My previous post related to the aromatic electrophilic substitution of benzene using as electrophile phenyl diazonium chloride. Another prototypical reaction, and again one where benzene is too inactive for the reaction to occur easily, is the catalyst-free bromination of benzene to give bromobenzene and HBr.

Every introductory course or text on aromatic electrophilic substitution contains an explanation along the lines of the resonance diagram shown below. With an o / p directing group such as NH ₂ , it is argued that negative charge accumulates in those positions as a result of the resonance structures shown. MEP for PhNH2. Click for 3D.The opposite occurs for electron withdrawing groups.