Lewis acid
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
Dixon[K. 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.
