18.12 : 置換基の方向性効果: オルトパラ方向性基

Substituents present on the benzene ring that direct the incoming group to a position ortho or para to itself during electrophilic substitution are ortho–para directors.

These substituents work by donating electrons to the benzene ring, thereby increasing the reactivity of the ring towards the electrophile.

Groups with unshared electron pairs on the atom adjacent to the ring donate electrons through resonance.

For example, nitration of phenol occurs more rapidly at the ortho–para positions than at meta because ortho and para carbocation intermediates are more stable. They have more resonance forms, including one stabilized by the oxygen's nonbonding electrons.

Groups without unshared electron pairs donate electrons through the inductive effect.

For instance, during the nitration of toluene, the carbocation intermediates involved in the ortho–para substitution are more stable than meta. The positive charge on the tertiary carbon is stabilized by the electron-donating inductive effect of the methyl group.