By Sunil Bhardwaj

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According to Lewis, an acid is any species that acts as an electron pair acceptor and a base any species that acts as an electron pair donor. Thus an acid base reaction is the sharing of an electron pair provided by the base to the acid. For example, $$ \underset { Acid }{ { H }^{ + } } + :N{ H }_{ 3 } \longrightarrow { \left[ N{ H }_{ 4 } \right] }^{ + } $$ in this reaction the Lewis acid \({ H }^{ + }\) accepts a lone pair of electrons donated by the Lewis base :\(N{ H }_{ 3 }\).

Lewis acid can be classified into the following categories:

(i) Simple Cations: All cations are potential lewis acids, typical acid-base reactions involving cations are $$ \underset { Acid }{ { Al }^{ +3 } } + 6{ H }_{ 2 }\overset { \cdot \cdot }{ \underset { \cdot \cdot }{ O } } \longrightarrow { \left[ Al\left( { H }_{ 2 }O \right) _{ 6 } \right] }^{ +3 } $$ An increase in the charge density ofcations increases the capacity to accept the pair ofelectrons and therefore increases the acid character.

(ii) Compound in which the Central Atom has an Incomplete Octet: Typical examples of such Lewis acids are \(B{ F }_{ 3 }, BCl_{ 3 }, AlCl_{ 3 },\) etc. $$ F-Be-F + 2F^{ - } \longrightarrow { \left[ BeF_{ 4 } \right] }^{ -2 } $$ In this reaction Be is having only 4 electrons in its outer most orbital and can accept two pair of electrons from \(F^{ - }\) ions.

(iii) Compounds in which the Octet of the Central Atom can be Expanded: \(SiCl_{ 4 }\) or \(SiF_{ 4 }\) acts as a Lewis acid. Si with its vacant d ortibals can accept electron pairs by expanding its octet. This is shown by the action of \(SiF_{ 4 }\) with \(F^{ - }\) to form fluosilicate ion. $$ SiF_{ 4 } + 2F^{ - } \longrightarrow { \left[ SiF_{ 6 } \right] }^{ -2 } $$ (iv) Molecules with a Multiple Bond Between Atoms of Dissimilar Electronegativities: Examples of such substances are \(CO_{ 2 }, SO_{ 2 }\), etc. In \(CO_{ 2 }\), oxygen atoms are more electronegative than the carbon atom. As a result, the electron density due to the \(\pi\) electrons is displaced away from the carbon atom towards the oxygen atoms. The carbon atom is electron deficient and can accept electron pair from a Lewis base. The reaction of \(CO_{ 2 }\) with hydroxyl ions can be represented as $$ O=C^{ +\delta }={ O }^{ -\delta } + { \left[ \overset { \cdot \cdot }{ \underset { \cdot \cdot }{ :O } } -{ H } \right] }^{ - } \longrightarrow { \left[ \overset { O || }{ \underset { | O }{ C } } -\overset { \cdot \cdot }{ \underset { \cdot \cdot }{ O } } -H \right] }^{ - } $$ Similarly, lewis bases can be classified into three categories:
(i) All the anions are Irwis bases,
(ii) Molecules in which an atom has one or two unshared pairs of electron,
(iii) Molecules with carbon-carbon double bonds are lewis bases.