question_answer 62)

  Why is group of aniline acetylated before carrying out nitration?

Answer:

                  Due to the strong activating effect of  group, aromatic amines readily undergo electrophilic substitution reactions and it is difficult to stop the reaction at the monosubstitution stage. Usually the reaction occurs to give 2, 4, 6-trisubstituted amines. For- example, aniline on treatment with bromine water gives 2, 4, 6- tribromoaniline.                                                                               If, however, we wish to stop the reaction at the monosubstitution stage, the activating effect of the - group is reduced by acetylation. The acetyl group being       electron-withdrawing attracts the lone pair of electrons on the N-atom towards itself. As a result, the lone pair of electrons on the N-atom is not exclusively available for donation to the benzene ring and hence the activating electrons of the ? group is reduced. This method is known as protection of the amino group by acetylation and can be used to stop the reaction at the monosubstitution stage by preventing the formation of di- and trisubstitution products. For example, acetylation of aniline gives acetanilide. Bromination of acetanilide with followed by acid hydrolysis gives p-bromoaniline as the major product. Similarly, nitration of acetanilide with a mixture of cone.  + cone.  followed by acid hydrolysis give p-nitroaniline as the major product. For equations, refer to page 13/35.