Question

Account for the following.

Although amino group is o- and p-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.

Answer 1

Nitration is usually carried out with a mixture of conc. HNO₃ and conc. H₂SO₄. In the presence of these acids, most of the aniline gets protonated to form an anilinium ion. Therefore, in the presence of acids, the reaction mixture consists of aniline and anilinium ions.

Now –NH₂, a group in aniline is O, P-directing and activating while the –NH₃ group is anilinium ion is meta-directing and deactivating. Whereas nitration of aniline (due to steric hindrance at o-position) mainly gives p-nitroaniline, the nitration of anilinium ion gives m-nitro aniline. In actual practice, approximately a 1 : 1 mixture of P and m-nitro aniline is formed.