Question

What are the three types of Substitution?

Answer 1

Reaction Intermediates or Reagents

1. Electrophile - They are usually electron-deficient and account for their affinity towards electrons. For example, ${\mathrm{AlCl}}_3,{{\mathrm{NO}}_2}^{+},\mathrm{etc}.$ Such species when undergoing a chemical reaction are called Electrophilic Reactions.
2. Nucleophile - They are usually electron-rich species and account for sharing a pair of electrons. For example, ${\mathrm{NH}}_3,\mathrm{R}-\mathrm{CO}-{\mathrm{O}}^{-},\mathrm{etc}.$ Such species when undergoing a chemical reaction are called Nucleophilic Reactions.
3. Free Radical - Chemical species having one or more unpaired electrons are called free radicals. For example, ${{\mathrm{CH}}_3}^{.},{\mathrm{Cl}}^{.},\mathrm{etc}.$ Such species when undergoing a chemical reaction are called Free radical Reactions.

Substitution reaction

A chemical reaction in which an atom, nucleus, or group of atoms or ions is substituted by another atom, ion, or group of atoms or ions in a molecule of a compound is called a Substitution reaction.

Substitution reaction is of three types

1. Nucleophilic Substitution - In this, the new atom formed is electron-rich or acts as a nucleophile. In this, nucleophile gets attached to the positively charged atoms or molecules of the other substance. For example, hydrolysis of alkyl halide where nucleophile is a base and leaving group is Bro mine ion.
   $$\begin{gathered} {\mathrm{CH}}_3\mathrm{Br}\stackrel{\mathrm{Aq}\mathrm{NaOH}}{\to }{\mathrm{CH}}_3\mathrm{OH}+\hspace{0.17em}{\mathrm{Br}}^{-} \\ \mathrm{Methyl}\mathrm{Bromide}\mathrm{Methanol} \end{gathered}$$
2. Electrophilic Substitution - In this, the new atom formed is electron-deficient or acts as an electrophile. Generally, it replaces the functional groups in an aromatic compound except for hydrogen. For example, Substitution of a proton with nitro group in benzene to form Nitrobenzene.$$\begin{gathered} \mathit{}\mathit{}\mathit{}{C}_{\mathit{6}}{H}_{\mathit{6}}\mathit{}\underset{\mathit{C}\mathit{o}\mathit{n}\mathit{c}\mathit{.}\mathit{}\mathit{H}\mathit{N}{\mathit{O}}_{\mathit{3}}}{\overset{\mathit{C}\mathit{o}\mathit{n}\mathit{c}\mathit{}{\mathit{H}}_{\mathit{2}}\mathit{S}{\mathit{O}}_{\mathit{4}}}{\mathit{\to }}}{C}_{\mathit{6}}{H}_{\mathit{6}}N{O}_{\mathit{2}}\mathit{}\mathit{+}\hspace{0.17em}2{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Benzene}\mathrm{Nitrobenzene} \end{gathered}$$
3. Free Radical Substitution - In this reaction free radicals act as an intermediate. For example,
   $$\begin{gathered} {\mathrm{CH}}_4+\hspace{0.17em}{\mathrm{Cl}}^{·}\to {{\mathrm{CH}}_3}^{.}+\hspace{0.17em}\mathrm{HCl} \\ {\mathrm{CH}}_3^{·}+\hspace{0.17em}{\mathrm{Cl}}^{·}\to {\mathrm{CH}}_3\mathrm{Cl} \\ \mathrm{Methyl}\mathrm{Chloride} \end{gathered}$$