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Lewis acids and bases
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==History== [[Image:Dative covalent bond2.png|thumb|[[MO diagram]] depicting the formation of a dative covalent bond between two atoms]] The concept originated with [[Gilbert N. Lewis]] who studied [[chemical bonding]]. In 1923, Lewis wrote ''An acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms.''<ref name="lewisvalence"/><ref name="lewis_1">Miessler, L. M., Tar, D. A., (1991) p. 166 – Table of discoveries attributes the date of publication/release for the Lewis theory as 1923.</ref> The [[Brønsted–Lowry acid–base theory]] was published in the same year. The two theories are distinct but complementary. A Lewis base is also a Brønsted–Lowry base, but a Lewis acid does not need to be a Brønsted–Lowry acid. The classification into hard and soft acids and bases ([[HSAB theory]]) followed in 1963. The strength of Lewis acid-base interactions, as measured by the standard [[enthalpy]] of formation of an adduct can be predicted by the Drago–Wayland two-parameter equation. ===Reformulation of Lewis theory=== Lewis had suggested in 1916 that two [[atom]]s are held together in a chemical bond by sharing a pair of electrons.<ref>{{cite journal |last1=Lewis |first1=Gilbert N. |title=The atom and the molecule |journal=Journal of the American Chemical Society |date=April 1916 |volume=38 |issue=4 |pages=762–785 |doi=10.1021/ja02261a002 |bibcode=1916JAChS..38..762L |s2cid=95865413 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hs1t2w;view=1up;seq=772}}</ref> When each atom contributed one electron to the bond, it was called a [[covalent bond]]. When both electrons come from one of the atoms, it was called a dative covalent bond or [[coordinate bond]]. The distinction is not very clear-cut. For example, in the formation of an ammonium ion from ammonia and hydrogen the [[ammonia]] molecule donates a pair of electrons to the [[hydron (chemistry)|proton]];<ref name="hydron">Traditionally, but not precisely, [[hydron (chemistry)|H<sup>+</sup> ions]] are referred as "[[proton]]s". See {{GoldBookRef|title=hydron|file=H02904}}</ref> the identity of the electrons is lost in the [[ammonium]] ion that is formed. Nevertheless, Lewis suggested that an electron-pair donor be classified as a base and an electron-pair acceptor be classified as acid. A more modern definition of a Lewis acid is an atomic or molecular species with a localized empty [[atomic orbital|atomic]] or [[molecular orbital|molecular]] orbital of low energy. This lowest-energy unoccupied molecular orbital ([[LUMO]]) can accommodate a pair of electrons. === Comparison with Brønsted–Lowry theory === A Lewis base is often a Brønsted–Lowry base as it can donate a pair of electrons to H<sup>+</sup>;<ref name="hydron"/> the proton is a Lewis acid as it can accept a pair of electrons. The conjugate base of a Brønsted–Lowry acid is also a Lewis base as [[deprotonation|loss of H<sup>+</sup>]] from the acid leaves those electrons which were used for the A—H bond as a lone pair on the conjugate base. However, a Lewis base can be very difficult to [[protonation|protonate]], yet still react with a Lewis acid. For example, [[carbon monoxide]] is a very weak Brønsted–Lowry base but it forms a strong adduct with BF<sub>3</sub>. In another comparison of Lewis and Brønsted–Lowry acidity by Brown and Kanner,<ref>{{cite journal |doi=10.1021/ja00957a023 |title=Preparation and Reactions of 2,6-Di-t-butylpyridine and Related Hindered Bases. A Case of Steric Hindrance toward the Proton |year=1966 |last1=Brown |first1=Herbert C. |last2=Kanner |first2=Bernard |journal=Journal of the American Chemical Society |volume=88 |issue=5 |pages=986–992 |bibcode=1966JAChS..88..986B }}</ref> [[2,6-Di-tert-butylpyridine|2,6-Di-''tert''-butylpyridine]] reacts to form the hydrochloride salt with HCl but does not react with BF<sub>3</sub>. This example demonstrates that steric factors, in addition to electron configuration factors, play a role in determining the strength of the interaction between the bulky di-''t''-butylpyridine and tiny proton.
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