Lewis Acids and Bases: Concepts and Applications

Lewis Acids and Bases

Introduction to Lewis Acid-Base Theory

The Lewis acid-base definition, introduced in the 1920s by Gilbert N. Lewis, expands the concept of acids and bases beyond the traditional Brønsted-Lowry and Arrhenius models. This theory focuses on the transfer of electron pairs rather than protons.

• Lewis Acid: An electron pair acceptor.

• Lewis Base: An electron pair donor.

Comparison of Acid-Base Theories

The table below summarizes the differences between Arrhenius, Brønsted-Lowry, and Lewis definitions of acids and bases, including examples for each.

Concept	Arrhenius	Brønsted-Lowry	Lewis	Examples
Acid	Produces H+ in water	Proton (H+) donor	Electron pair acceptor	HCl, H2SO4, BF3
Base	Produces OH- in water	Proton (H+) acceptor	Electron pair donor	NH3, OH-, H2O

Additional info: The Lewis definition is the most general, encompassing all Brønsted-Lowry and Arrhenius acids and bases, as well as species that do not fit those definitions.

Lewis Acid-Base Reaction

A Lewis acid-base reaction involves the formation of a coordinate covalent bond, where the Lewis base donates an electron pair to the Lewis acid.

• Product: The resulting species is called an adduct.

Example Reaction:

NH3 (Lewis base) + BF3 (Lewis acid) → F3B:NH3 (adduct)

Identifying Lewis Acids and Bases

• All Brønsted-Lowry acids and bases are also Lewis acids and bases.

• However, many Lewis acids and bases are not classified as acids or bases under Brønsted-Lowry or Arrhenius models.

Examples

• Lewis Acid: BF3 (accepts electron pair)

• Lewis Base: NH3 (donates electron pair)

• Both Brønsted-Lowry and Lewis: H2O (can act as both a proton acceptor and electron pair donor)

Practice Problem

Identify the Lewis acid and Lewis base in the following reaction:

CaO (s) + CO2 (g) → CaCO3 (s)

• Lewis Acid: CO2 (accepts electron pair from O2- in CaO)

• Lewis Base: CaO (provides electron pair via O2-)

Summary Table: Acid-Base Definitions

Definition	Acid	Base
Arrhenius	Produces H+ in water	Produces OH- in water
Brønsted-Lowry	Proton donor	Proton acceptor
Lewis	Electron pair acceptor	Electron pair donor

Key Equations

• General Lewis acid-base reaction: $\text{Lewis base} + \text{Lewis acid} \rightarrow \text{Adduct}$

Additional info: Lewis acids are often electron-deficient species (such as metal ions or molecules with incomplete octets), while Lewis bases typically have lone pairs available for bonding.