The history of Lewis acids and bases

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Lewis had suggested in 1916 that two atoms are held together in a chemical bond by sharing a pair of electrons. 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 clear-cut as the diagram at the right shows; although the ammonia molecule donates a pair of electrons to the hydrogen ion, 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.
The modern definition of a Lewis acid is an atomic or molecular species that has an empty atomic or molecular orbital of low energy (LUMO) that can accommodate a pair of electrons, as illustrated in the molecular orbital diagram at the right.
A Lewis base is often a Brønsted-Lowry base as it can donate a pair of electrons to a proton; 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 loss of a proton 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 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 BF₃.
In another comparison of Lewis and Brønsted-Lowry acidity by Brown and Kanner,[2] 2,6-di-t-butylpyridine reacts to form the hydrochloride salt with HCl but does not react with BF₃. This example demonstrates that steric factors, in addition to electronic factors, play a role in determining the strength of the interaction between the bulky di-t-butylpyridine and tiny proton.
A typical example of a Lewis acid in action is in the Friedel-Crafts alkylation reaction. The key step is the acceptance by Aluminum chloride of a chloride ion lone-pair, forming AlCl₄- and creating the strongly acidic, that is, electrophilic, carbonium ion.