronsted bases are also Lewis bases, but all Bronsted acids are not Lewis acids.

Lewis Acid - Base Reaction

Whenever a Lewis acid reacts with a Lewis base, a covalent bond is formed.

For example, CO₂ (Lewis acid) reacts with OH^- (Lewis base) to give HCO₃^- (bicarbonate ion), BF₃ (boron trifluoride) behaves as a Lewis acid and NH₃ (ammonia) as a Lewis base.

Calcium oxide (CaO) behaves as a Lewis base, and sulphur trioxide (SO₃) as a Lewis acid in the reaction between the two.

The Lewis concept of acids and bases suffers from the following limitations.

• It fails to account for the relative strength of acids and bases, as it is not based on ionization.

• An acid-base reaction being an electron-transfer reaction, should be quite fast. However, many Lewis acid-base reactions are slow.