Question

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

Answer 1

Hint: Chlorine donates lone pairs of electrons to the aromatic ring and hence it increases the electron density at Ortho and para positions and the lone pair of an electron in chlorine atom stabilizes the intermediate carbocation due to resonance.


Complete step by step solution:
The -I effect of chlorine withdraws electrons from the Benzene ring and Hence tends to destabilize the intermediate carbocation formed during Electrophilic substitution.
Conversely, Chlorine donates lone pairs of electrons to the aromatic ring
Hence it increases the electron density at Ortho and para positions
The lone pair of an electron in a chlorine atom stabilizes the intermediate carbocation due to resonance.
Since- I effect of Chlorine is stronger than it's +R effect hence Cl causes net deactivation.
Further Resonance effect opposes the inductive effect, it makes deactivation less at Ortho and para positions.
Since in Chlorobenzene, reactivity is controlled by stronger -I effect while the orientation of incoming substituent group is controlled by a weaker resonance effect.
Chlorine withdraws electrons through inductive effect and releases electrons through resonance.
Hence, chlorine is ortho, para-directing in electrophilic aromatic substitution reaction.

Note: Students can confuse between -I and +R effects of chlorine atoms on benzene rings. Make sure to remember that -I of chlorine is much stronger than +R effect and it takes place in case of chlorine otherwise +R effect is much stronger than -I effect.