Although chlorine is an electron withdrawing group, yet it is ortho, para-directing in electrophilic aromatic substitution reaction. Explain why it is so?

Chlorine is indeed an electron-withdrawing group, and in electrophilic aromatic substitution (EAS) reactions, it generally acts as a meta-directing group. However, when it comes to chlorobenzene or benzene derivatives with chlorine substituents, the situation is a bit different. Chlorine is ortho, para-directing in certain cases due to resonance effects.

In the case of chlorobenzene, the lone pairs on the chlorine atom can participate in resonance with the benzene ring. The resonance structures that result show that the negative charge (or electron density) is located at the ortho and para positions relative to the chlorine substituent. This is because the chlorine lone pairs can delocalize into the aromatic ring, leading to resonance-stabilized structures.

The resonance structures depict a partial double bond character between carbon and chlorine, and this helps distribute the negative charge across the ortho and para positions, making those positions more nucleophilic. As a result, in electrophilic aromatic substitution reactions on chlorobenzene, the electrophile is more likely to attack at the ortho and para positions due to the increased electron density in those regions.

It's important to note that this ortho, para-directing behavior is specific to chlorobenzene or benzene derivatives with chlorine substituents, and electron-withdrawing groups in general are usually meta-directing in EAS reactions. The unique behavior of chlorobenzene is a consequence of the resonance stabilization provided by the interaction between the chlorine lone pairs and the benzene ring.