Lewis acid

A Lewis acid (LA) can accept a pair of electrons and form a coordinate covalent bond. The Lewis acid and Lewis base theory, named after the American chemist Gilbert Lewis, is one of several acid-base reaction theories. Therefore the term acid, per se, is ambiguous; it should always be clarified as being a Lewis acid or a Brønsted-Lowry acid.

An electrophile or electron acceptor is a Lewis acid. A Lewis acid usually has a low-energy LUMO, which interacts with the HOMO of the Lewis base. Unlike a Brønsted-Lowry acid, which always transfers a hydrogen ion (H⁺), a Lewis acid can be any electrophile (including H⁺). Although all Brønsted-Lowry acids are Lewis acids, in common usage the term Lewis acid is often reserved for those Lewis acids which are not Brønsted-Lowry acids.

The reactivity of Lewis acids can be judged from the Hard-Soft Acid-Base concept. There is no universally valid description of Lewis acid strength, because Lewis acid strength depends on the specific Lewis base. One model ^[1] has predicted Lewis acid strength based on a computational model of gas-phase affinity for fluoride, and out of a selection of common isolable Lewis acids they found that SbF₅ had the strongest fluoride affinity. Fluoride is a "hard" Lewis base; chloride and "softer" Lewis bases are very difficult to study because of limitations of the computational methods, and Lewis acidity in solution suffers from the same restriction.^[2]

Some common Lewis acids include aluminium chloride, iron(III) chloride, boron trifluoride, niobium pentachloride and the lanthanide triflates such as ytterbium(III) triflate.

Lewis acids may be corrosive. Zinc chloride, which is corrosive, particularly towards cellulose (paper and cotton), is a notable example of Lewis acidity itself causing a corrosive effect. As water is Lewis basic, common Lewis acids react rapidly with water to give hydrates, which are Brønsted acidic. Thus, solutions of many common Lewis acids are also Brønsted acidic. Hydrates have strong chemical bonding between the Lewis acid and water, and it is usually not possible to "dry" them, i.e. the hydrate forms a separate chemical compound. For example, attempted drying of a metal chloride gives vapors of hydrogen chloride and metal oxychloride.

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Ate complexes

An ate complex is a salt formed by reaction of a Lewis acid with a base whereby the central atom increases its valence ^[3]. Often in chemical nomenclature the phrase ate is suffixed to the element in question. For example, the ate complex of a boron compound is called a borate. Thus trimethylborane and methyllithium react to form the ate compound Me₄B^-Li⁺. This concept was introduced by Georg Wittig in 1958 ^[4]. Similarly Lewis bases form onium salts.

References

Further reading

• The Lewis acid-base concepts : an overview, 1980, ISBN 0471039020
• Selectivities in Lewis acid promoted reactions, 1989, ISBN 0792304527
• Lewis acid reagents : a practical approach, 1999, ISBN 0198500998

See also

• Acid-base reaction
• Base (chemistry)
• Brønsted-Lowry acid-base theory
• Lewis base