Lewis acid

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In chemistry, a Lewis acid can accept a pair of electrons and form a coordinate covalent bond, named after the American chemist Gilbert Lewis. The Lewis acid and Lewis base theory is one of several acid-base reaction theories, therefore the term acid 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. Christe and DixonK. O. Christe, D. A. Dixon, D. McLemore, W. W. Wilson, J. A. Sheehy and J. A. Boatz, (2000). "On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry", Journal of Fluorine Chemistry, pp. 101, 151-153. have 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.Discussions involving Christe and Dixon mentioned in reference 1 at the American Chemical Society 16th Winter Fluorine Conference, St. Pete Beach, Florida, January 12-17, 2003.

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

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