18.15 : Désactivateurs ortho–para-directeurs : Halogènes

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.