Lewis acid
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In chemistry, a Lewis acid, named after the American chemist Gilbert Lewis, can accept a pair of electrons and form a coordinate covalent bond. 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 Dixon[1] 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.[2]
Some common Lewis acids include aluminium chloride, iron(III) chloride, boron trifluoride, niobium pentachloride and ytterbium(III) triflate.
[edit] References
- ^ Christe, K.O.; Dixon, D.A.; McLemore, D.; Wilson, W.W.; Sheehy, J.A.; and Boatz, J.A. (2000). "On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry". Journal of Fluorine Chemistry 101 (2): 101, 151-153. ISSN 0022-1139.
- ^ 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.
