What is the Lewis theory of acids and bases?

Short Answer

The Lewis theory of acids and bases explains acids and bases based on electron pair transfer, not proton transfer. According to this theory, a Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. This idea is broader than the Arrhenius and Bronsted–Lowry theories.

For example, aluminium chloride (AlCl₃) is a Lewis acid because it accepts an electron pair, while ammonia (NH₃) is a Lewis base because it donates an electron pair. This theory helps explain many reactions that do not involve hydrogen ions.

Detailed Explanation :

Lewis Theory of Acids and Bases

The Lewis theory of acids and bases was proposed by American scientist Gilbert N. Lewis in 1923. This theory expanded the understanding of acids and bases beyond hydrogen ions (H⁺) and hydroxide ions (OH⁻). The earlier theories—Arrhenius and Bronsted–Lowry—focused mainly on the transfer of ions or protons. However, many reactions in chemistry do not involve hydrogen at all. The Lewis theory explains such reactions by focusing on electron pairs.

Lewis theory is one of the most general and widely applicable acid–base theories in chemistry. It forms the basis for understanding complex formation, organic reactions, and industrial chemical processes.

Meaning of Lewis Acids and Lewis Bases

According to Lewis:

  • A Lewis acid is a substance that accepts an electron pair.
  • A Lewis base is a substance that donates an electron pair.

This definition focuses on electron movement during reactions. The Lewis acid must have an empty orbital or space to accept the electron pair, while the Lewis base must have a lone pair of electrons to donate.

This broad definition makes it possible to classify many substances as acids or bases that do not fit into older theories.

Characteristics of Lewis Acids

A Lewis acid has certain features:

  1. Electron pair acceptor:
    It has an empty orbital to accept a pair of electrons.
  2. Electron-deficient molecules:
    Examples include BF₃, AlCl₃, and Fe³⁺, which lack a complete outer shell.
  3. Positive ions (cations):
    Metal ions like Cu²⁺, Fe³⁺, and Ag⁺ can act as Lewis acids because they attract electrons.
  4. Molecules with multiple bonds:
    Carbon dioxide (CO₂) and sulfur trioxide (SO₃) can also act as Lewis acids.

Examples of Lewis acids:

  • BF₃
  • AlCl₃
  • Fe³⁺
  • H⁺
  • CO₂

Characteristics of Lewis Bases

A Lewis base has different features:

  1. Electron pair donor:
    It must have a lone pair of electrons.
  2. Neutral molecules with lone pairs:
    Examples include NH₃, H₂O, and PH₃.
  3. Negative ions (anions):
    Ions like OH⁻, CN⁻, and Cl⁻ can donate electrons, making them Lewis bases.
  4. Organic compounds with nitrogen or oxygen:
    Amines, alcohols, and ethers often behave as Lewis bases.

Examples of Lewis bases:

  • NH₃
  • OH⁻
  • H₂O
  • CN⁻
  • Cl⁻

Formation of Lewis Acid–Base Complex

When a Lewis acid accepts an electron pair from a Lewis base, they form a Lewis acid–base complex (also called a coordinate bond or dative bond).

Example:
NH₃ + BF₃ → NH₃ → BF₃ (a coordinate bond forms)

Here:

  • NH₃ donates an electron pair → Lewis base
  • BF₃ accepts the electron pair → Lewis acid

This forms a stable complex through the shared electron pair.

Why Lewis Theory Is Important

The Lewis theory is significant for several reasons:

  1. Broadest definition of acids and bases

Unlike Arrhenius and Bronsted–Lowry theories, Lewis theory includes reactions that do not involve hydrogen ions at all.

  1. Explains complex formation

Metal ions often form complexes with electron-rich molecules. Lewis theory explains how these complexes form.

  1. Useful in organic chemistry

Many organic reactions involve electron pair donation, such as nucleophilic and electrophilic reactions.

  1. Explains behaviour of many inorganic compounds

Compounds like BF₃, AlCl₃, and metal ions cannot be explained by older theories but fit perfectly under Lewis theory.

  1. Works in all types of solvents

Lewis theory is not limited to water. It works in organic solvents, gases, and even solid-state chemistry.

Examples of Lewis Acid–Base Reactions

  1. Ammonia and Boron Trifluoride

NH₃ + BF₃ → NH₃ → BF₃

  • NH₃ donates electron pair → Lewis base
  • BF₃ accepts electron pair → Lewis acid
  1. Hydrogen Ion Reaction

H⁺ + NH₃ → NH₄⁺

  • H⁺ is Lewis acid
  • NH₃ is Lewis base
  1. Silver Ion and Ammonia

Ag⁺ + 2 NH₃ → [Ag(NH₃)₂]⁺
This complex formation is explained only by Lewis theory.

Conclusion

The Lewis theory of acids and bases defines acids as electron pair acceptors and bases as electron pair donors. This idea is broader and more flexible than earlier theories because it explains reactions that do not involve hydrogen ions. The theory helps understand complex formation, organic reactions, and many industrial processes. With its focus on electron transfer, the Lewis concept is a key part of modern chemistry.