Lewis Theory
Lewis theory is based on the tendency to transfer electron pair. Therefore it is called as electronic theory.
The substance that can accept a pair of electrons to form a coordinate covalent bond is called is Lewis acid. So electron pair acceptor is Lewis acid.
Ex. BCl3, AlCl3, CH3+ etc.
The substance that can donate a pair of electrons to form a coordinate covalent bond is called is Lewis base. So electron pair donor is Lewis base.
Ex. NH3, H2O, Cl- etc.
Neutralisation:
The transfer of electron pair from an donor to acceptor is called neutralisation. It results the formation of coordinate covalent bond.
- BF3 + :NH3 (BF3 NH3)
Lewis acids are electrophiles i.e. electron loving or electron seeking.
Lewis bases are nucleophiles i.e. nucleus seeking or nucleus rich.
Lewis acid contains a vacant orbital.
Lewis base contains a lone pair of electrons.
| Lewis Acids | Lewis Bases |
|---|---|
| All cations are Lewis acids since they are able to accept electrons. Ex: Cu+2, Fe+2, Fe+3 | The most common Lewis bases are anions. Ex: Cl-, OH-, NH2- |
| Lewis acids are compounds with incomplete octet. Ex: BCl3, AlCl3 | Lewis bases are molecules with one or two lone pairs of electrons on the central atom. Ex: NH3, H2O |
| Compounds with double bonds other than carbon-carbon c=c bond. Ex: SO2, CO2 | Compounds with c=c and c≡c multiple bonds. Ex: C2H4, C2H2 |
| Molecules in which central atom can expand its octet using vacant d-orbitals present in its valency shell. Ex: SiF4, SiCl4 |
Limitations of Lewis theory:
According to lewis theory, neutralisation reactions occur very slowly. But in fact, the acid base neutralisation reactions occur quickly.
Lewis theory could not explain the strengths of acids and bases.
Lewis theory could not explain the catalytic activity of H+ ion.
All the acid base reactions do not involve formation coordinate covalent bond.
- NaOH + HNO3 NaNO3 + H2O