The basicity of aromatic amines depends on the position and nature of the substituent on the benzene ring. The net effect of the electron density of nitrogen is determined by the ortho and para positions (not the meta position) of the group. The inductive effect (-I or +I) or resonance effect (-R or +R) of the group, which operates at the position of the aromatic ring, determines its nature.
It has been found that
1. Electron releasing group:
Such as -CH3, -OCH3, -OH, and -NH2 increase the basicity of amines. This is due to the fact that electron-releasing groups tend to increase the electron density in the amino group. Thus, the lone pair of electrons on nitrogen becomes more available for sharing with an acid. At the same time, the substituent also stabilises the substituted anilinium ion by the dispersal of positive charge on nitrogen and thus increases its stability relative to parent amine. It is, therefore, quite natural that electron-releasing substituents increase the basicity of aromatic amines.
2. Electron withdrawing group:
Such as -NO2, -CN, and -X, on the other hand, decrease the basic strength by decreasing the electron density on the amino group, and thus, the lone pair of electrons becomes less available for sharing with an acid. Moreover, these substituents also do not satisfy the positive charge on substituted anilinium ion and decrease the stability relative to parent amine.