Question

Account for the following:

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

Give reasons for the following:

Although the –NH₂ group is o/p directing in an electrophilic substitution reaction, yet aniline on nitration gives a good yield of m-nitroaniline.

Answer 1

Nitration is carried out in an acidic medium. In an acidic medium, aniline is protonated to give anilinium ions (which are meta-directing).

For this reason, aniline on nitration gives a substantial amount of m-nitroaniline.

Answer 2

Nitration is usually carried out with a mixture of cone HNO₃ + 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 group in aniline is activating and p-directing, while the \[\ce{-NH+3}\] group in the anilinium ion is deactivating and m-directing. Nitration of aniline (due to steric hindrance at the o-position) mainly gives p-nitroaniline; the nitration of anilinium ions gives m-nitroaniline. In actual practice, approx. a 1 : 1 mixture of p-nitroaniline and m-nitroaniline is obtained. Thus, nitration of aniline gives a substantial amount of m-nitroaniline due to protonation of the amino group.