Aromatic amines exhibit basicity due to the presence of the amino group (-NH2) attached to the benzene ring. This amino group can accept a proton (H⁺) to form an ammonium ion (NH3+). Bing is basic in nature; they react with acid to form salt.
In aliphatic amines, the non-bonding electron pair of nitrogen is localized and fully available for coordination with a proton. In contrast, in aromatic amines, the non-bonding electron pair is delocalized into a benzene ring through resonance.
The non-bonding electron pair in the hybrid is dispersed over the benzene ring, making it less available for protonation. Therefore, aromatic amines are less basic than aliphatic amines, which do not exhibit resonance.
Several factors influence the basicity of aromatic amines:
1. Resonance Stabilization: The lone pair of electrons on the nitrogen atom in the amino group can participate in resonance with the benzene ring, stabilizing the resulting ammonium ion. This resonance delocalization enhances the basicity of the amine.
2. Inductive Effect: The amino group can donate electron density to the benzene ring through the sigma bonds, making the ring more electron-rich. This electron-donating effect increases the basicity of the amine.
3. Substituent Effects: Substituents on the benzene ring can influence the basicity of aromatic amines. Electron-donating groups (e.g., alkyl groups) increase the basicity by further enhancing the electron density on the amino group. Conversely, electron-withdrawing groups (e.g., nitro groups) decrease the basicity by withdrawing electron density from the amino group.
4. Steric Hindrance: Bulky substituents near the amino group can hinder the approach of a proton, reducing the basicity of the amine.
In conclusion, the basicity of aromatic amines depends on the position and nature of the substituent present on the benzene ring. The ortho and para positions of the group, not the meta position, determine its net effect on the electron density of nitrogen. The nature of the group determines its inductive effect (whether it is electron-withdrawing (-I) or electron-donating (+I)) or resonance effect (-R or +R) which operates at the position on the aromatic ring.