Heterolytic reaction in which an entering group adds to the electrophilic part of the substrate and in which the leaving group, or nucleofuge, retains both electrons of the bond that is broken, whereupon it becomes another potential nucleophile.

Example: \[\ce{CH3Br (substrate) + HO^{-} (nucleophile) -> CH3OH + Br^{-} (nucleofuge)}\]

Notes:

1. It is arbitrary to emphasize the nucleophile and ignore the feature that this is also an electrophilic substitution, but the distinction depends on the nucleophilic nature of the reactant that is considered to react with the substrate.
2. Nucleophilic substitution reactions are designated as \(\rm{S}_{\rm{N}}1\) or \(\rm{S}_{\rm{N}}2\), depending on whether they are unimolecular or bimolecular, respectively. Mechanistically, these correspond to two-step and one-step processes, respectively. \(\rm{S}_{\rm{N}}1\) reactions follow first-order kinetics but \(\rm{S}_{\rm{N}}2\) reactions do not always follow second-order kinetics.