Question

Why is nitration of aniline difficult?

Answer 1

Hint : During the nitration of aniline the Nitric Acid protonated the aniline to form the Anilinium ion. Since the nitrogen atom has no lone pair to conjugate, it has no mesomeric effect on the ring, but as nitrogen is protonated, and it has a high negative inductive effect. The nitration of aniline is difficult because aniline gets oxidized into protonated aniline.

Complete Step By Step Answer:
In case of nitration in aniline, the Nitric Acid protonates aniline to form the Anilinium ion. The positive charge in sigma complex stays the farthest in para position from the anilinium ion hence the most stable and is the major product
Direct nitration of aniline is not a feasible process because nitric acid oxidizes most of aniline to give tarry oxidation products along with only a small amount of nitrated products. By doing this it gets oxidized into protonated aniline which gives $47\%$ m-nitroaniline.
Aniline is also protected before nitration because the hurdles for the nitration of aniline are overcome by the protection of the amino group by acetylation. The acetyl group reduces the reactivity of the ring and thus its oxidation does not occur easily with nitric acid $HN{O_3}$ .

Note :
Aniline is ortho and para directing because the $N{H_2}$ group in aniline is ortho and para guiding group because due to resonance, they will release electrons to the ring and at the same time remove the electrons towards themselves due to $+ 1$ impact from the aromatic ring.