Aniline upon heating with conc. HNO_3 and conc. H_2SO_4 mixture gives

The correct answer is Option (3) → Black tarry mass

The –\( \ce{NH2} \) group makes the ring very electron rich and easily oxidised. Concentrated \( \ce{HNO3} \) is a strong oxidising agent, so direct nitration of aniline mostly gives tarry oxidation products (a black tarry mass), along with some nitro derivatives. In the strongly acidic mixture aniline is also protonated to the anilinium ion, a meta director:

    \( \displaystyle \ce{C6H5NH2 + H+ -> C6H5NH3+} \)

To get p-nitroaniline, the –\( \ce{NH2} \) group is first protected by acetylation, then nitrated and hydrolysed.

Why the other options are incorrect

• Option (1) o- and p-nitroaniline — Incorrect. A clean o/p mixture is not obtained; oxidation dominates and protonation also gives a large amount of the meta product.

• Option (2) o-Nitroaniline — Incorrect. It is a minor product at best (about 2%).

• Option (4) 2,4,6-trinitroaniline — Incorrect. Trisubstitution is what bromine water does (2,4,6-tribromoaniline); with the nitrating mixture the ring is oxidised instead.

NCERT (2026-27 edition):
Nitration – “Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amount of meta derivative is also formed.”