Directive Influence of Halogen in Electrophilic Substitution

A halogen substituent on a benzene ring shows a combination of electronic effects unusual among ring substituents: it deactivates the ring towards electrophilic substitution (making the reaction slower than with benzene itself) while simultaneously directing the incoming electrophile to the ortho and para positions.

Why halogens deactivate. A halogen's strong inductive electron-withdrawal (it is far more electronegative than carbon) pulls electron density out of the ring through the sigma-bond framework, lowering the overall electron density available to stabilise the positively charged arenium-ion (sigma-complex) intermediate that forms during electrophilic attack. Since this inductive effect operates through every position of the ring, it slows electrophilic substitution at every position relative to unsubstituted benzene -- chlorobenzene, for instance, reacts more slowly with electrophiles than benzene does.

Why halogens still direct ortho/para. Despite this net deactivation, one of the halogen's lone pairs can donate into the ring by resonance specifically when the electrophile attacks the ortho or para position, because only for attack at those positions does a resonance structure exist in which the halogen's lone pair directly stabilises the positive charge of the arenium-ion intermediate (by forming a temporary $\text{C=X}^+$-type resonance contributor). No such stabilising resonance structure is available for attack at the meta position, so the meta arenium ion is comparatively higher in energy (less stabilised) and meta substitution is strongly disfavoured. The resonance donation, though weaker than the inductive withdrawal in its effect on overall rate, is what controls where substitution occurs. …