Question

Write short notes on ortho, para directors in aromatic electrophilic substitution reactions.

Answer 1

1. All the activating groups are ‘ortho-para’ directors.
   Example: – OH, – NH₂, -NHR, -NHCOCH₃, -OCH₃ -CH₃ – C₂H₅ etc.
2. Let us consider the directive influences of phenolic (-OH) group. Phenol is the resonance hybrid of following structures.
3. In these resonance structures, the (-) charge residue is present on ortho and para position of ring structure. It is quite evident that the lone pair of electron on the atom which is attached to the ring involves in resonance and makes the ring more electron rich than benzene. The electron density at ortho and para positions increases as compared to the meta position. Therefore phenolic group activates the benzene ring for electrophilic attack at ‘ortho’ and ‘para positions and hence -OH group is an ortho-para director and activator.
4. In aryl halides, the strong -I effect of the halogens (electron withdrawing tendency) decreases the electron density of benzene ring, thereby deactivating for electrophilic attack. However the presence of lone pair on halogens involved in the resonance with pi electrons of benzene ring, increases electron density at ortho and para position. Hence the halogen group is an ortho-para director and deactivator.