(i) The lone pair of nitrogen will get involved in resonance with the carbonyl group. Hence it will reduce the activity of benzene ring in aniline. The resonance involved is as under : (ii) Aromatic amines are far less basic than aliphatic amines. This can be explained as follows:
Resonance stabilization is there in aniline. It can be regarded as a resonance hybrid of these structures:
Hence, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring and thus is less available for protonation. The electron density on the nitrogen atom is increased by electron-donating inductive effect of the alkyl groups. As a result, aliphatic amines are much stronger bases than aniline.
(iii) Nitration is usually carried out with a mixture of concentrated HNO3 and concentrated H2SO4. In the presence of these acids, most of aniline gets protonated to form anilinium ion. Therefore, in presence of acids, the reaction mixture consists of aniline and anilinium ion. Nitration of aniline due to stearic hindrance at ortho position, mainly gives para nitroaniline and the nitration of anilinium ion gives m-nitroaniline. In actual practice, approximately 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.