Lewis Concept of Acid and Base

To overcome the drawbacks of  Bronsted -Lowrey  concept, a new  and broad concept of acid and base was proposed by GN Lewis called Lewis Concept of Acid and base.

According to this concept, an acid is a substance which can accept a lone pair of electron and a base is a substance which can donate a lone pair of electron.

Lewis acid base concept

Following species can be regarded as Lewis Acid:

  1. All cations like H+, Na+ NH4 + CH3+, C2H5+  etc.
  2. Compounds with electron deficient centre like BF3, AlCl3 etc.
  3. Compounds containing central atoms with vacant D-orbital. Example: SIF4 , PF3, SnCl4 etc.
  4. Compound containing central atom bonded with more electronegative atom by multiple bond like HCO3 .

Following species can be regarded as Lewis base:

  1.  All the anions like OH, Cl , Br, SO4, CO3 etc are called Lewis base.
  2.  All the compound containing central atom with lone pair of electron are lewis base. Example: NH3, H2O etc.
  3.  All the Lewis bases are also Bronsted-Lowry base.

According to this concept, Neutralization is the donation of pair from Lewis base to Lewis acid in order to form a coordinate compound. For Example:

$$\ce{H2O +H^+  -> H3O}$$

Merits of Lewis concept:

  • This concept is able to explain the acidity of the compounds like CO2, AlCl3, BF3 and the basicity of compounds like CaO, MgO etc.
  •  This concept is applicable to interpret the reaction mechanism.
  • It gives the broader interpretation of acid-base behavior.

Limitations of Lewis concept:

  • It could not explain the neutralization of NaOH with HCl to give NaCl and water since no coordinate covalent bond is formed here.
  • The relative strength of acid and base cannot be explained on the basis of Lewis concept.

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