18.15:
ortho–para-Directing Deactivators: Halogens
Halogens are ortho–para directors. However, they deactivate the aromatic ring towards electrophilic substitutions.
This unusual behavior results from the interplay of their electron-withdrawing inductive effect and electron-donating resonance effect.
Halogens are more electronegative than carbon. When present as substituents, they withdraw electrons from the aromatic ring by the inductive effect and deactivate it.
In comparison, the electron-donating resonance effect of halogens influences the orientation.
When an electrophile adds to the aromatic ring at the ortho or para position, a carbocation forms adjacent to the halogen atom. The halogen then donates its lone pair and delocalizes the charge through a halonium ion, followed by other contributing resonance structures.
In contrast, a meta attack does not generate the halonium ion resonance form.
Consequently, the ortho and para forms are lower in energy and form faster.
Overall, halogens have a stronger electron-withdrawing inductive effect but a weaker electron-donating resonance effect; they are deactivators yet ortho–para directors.
18.15:
ortho–para-Directing Deactivators: Halogens
Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate through the halonium ion. The resonance forms arising from the meta attack do not generate the halonium ion. Subsequently, the energies of ortho and para forms are lower, and these form faster. In summary, a weaker electron-donating resonance effect of the halogens makes them ortho–para directors, and a more substantial electron-withdrawing inductive effect makes them deactivators.