A Lewis acid is a molecular entity (and the corresponding chemical species) that is an electron-pair acceptor and therefore able to react with a Lewis base. This reaction forms a Lewis adduct by sharing the electron pair furnished by the Lewis base. Thus, H+ is a Lewis acid, because it can accept a lone pair to complete its stable form, which requires two electrons.
A Lewis base, therefore, is any species that donates a pair of electrons to a Lewis acid to form a Lewis adduct. For example, OH− and NH3 are Lewis bases, because they can donate a lone pair of electrons. Some compounds, such as H2O, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction (Figure 1).
Terminology and Symbols
The terms Lewis acid and Lewis base are usually defined within the context of a specific chemical reaction. For example, in the reaction
In many cases, the interaction between the Lewis base and Lewis acid is indicated by an arrow. For example,
A center dot may also be used:
Comparison to Classical Acid-Base Reaction
In the Lewis system, an acid does not exchange atoms with a base, but combines with it. For example, consider this classical aqueous acid–base reaction:
The Lewis definition does not regard this reaction as the formation of salt and water or the transfer of H+ from HCl to OH−. Instead, it regards the acid to be the H+ ion itself, and the base to be the OH− ion, which has an unshared electron pair. Therefore, the acid–base reaction here, according to the Lewis definition, is the donation of the electron pair from OH− to the H+ ion. This forms a covalent bond between H+ and OH−, thus producing water (H2O).
By treating acid–base reactions in terms of electron pairs instead of specific substances, the Lewis definition can be applied to reactions that do not fall under other definitions of acid–base reactions. For example, a silver cation behaves as an acid with respect to ammonia, which behaves as a base, in the following reaction:
The result of this reaction is the formation of an ammonia–silver adduct.
In reactions between Lewis acids and bases, an adduct forms when the highest occupied molecular orbital (HOMO) of a molecule with available lone electron pair(s) donates lone pairs of electrons to the electron-deficient molecule's lowest unoccupied molecular orbital (LUMO) through a co-ordinate covalent bond; in such a reaction, the HOMO-interacting molecule acts as a base, and the LUMO-interacting molecule acts as an acid. Adducts involving metal ions are referred to as coordination compounds.
Application to Organic Chemistry
In organic chemistry, it is useful to understand that nucleophiles are Lewis bases and electrophiles are Lewis acids. Nearly all heterolytic reactions in organic chemistry can be thought of as Lewis acid-base processes.