Lewis Acid vs Base

Acids and bases are very different from each other. There are different definitions for acids and bases, but Lewis acid refers specifically to the definition of an acid which was published in 1923 by Gilbert N. Lewis. In general terms, Lewis acid is considered to be an acceptor of electron-pairs, whereas the Lewis base is considered to be donor of electron-pairs.

Lewis acid

Lewis acid is an acid substance which accepts a lone or single pair of electrons from some other molecule to complete its own stable group atom. For example, H+ can accept an electron pair to complete its stable group, thus it is a Lewis acid because H+ requires 2 electrons.

Another way of defining Lewis acid, which has been approved by IUPAC is by acknowledging that Lewis acid is a molecular entity which accepts an electron-pair, and thus reacts with Lewis base in order to form a Lewis adduct. The reaction that takes place between Lewis acid and Lewis base is that acids accept the electron-pair, whereas the Lewis base donates them. The main criteria behind the reaction is the production of an “adduct” and not a displacement reaction.

Lewis acid is classically restricted to species which have empty p orbital and are called trigonal planar species, for example BR3. Here R can be either halide or organic substituent.

Lewis base

Lewis base can be defined as a species or as a  basic substance that donates a lone pair of electrons to Lewis acids, in order to form a Lewis adduct. Let’s see the example of NH3 and OH-. They are both Lewis bases because they can donate a pair of electron to Lewis acids.

NH3 gives a lone electron pair to Me3B in a chemical reaction and forms Me3BNH3 which is a Lewis adduct.  Me3B is a Lewis acid which accepts a pair of electron from NH3.

There are some compounds which act both as Lewis acids and Lewis bases. These species have the ability to either accept an electron–pair or donate an electron pair. When they accept a pair of electrons or lone pair of electrons they act as  Lewis acid. When they donate a lone pair of electrons, they act as Lewis base; for example, water and H2O. These compounds act like both Lewis acid or as Lewis base depending upon chemical reaction taking place.

Summary

1. Lewis acid is an acid substance which accepts a lone or single pair of electrons from some other molecule to complete its own stable group of atoms ( for example H+). Lewis acid is classically restricted to any species which have an empty p orbital and are called trigonal planar species. Lewis base can be defined as a species or basic substance which donates a lone pair of electrons to Lewis acids in order to form a Lewis adduct.

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