Heterolytic reaction in which the entering group adds to a nucleophile and in which the leaving group, or electrofuge, relinquishes both electrons to its reaction partner, whereupon it becomes another potential electrophile.

Example: Azo coupling: $$\begin{array}{r}\mathrm{RN}{\phantom{A}}_2{\phantom{A}}^{+}+\mathrm{H}{\phantom{A}}_2\mathrm{NC}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5⟶\mathit{\text{p}}\text{-}\mathrm{H}{\phantom{A}}_2\mathrm{NC}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_4\mathrm{N}=\mathrm{NR}+\mathrm{H}{\phantom{A}}^{+}\\ (\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{i}\mathrm{l}\mathrm{e})(\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{i}\mathrm{l}\mathrm{e})(\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{u}\mathrm{g}\mathrm{e})\end{array}$$

Note: It is arbitrary to emphasize the electrophile and ignore the feature that this is also a nucleophilic substitution, but the distinction depends on the electrophilic nature of the reactant that is considered to react with the substrate.