Nucleophilic substitutions are chemical reactions where a nucleophile, which is a species that donates an electron pair, replaces a leaving group in a molecule. This process typically occurs in organic compounds, where the nucleophile attacks a carbon atom that is bonded to a leaving group, resulting in the formation of a new bond and the departure of the leaving group. There are two main types of nucleophilic substitutions: S_N1 and S_N2. In S_N1 reactions, the leaving group departs first, creating a carbocation intermediate, while in S_N2 reactions, the nucleophile attacks the carbon simultaneously as the leaving group departs. These mechanisms are fundamental in organic chemistry and are essential for synthesizing various compounds.