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.

The below diagram shows the position of ortho(o), para(p) and meta(m) derivatives of amino group:

Electrophilic addition reaction of amines: In addition to the reaction of the amino group (NH₂ group), aromatic amines also undergo typical electrophilic substitution reactions of the aromatic ring. In all these reactions, the NH₂ group strongly activates the aromatic ring through delocalization of the lone pair of electrons of the N-atom over the aromatic ring.

As a result, electron density increases more at ortho and para positions. Therefore NH₂ group directs the incoming group to ortho and para positions, i.e., NH₂ is an o-, p-directing group.

But it has been observed that on nitration of aniline gives a substantial amount of m-nitroanilne.

Explanation: Nitration is usually carried out in acidic medium in the presence of concentrated HNO₃ and concentrated H₂SO₄. As a result, most of the aniline is converted into anilinium ion(gets protonated) and since - ⁺NH₃ is m-directing group, therefore, an unexpected large amount of m-nitroaniline is obtained.

Nitration of anilne gives mainly p-nitroaniline

Nitration of anilinium ions give m-nitroanilne(due to protonation)

Hence, nitration of aniline gives a mixture of p-nitroaniline and m-nitroaniline in approx. 1:1 ratio.