Question

Chlorobenzene is o, p – directing in electrophilic substitution reaction. The directing influence is explained by which of the following?
A.+M of Ph
B.+I of Cl
C.+M of Cl
D.+I of Ph

Answer 1

The ortho/ para directing ability in electrophilic substitution reaction of chlorobenzene is due to the positive resonance effect. In order to solve the question, we must remember that chlorine is ortho and para directing in electrophilic substitution reaction as it is an electron withdrawing group.

Complete step-by-step answer: - Positive resonance effect, also known as the +R or +M effect is expressed by the electron donating groups. These electron withdrawing groups withdraw electrons from the rest of the molecule by delocalization of electrons within the molecule.
- Negative resonance effect happens when groups withdraw elements from the molecule, due to the process of delocalization. These groups are usually denoted by -R or -M.
- Positive inductive effect or +I effect occurs when an alkyl group which has the tendency to release or donate electrons, is introduced to a carbon chain, the charge is relayed through the chain.
- Negative inductive effect or -I effect of say, chlorine withdraws electrons from benzene rings. Hence tends to destabilize the intermediate carbocation formed during the electrophilic substitution.
- In chlorobenzene, orientation of incoming substitution is controlled by the weaker resonance effect while reactive is controlled by stronger -I effect.
- The -I effect of chlorine is stronger than its +R effect, hence chlorine causes net deactivation.
- Since resonance effect opposes the inductive effect, it makes deactivation less at ortho and para positions.
- Therefore, in electrophilic substitution reactions of chlorobenzene, the ortho/ para directing ability of chlorine is due to the positive resonance effect (+M of Cl).

So, the correct option is C.

Note: The groups that show +M effect are activating but halogens are deactivating due to -I. generally groups that show +M also show +I and the groups that show -M also show -I but halogens show +M with -I. Resonance effects tell the electron donation of withdrawing power on the basis of lone pair or charge.