Nucleophilic substitution is a chemical reaction in which a nucleophile replaces a leaving group in a molecule.

Amines are capable of undergoing nucleophilic substitution reactions due to the presence of a lone pair of electrons on the nitrogen atom. In an $S_N2$ reaction, the nucleophile directly attacks the carbon atom that is attached to the leaving group. This interaction causes the leaving group to depart while forming a new bond with the nucleophile, resulting in the creation of a new amine molecule. Conversely, in an $S_N1$ reaction, the leaving group first detaches, leading to the formation of a carbocation intermediate. Subsequently, this carbocation is attacked by the nucleophile, yielding the final product.

The mechanism of nucleophilic substitution consists of several distinct steps. Initially, the nucleophile attacks the carbon atom bonded to the leaving group, which prompts the departure of the leaving group. This action creates a transition state in which both the nucleophile and the leaving group are partially bonded to the carbon atom. In the subsequent step, the nucleophile forms a new bond with the carbon atom, resulting in the formation of the final product.

In addition to reacting with carbon atoms attached to leaving groups, amines can also participate in nucleophilic substitution with other electrophiles, such as alkyl halides, acyl halides, and carbonyl compounds. In these scenarios, the lone pair of electrons on nitrogen acts as a nucleophile, attacking the electrophilic carbon atom. This results in the formation of a new bond and the displacement of a leaving group.

In summary, nucleophilic substitution is a significant reaction for the synthesis and modification of amines. Understanding its mechanism is essential for comprehending the reactivity of these important biomolecules.