Lewis theory and explanation of acids and bases
Lewis Theory of Acids and Bases
Based on the electronic theory of valency, the scientist G. N. Lewis proposed the electronic concept of acids and bases in 1923. In general terms, neutral molecules or ions that can accept an electron pair are called acids, and those that can donate an electron pair are called bases.
Lewis Acid
Compounds or ions that can accept one or more lone pair electrons from another electron-donating substance are called Lewis acids. In other words, an electron-pair acceptor is an acid.
Types and Examples of Lewis Acids:
- Incomplete Octet Compounds: BF3, BCl3, AlCl3, FeCl3, etc.
- Expanded Octet Compounds (with vacant d-orbitals): PCl5, SnCl2, SO2, SiF4, etc.
- Simple Cations: H+, Ag+, Cu2+, Fe2+, Fe3+, Zn2+, etc.
Lewis Base
Compounds or ions that can donate one or more lone pair electrons to another electron-accepting substance to form a coordinate covalent bond are called Lewis bases. In other words, an electron-pair donor is a base.
Types and Examples of Lewis Bases:
- Neutral Molecules with Lone Pairs: NH3, H2O, PH3, R-OH (Alcohol), CO, Ethylenediamine, etc.
- Anions (Negatively charged particles): OH–, Cl–, Br–, I–, CN–, SH–, NH2–, etc.
Example and Mechanism of Lewis Reaction (Diagram)
Figure: Formation of a coordinate compound through donation of a lone pair of electrons by ammonia to boron trifluoride.
SiF4 (Lewis Acid) + 2F– (Lewis Base) → [SiF6]2-
The Lewis concept of acids and bases is completely fundamental and extraordinarily comprehensive. There are many complex reactions or coordination complex formation processes where the exchange or transfer of a proton (H+) does not take place at all. Reactions that could not be categorized as acid-base interactions by the Arrhenius or Brønsted-Lowry theories have been successfully integrated into acid-base chemistry by the Lewis theory.
Practical Examples of Protonless Cations and Complex Compound Formation:Here, ammonia (Lewis base) donates its lone pair to the boron atom (Lewis acid).
Two ammonia molecules donate electron pairs to the vacant orbital of the silver ion to form the diamminesilver(I) complex ion.
The reaction of cyanide ions with the ferrous ion produces the complex ferrocyanide ion.
* Protons play absolutely no role in any of the above reactions. Therefore, the contribution of Lewis’s electronic theory is extremely significant in chemistry for flawlessly explaining the acid-base nature and complex compound formation in protonless reactions.
