A Rationale for the Ortho Effect in Electrophilic Aromatic Substitutions

The ortho effect arises in certain electrophilic aromatic substitutions when a meta director is meta to an ortho/para director. Among the three expected isomer products, only the two isomers ortho to the meta director are produced normally. To find an explanation for the ortho effect, nitrations of two properly substituted benzenes were explored. Evidence supported an explanation based on resonance involving the meta director.