Write a note on nucleophilic substitution reactions.

(N/A) reaction in which a nucleophile replaces an already existing nucleophile in a molecule is called a nucleophilic substitution reaction.
In these reactions,haloalkanes are the substrate and the nucleophile reacts with a substrate having a partial positive charge on the carbon atom bonded to the halogen atom. $A$ substitution reaction takes place and the halogen atom,called a leaving group,departs as a halide ion. Since the substitution reaction is initiated by a nucleophile,it is called a nucleophilic substitution reaction $(S_N)$.
The general reaction is represented as:
$:Nu^- + -C^{\delta+} - X^{\delta-} \rightarrow -C - Nu + X:^-$
The nucleophilic substitution reactions are of two types:
$(i)$ Unimolecular Nucleophilic Substitution reaction $(S_N^1)$.
$(ii)$ Bimolecular Nucleophilic Substitution reaction $(S_N^2)$.