18.12:
Directing Effect of Substituents: ortho–para-Directing Groups
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.
18.12:
Directing Effect of Substituents: ortho–para-Directing Groups
Ortho–para directors are substituent groups attached to the benzene ring and direct the addition of an electrophile to the positions ortho or para to the substituent. All electron-donating groups are considered ortho–para directors. They donate electrons to the ring and make the ring more electron-rich. The ring is therefore susceptible to the addition of electrophiles. Substituents such as amino, hydroxy, or alkoxy, containing lone pairs on the atom adjacent to the ring, donate electrons through resonance. For instance, phenol can undergo nitration at either ortho, meta, or para positions. However, the ortho and para carbocation intermediates are more stable than the meta intermediate because they have more resonance forms. Moreover, a particular favorable form in ortho and para carbocation intermediates exists—the one arising from the donation of oxygen’s nonbonding electrons.
Alkyl substituents without lone pairs also function as ortho–para directors. They donate electrons through an inductive effect. The carbocation intermediates involved in the ortho–para attack are more stable than meta. The ortho–para directors function by stabilizing a positive charge directly on the methyl-substituted carbon. Since it is a tertiary position, the positive charge can be stabilized by the electron-donating inductive effect of the methyl group.