What Is a Lewis Acid?

The term Lewis acid is named after the American chemist Gilbert N. Lewis. Early chemists recognized an acid as a substance with a sour taste that reacts with some metals and that neutralizes bases, or alkalis, producing a salt. Since the late 19th century, however, attempts have been made to define acids and bases in a more rigorous way, and one that explains what actually happens in an acid-base reaction. Lewis’s is the broadest definition.

In 1883, the Swedish chemist Svante Arrhenius defined an acid as a substance that forms hydrogen ions (H⁺) in aqueous solution, and a base as a substance that forms hydroxide (OH^-) ions. The H⁺ ions — which are simply protons — are too reactive to exist in an aqueous solution, and associate with water molecules to form hydronium (H₃O⁺) ions. Arrhenius’s definition proved very useful and covers most of the compounds commonly regarded as acids. For example, hydrochloric acid, a solution of the gas hydrogen chloride in water, provides H⁺ ions that form hydronium ions in solution: HCl + H₂O → H₃O⁺ + Cl^-. This definition remained the standard until well into the 20th century and is still frequently used today.

A defining characteristic of all acids is that they neutralize bases to produce salts. An example is the reaction of hydrochloric acid with sodium hydroxide (NaOH) to produce sodium chloride and water (H₂O): H₃O⁺Cl^- + Na⁺OH^- → Na⁺Cl^- + H₂O. Here, the H⁺ ions provided by the hydrochloric acid have combined with the OH^- ions provided by the sodium hydroxide to produce water, while the Na⁺ and Cl^- ions have combined to produce salt, in accordance with Arrhenius’s theory; however, similar reactions can occur between compounds that do not fit the Arrhenius definitions of acids and bases. For example, gaseous hydrogen chloride can react with gaseous ammonia to form the salt ammonium chloride: HCl + NH₃ → NH₄⁺Cl^-. Two compounds have combined to form a salt, but since they are not in solution, there are no H⁺ or OH^- ions present, so the reactants do not qualify as an acid and a base according to Arrhenius.

In 1923, two chemists — Johaness Bronsted and Thomas Lowry — independently came up with a new definition. They suggested that an acid was a proton donor and a base a proton acceptor. In an acid-base reaction, the acid provides a proton, or H⁺ ion, to the base; however, neither reactant need be in solution, with H⁺ or OH^- ions actually present prior to the reaction. This definition includes all Arrhenius acids and bases, but also explains the combining of gaseous hydrogen chloride and ammonia as an acid-base reaction: the covalent hydrogen chloride has provided a proton to the ammonia to form an ammonium (NH₄⁺) ion, which forms an ionic compound with the Cl^- ion.

The American chemist Gilbert N. Lewis suggested, also in 1923, an extended concept of acids and bases as electron pair acceptors and donors, respectively. By this definition, an acid-base reaction involves the reactants forming a co-ordinate bond — a covalent bond where both the shared electrons come from the same atom — with the electrons coming from the base. In the HCl–NaOH reaction described above, the H⁺ ion provided by the HCl accepts an electron pair from the OH^- ion provided by the NaOH to form water.

According to this theory, therefore, a Lewis base is a compound that has an unbonded electron pair available for bonding. The Lewis acid structure is such that it can achieve a stable configuration by forming a co-ordinate bond with a Lewis base. Bases need not contain hydroxide ions or accept protons, and a Lewis acid need not contain hydrogen or donate protons. The Lewis acid definition includes all Arrhenius and Bronsted-Lowry acids and also many substances that do not meet the Bronsted-Lowry or Arrhenius criteria.

A good example of such a substance is boron trifluoride (BF₃). In this compound, boron, which normally has three electrons in its outer shell, has formed covalent bonds, sharing an electron pair with each of the three fluorine atoms. Although the compound is stable, it has room for two more electrons in its outer shell. It can thus form a co-ordinate bond with an electron pair donor — in other words, a base.

For example, it can combine with ammonia (NH₃), which has a nitrogen atom with an unbonded electron pair, as three of the five electrons in the nitrogen’s outer shell are in covalent bonds with the three hydrogen atoms. The combination of boron trifluoride and ammonia is thus as follows: BF₃ + :NH₃ → BF₃:NH₃ — the “:” represents the electron pair from the ammonia’s nitrogen atom. Boron trifluoride is thus behaving as a Lewis acid and ammonia as a base.