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

Explanation

Aniline is very reactive. The direct nitration of aniline is unsatisfactory because of the susceptibility of the ring towards oxidation with nitric acid. However, to carry out nitration, the activation of benzene ring is reduced by first acetylating aniline with acetyl chloride.

Explanation of reduced reactivity of `NH_(2)` group in acetanilide. In acetanilide the oxygen of the group withdraws electrons from `NH_(2)` group as shown below:

As a result, the electron pair on nitrogen gets displaced to the carboxyl group. Therefore, the unshared pair of electrons on nitrogen is less available for donation to the aromatic ring. Consequently, the electron density at ortho and para positions in the benzene ring gets reduced which in turn results in reduced reactivity towards electrophilic substitution of benzene.