12.2 Electrophillic substitution reactions

• Aryl halides are compounds where the halogen is directly attached to the benzene ring. Halogens are electron withdrawing in nature as they show $-I$ effect. (that is they tend to pull the electron pair towards themselves). Hence they are ring deactivators.
  

• Ring deactivators are species which decrease the reactivity of the ring towards electrophilic substitution. In electrophilic substitution, carbocation formation takes place. The presence of an electron withdrawing group on the ring destabilizes the intermediate formed, and hence it deactivates the ring. When an electron withdrawing group is attached to the benzene ring, electrophilic attack usually takes place at the meta position.
  

• However, though halogens are ring deactivators, they attack at the ortho and para position. This is because halogens show an electron releasing nature due to resonance effect. The lone pair on the halogens participates in delocalization with the pi-electrons of the benzene ring. This electron release, towards the ring, through resonance, stabilizes the intermediate carbocation formed by ortho and para attack. As a result, electrophilic substitution is favoured at ortho and para positions.
  

• From the diagrams below, it is evident that electrophilic substitution is facoured at ortho and para positions the lone pair of the halogen participates in delocalization to stablilze the carbocation formed.
  
  
  
  

Hence, in all aromatic electrophilic substitution involving haloarenes, the electrophile will attack at ortho and para positions only.

Aryl halides undergo the following electrophilic substitution reactions:

• Nitration

• Halogenation

• Sulphonation

• Friedel-Craft's alkylation

• Friedel-Craft's acylation

The electrophilic substitution reaction occurs slowly in haloarenes and requires more drastic conditions than electrophilic substitution reactions in benzene. This is because of resonance in haloarenes. Resonance causes the electron density at the ortho and para positions to be greater than that at meta position. Additionally, the halogen has a tendency to withdraw electrons from benzene ring and thus the ring gets deactivated. Thus, the electrophilic substitution reactions occur more slowly as compared to those in benzene.

Through $-I$ effect, chlorine withdraws electrons and it releases electrons through resonance. The formation of carbocation during electrophilic substition is destabalised due to the inductive effect of $Cl$ or any other halogen. Through resonance, halogen tends to stabilize the carbocation.

Destabalization of the intermediate carbocation because of inductive effect.

(a) Attack at ortho position

(b) Attack at para position

• In electrophilic substitution of chlorobenzene, para substituted product is the major product.

• Reactivity is controlled by the inductive effect and orientation is controlled by the resonance effect, i.e., the inductive effect of halogens deactivates the ring and decreases its reactivity towards electrophilic substitution and the $+R$ effect of halogens makes it an ortho and para director.