What is meant by a substitution reaction? Give an example (with equation) of the substitution.

Hint: There are two types of reaction viz. Addition reaction and substitution reaction. Substitution means, removal of existing species and addition of a foreign species to the substrate. It involves displacement of an existing group with the incoming group.

Complete Solution:
Let us understand what a substitution reaction is. The substitution reaction is the reaction in which the functional group of one chemical compound is substituted or replaced by the functional group of another chemical compound.
- These types of reaction are said to possess primary importance in the field of organic chemistry. For example, when \[C{H_3}Cl\] is reacted with the hydroxyl ion \[O{H^ - }\], it will lead to the formation of the original molecule called methanol with that hydroxyl ion. The following reaction is as shown below-
\[C{H_3}Cl + O{H^ - } \to C{H_3}OH + C{l^ - }\]
- Another example of substitution reaction will be the reaction of Ethanol with the hydrogen iodide which forms iodoethane along with water. The reaction is as shown as-
\[C{H_3}C{H_2}OH + HI \to C{H_3}C{H_2}OI + {H_2}O\]

There are certain conditions for a substitution reaction to occur. They are
- Maintaining low temperatures such as room temperature.
- The strong base such as NaOH has to be in dilute form. If NaOH is in the concentrated form, there are chances for the dehydrohalogenation to take place.
- The solution needs to be in an aqueous state such as water.
Substitution reactions are of two types, depending upon the attacking species. The two types are nucleophilic substitution and electrophilic substitution.

Note: Nucleophile substitution is of two types, depending upon uni molecularity and bi molecularity. \[{S_N}2\] is a bimolecular reaction, in this reaction, the elimination of the leaving group and the addition of the nucleophile occur simultaneously. In the \[{S_N}1\] reaction, there is an intermediate formation after removal of the leaving group, and then nucleophile is added. It is a unimolecular reaction.