Question

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

Answer 1

• The -I effect of chlorine withdraws electron from Benzene ring. Hence tends to destabilize the intermediate carbocation formed during Electrophilic substitution. Conversely, Cl donates it's lone pair electron to the aromatic ring and hence increase the electron density at Ortho and para positions. Thus the lone pair of an electron on chlorine atom stabilize the intermediate carbocation due to resonance.
  
• Since -I effect of Chlorine is stronger than it's +R effect hence Cl cause net deactivation. Also further since Resonance effect opposes the inductive effect, it makes deactivation less at Ortho and para positions.
• Thus in Chlorobenzene, reactivity is controlled by stronger -I effect while the orientation of incoming substituent group is controlled by a weaker resonance effect.