Although chlorine is an electron withdrawing group, yet it is ortho-, para- directing in electrophilllic aromatic subsititution reactions. Why?

Explanation

This can be explained by considering relative stabilities of the `sigma` - complexes formed during ortho-, para- and metasubstitution.

The `sigma` -complexes for ortho - and para- substitution are stabilised by four resonating structures, while the `sigma` -complex for meta-substitution is stabilised by only three resonating structures. In order words, the `sigma` -complexes for o- and p- substitution are more stable than that for the metasubstitution. Hence, chlorobenzene undergoes electrophillic substitution preferably at the ortho and para positions.
However, Cl -atom withdraws electron from the ring by -1 effect and at the same time it icreasese electron density in the ring by +R effect. Since -I effect of Cl is stonger than that of its +R effect, the ring system becomes deactivated. Hence, chlorobenzene undergoes electrophilic substitution at a much slower rate than unsubstituted benzene.