Lewis acids and bases

Lewis bases and acids

NH3-BF3-adduct-bond-lengthening-2D

Partial Roche Synthesis of Mibefradil.tif

Dative covalent bond2

Lewis acids and bases are a concept in chemistry that extends the definition of acids and bases beyond the traditional Brønsted–Lowry acid–base theory. The concept was introduced by Gilbert N. Lewis in 1923, and it defines acids and bases in terms of electron pair exchanges rather than proton exchanges.

Definition

A Lewis acid is defined as an electron pair acceptor, whereas a Lewis base is an electron pair donor. This definition encompasses a wider variety of chemical reactions than the Brønsted–Lowry definition, as it includes reactions that do not involve hydrogen ions (H+). Lewis acids and bases can form a Lewis adduct by sharing a Lewis base's electron pair.

Examples

• Lewis Acids: Metal ions like Fe^3+, Al^3+, and BF_3 (boron trifluoride) are typical examples of Lewis acids. They can accept an electron pair from a Lewis base to form a coordinate covalent bond.
• Lewis Bases: Molecules or ions with a lone pair of electrons, such as NH_3 (ammonia), H_2O (water), and F^- (fluoride ion), are examples of Lewis bases. They can donate an electron pair to a Lewis acid.

Applications

Lewis acids and bases are used in various chemical reactions and processes. For example, in catalysis, Lewis acid catalysts are used to increase the rate of a reaction by accepting an electron pair. In organic synthesis, Lewis acids can help in the formation of new compounds by facilitating the addition of nucleophiles to electrophiles.

Reactivity and Strength

The strength of a Lewis acid or base is determined by its ability to accept or donate electron pairs. The strength can be influenced by several factors, including the acidity or basicity of the solvent, the presence of substituents on the Lewis acid or base, and the stability of the Lewis adduct formed.

Comparison with Other Acid-Base Theories

The Lewis concept is broader than the Brønsted–Lowry and Arrhenius theories, as it can explain reactions that do not involve protons but involve the transfer of electron pairs. However, all three theories are useful for understanding different aspects of acid-base chemistry.

See Also

• Acid-base reaction theories
• Coordination chemistry
• Electrophile
• Nucleophile