18.14 : Ativadores Orto-Para-Dirigentes: –CH_3, –OH, –⁠NH_2, –OCH_3

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.