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 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<sup>+</sup>), a Lewis acid can be any electrophile (including H<sup>+</sup>). 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.

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