Lewis Definition of Acids and Bases

In the Lewis definition, an acid is an electron-pair acceptor and a base is an electron-pair donor, and an acid-base reaction is the formation of a coordinate covalent bond in which the base donates a lone pair to the acid. In brief: boron trifluoride (BF₃), which has an empty orbital, reacts with ammonia (NH₃), which has a lone pair, to form F₃B←NH₃; here BF₃ is a Lewis acid and NH₃ is a Lewis base, although no H⁺ or OH⁻ is involved.

The Lewis definition is the most general of the three common theories. It includes all Brønsted-Lowry acids and bases and also reactions that do not involve protons, such as the formation of metal complexes and many organic reactions. This article explains the definition, shows how to recognize Lewis acids and bases, compares the three definitions, lists examples in a table, works through examples that include metal ions and organic reactions, describes applications and gives the history.

What is the Lewis definition?

A Lewis base is a species that can donate a pair of electrons to form a covalent bond. A Lewis acid is a species that can accept a pair of electrons to form a covalent bond. The product is called an adduct, and the new bond is a coordinate (dative) bond. In the reaction H⁺ + :NH₃ → NH₄⁺, the proton is a Lewis acid (it has an empty 1s orbital), and ammonia is a Lewis base (it has a lone pair).

How do you recognize Lewis acids and bases?

• Lewis bases have at least one lone pair, or a pi bond, that can be donated. Typical examples are anions (OH⁻, Cl⁻, CN⁻) and neutral molecules with lone pairs (NH₃, H₂O, CO, alcohols, ethers).
• Lewis acids have an empty orbital or can make one available. They include: cations, especially small ones with a high charge (H⁺, Al³⁺, Fe³⁺, Cu²⁺); molecules whose central atom has an incomplete octet (BF₃, AlCl₃); molecules with a central atom that can expand its octet (SiF₄, SnCl₄); and molecules with polar multiple bonds, such as CO₂ and SO₃, in which the carbon or sulfur atom is electron-poor.

Examples of Lewis acids and bases

How do the three definitions compare?

Every Arrhenius acid or base is also a Brønsted-Lowry one, and every Brønsted-Lowry acid or base is a Lewis acid or base. The reverse is not true.

Worked examples

1. Identify the acid and base. In AlCl₃ + Cl⁻ → AlCl₄⁻, aluminum in AlCl₃ has 6 electrons and an empty p orbital, so AlCl₃ is the Lewis acid. Chloride has lone pairs, so it is the Lewis base.
2. Carbon dioxide in basic solution. CO₂ + OH⁻ → HCO₃⁻. The hydroxide ion donates a lone pair to the carbon atom, which is electron-poor because it is bonded to two electronegative oxygen atoms. CO₂ is the Lewis acid, and OH⁻ is the Lewis base. This reaction explains why a base absorbs CO₂ from the air.
3. Metal ions in water. Fe³⁺ accepts electron pairs from 6 water molecules to form [Fe(H₂O)₆]³⁺. The strong attraction of the Fe³⁺ ion for the oxygen lone pairs weakens the O–H bonds, and so the hydrated ion can lose a proton: [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺. This is why solutions of iron(III) and aluminum salts are acidic.
4. Count the bonds in an adduct. In F₃B←NH₃, boron has 4 bonds, and nitrogen has 4 bonds, with no lone pair. The formal charge of N is +1 and that of B is −1, and the molecule is neutral overall.

5. Which is the Lewis acid? BF₃ or NH₃? BF₃ has only 6 electrons around boron, so it accepts a pair. NH₃ has a lone pair, so it is a donor.
6. Organic chemistry. In the Friedel-Crafts reaction, AlCl₃ (Lewis acid) accepts a chloride ion from CH₃Cl to form the electrophile CH₃⁺ (as a complex with AlCl₄⁻), which attacks benzene.

Where do Lewis acid-base reactions matter?

• Coordination chemistry: formation of metal complexes, such as the blue copper-ammonia complex, with ligands as Lewis bases.
• Catalysis: Lewis acids such as AlCl₃, BF₃ and TiCl₄ catalyze many industrial reactions, including polymerization and alkylation.
• Biochemistry: the binding of oxygen to the iron of hemoglobin, and the action of zinc in many enzymes.
• Environmental chemistry: the absorption of CO₂ by basic solutions or by amine solvents in carbon capture.
• Glass and ceramics: acidic and basic oxides (SiO₂, CaO) combine to form silicates.

Who proposed the definition?

Gilbert N. Lewis described his theory of acids and bases in 1923, in his monograph Valence and the Structure of Atoms and Molecules, but its importance was recognized much later. It was developed further in the 1930s, after Lewis called attention to the idea in a paper of 1938. The terms “hard” and “soft” acids and bases were added by Ralph Pearson in 1963 to predict which Lewis acids prefer which Lewis bases. Lewis’s definition connects acid-base chemistry with coordinate bonding, and it unifies large parts of inorganic and organic chemistry.

Which mistakes are common?

Mistake 1: Thinking that Lewis acids must contain hydrogen

BF₃, AlCl₃ and metal ions are Lewis acids and contain no hydrogen.

Mistake 2: Reversing the electron flow

The base donates the electron pair, and the acid accepts it.

Mistake 3: Forgetting that the Brønsted-Lowry acids are special cases

H⁺ itself is the Lewis acid in a proton transfer. HCl is a Lewis acid-base reaction in which the proton is accepted.

Mistake 4: Ignoring that the same species can be both

Water is a Lewis base (donates a lone pair from oxygen) and can also be a Lewis acid, when it accepts an electron pair on its hydrogen atoms, as in hydrogen bonds.