18.19: 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.
