Lewis acid-base theory

What is the Lewis acid-base theory?

In 1923, Lewis proposed a theory to explain the behavior of acidic and basic substances. This is because previous theories, such as Brønsted-Löwry, defined them in terms of proton acceptance/transfer (H).

However, substances were known which, lacking hydrogen in their molecule, such as SO3 and CO2, behave as acids. Thus these compounds, in the presence of basic oxides such as CaO and K2O and in the absence of solvent, so there is no proton transfer, react to form a salt.

SO3 + K2O -> SO4K2

SO3 + K2O -> SO4K2

In view of these facts, Lewis established a new concept of acid and base:

  • Acid: substance (atom, molecule or ion) capable of accepting the sharing of a pair of electrons (in a coordinated covalent bond) from a base.
  • Base: substance (atom, molecule or ion) capable of yielding a pair of electrons in a coordinated covalent bond.

Therefore a Lewis acid must have at least one free orbital and a base one unshared pair of electrons.

This electronic theory of Lewis is the most general, and naturally, it includes the two previous ones: Arrhenius acid-base theory and the Brønsted-Löwry acid-base theory.