Why is `NH_(2)` group of aniline acetylated before carrying our nitration

Explanation

Due to the strong activating effect of `NH_(2)` group, aromatic amines readily undergo ele3ctrophilic 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 effet of the `-NH_(2)` 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 effect of the -`NH_(2)` 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 `Br_(2)//CH_(3)COOH` followed by acid hydrolysis gives p-bromoaniline as the major products. Similarly, nitration of acetanilide with a mixture of conc. `HNO_(3)+conc.H_(2)SO_(4)` followed by acid hydrolysis gives pnitroaniline as the major product.