Aniline does not undergo
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Friedel – Crafts alkylation
The –\( \ce{NH2} \) group of aniline is strongly activating and ortho/para directing, so aniline undergoes most electrophilic substitution reactions very easily. Friedel-Crafts reactions, however, need the Lewis acid \( \ce{AlCl3} \) as catalyst. The nitrogen of aniline is a Lewis base: it donates its lone pair to \( \ce{AlCl3} \) and forms a salt.
\( \displaystyle \ce{C6H5NH2 + AlCl3 -> [C6H5NH2AlCl3]}\ (\text{salt}) \)
The nitrogen now carries a positive charge, which turns it into a strongly deactivating group, and the catalyst is used up. So aniline does not undergo Friedel-Crafts reactions (alkylation or acylation).
Option (1) Sulphonation — Aniline undergoes this reaction. With concentrated \( \ce{H2SO4} \) it forms anilinium hydrogensulphate, which on heating at 453–473 K gives sulphanilic acid (p-aminobenzenesulphonic acid).
Option (2) Friedel – Crafts alkylation — Aniline does not undergo this reaction. It forms a salt with the catalyst \( \ce{AlCl3} \); the positively charged nitrogen deactivates the ring.
Option (3) Bromination — Aniline undergoes this reaction. With bromine water at room temperature it gives a white precipitate of 2,4,6-tribromoaniline:
\( \ce{C6H5NH2 + 3Br2 -> C6H2Br3NH2 + 3HBr} \)
Option (4) Nitration — Aniline undergoes this reaction. Direct nitration gives o-, p- and a substantial amount of m-nitroaniline, because in the strongly acidic medium part of the aniline is protonated to the meta-directing anilinium ion.
NCERT (2026-27 edition):
Electrophilic Substitution – …
