Question

Why do haloarenes undergo electrophilic substitutions at o- and p-positions?

Answer 1

1. Haloarenes undergo electrophilic substitution preferentially at the ortho (o-) and para (p-) positions relative to the halogen substituent.
2. This regioselectivity is due to the electronic effects of the halogen atom attached to the aromatic ring.
3. Although halogens are electronegative and exhibit an electron-withdrawing inductive effect (-I effect), they also have lone pairs of electrons that can donate electron density to the aromatic ring through resonance (+R effect).
4. This resonance donation activates the ring towards electrophilic substitution and particularly stabilizes the intermediate sigma complex (arenium ion) formed when the electrophile attacks the ortho or para positions.