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. 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 the lanthanide triflates such as ytterbium(III) triflate.

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References

Further reading

See also

References

1. On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry, , K.O., Christe, Journal of Fluorine Chemistry,
2. 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.

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