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

Electrophilic aromatic substitution: The reaction in which an atom that is attached to an aromatic system is replaced by an electrophile (electron-deficient species) is called an electrophilic aromatic substitution reaction.

Example: The formation of bromobenzene $({\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{Br})$ from benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$. In this reaction, electrophile ${\mathrm{Br}}^{+}$ replaced hydrogen atom from benzene ring.

When a monosubstituted benzene undergoes an electrophilic substitution, the orientation of incoming group and the rate of reaction is determined by the substituent already present on the benzene ring.

• An ortho-, para- directing group activates the benzene ring by increasing the electron density on ortho and para positions through +R effect. Example: Aniline $({\mathrm{C}}_6{\mathrm{H}}_5-{\mathrm{NH}}_2)$

• A meta directing group deactivates the benzene ring by decreasing the electron density on ortho and para positions through -R effect. Example: Nitrobenzene $({\mathrm{C}}_6{\mathrm{H}}_5-{\mathrm{NO}}_2)$

Halogens are ortho-, para- directing groups but they deactivate the benzene ring for further electrophilic substitution. This is because there are two opposing effects i.e., +R effect and -I effect. Halogens deactivate benzene ring because they withdraw electrons through inductive effect more strongly than they donate electrons through resonance effect. These are ortho-, para- directing due to +R effect.