Lewis acid 222676 222343816 2008-06-28T21:02:47Z DOI bot 6652755 Citation maintenance. Formatted: pages. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. {{POV-check|date=May 2008}} {{Acids and Bases}} A '''Lewis acid''' ('''LA''') can accept a pair of [[electron]]s and form a [[coordinate covalent bond]]. The Lewis acid and [[Lewis base]] theory, named after the [[United States|American]] [[chemist]] [[Gilbert N. Lewis|Gilbert Lewis]], is one of several [[acid-base reaction theories]]. Therefore the term ''[[acid]]'', itself, 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 [[HSAB concept|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. One model <ref>{{cite journal | last = Christe | first = K.O. | coauthors = Dixon, D.A.; McLemore, D.; Wilson, W.W.; Sheehy, J.A.; and Boatz, J.A. | title = On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry | journal = Journal of Fluorine Chemistry | volume = 101 | issue = 2 | pages = 101, 151–153 | date = 2000 | id = ISSN 0022-1139 }}</ref> has 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 [[antimony pentafluoride|SbF<sub>5</sub>]] 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.<ref>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.</ref> Some common Lewis acids include [[aluminium chloride]], [[iron(III) chloride]], [[boron trifluoride]], [[niobium pentachloride]] and the [[lanthanide triflate]]s such as ytterbium(III) triflate. Lewis acids may be [[corrosive]]. [[Zinc chloride]], which is corrosive, particularly towards cellulose (paper and cotton), is a notable example of Lewis acidity itself causing a corrosive effect. As [[water]] is Lewis basic, common Lewis acids react rapidly with water to give hydrates, which are Brønsted acidic. Thus, solutions of many common Lewis acids are also Brønsted acidic. Hydrates have strong chemical bonding between the Lewis acid and water, and it is usually not possible to "dry" them, i.e. the hydrate forms a separate chemical compound. For example, attempted drying of a metal chloride gives vapors of [[hydrogen chloride]] and metal [[oxychloride]]. == Uses == Lewis acids are used in catalysis. For example, a Lewis acid coordinated to a carbonyl enhances the electrophilicity of the carbonyl. [[Esterification]] is a reaction that requires the carbonyl to act as an electrophile, and thus esterifications are catalyzed by Lewis acids. Lewis acidic metal salts can replace Brønsted acids like sulfuric acid in industrial-scale esterifications. ==Ate complexes== An '''ate complex''' is a [[salt]] formed by reaction of a Lewis acid with a base whereby the central atom increases its [[Valence (chemistry)|valence]] <ref>''Advanced organic Chemistry, Reactions, mechanisms and structure'' 3ed. Jerry March ISBN 0-471-85472-7 </ref>. Often in [[chemical nomenclature]] the phrase ''ate'' is [[suffix]]ed to the element in question. For example, the ate complex of a [[boron]] compound is called a [[borate]]. Thus [[trimethylborane]] and [[methyllithium]] react to form the ate compound Me<sub>4</sub>B<sup>-</sup>Li<sup>+</sup>. This concept was introduced by [[Georg Wittig]] in 1958 <ref>''Komplexbildung und Reaktivität in der metallorganischen Chemie'' [[Angewandte Chemie]] Volume 70, Issue 3, Date: 7 Februar '''1958''', Pages: 65-71 G. Wittig {{DOI|10.1002/ange.1760700302}}</ref>. Similarly Lewis bases form [[onium salt]]s. ==References== {{reflist}} ==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]] [[Category:Acid-base chemistry]] [[Category:Acids]] [[de:Lewis-Säure]] [[fr:Acide de Lewis]] [[it:Acido di Lewis]] [[he:חומצת לואיס]] [[nl:Lewis-zuur]] [[pt:Ácido de Lewis]] [[vi:Axít Lewis]] [[zh:路易斯酸]]