18.14:
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3
With the exception of halogens, all ortho–para directors are known as activating groups as they increase the reactivity of the aromatic ring towards electrophilic substitution.
For instance, nitration of anisole is faster than benzene under comparable conditions.
Recall that electrophilic aromatic substitution involves the formation of resonance-stabilized carbocation intermediates.
Electron-donating groups effectively stabilize the ortho and para intermediates through resonance effects via pi-donation, lowering the energy of the transition state and making the reaction go faster than the corresponding reaction of benzene.
The meta intermediate, on the other hand, has much higher energy.
Therefore, electron-donating groups activate the benzene ring towards ortho and para substitution.
In general, lone pair containing activators display an electron-donating resonance effect. They are stronger activating groups than groups that do not have lone pairs and show an electron-donating inductive effect.
18.14:
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3
All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding intermediates through resonance effects via pi-donation. As a result, the energy of the transition state is lowered for ortho and para intermediates, leading to an accelerated reaction.