19.8: Basicity of Aromatic Amines
The basicity of aromatic amines is much weaker than that of aliphatic amines due to the involvement of the lone pair of electrons over the N atom in resonance with the aryl rings. Generally, the electron-donating ability of any substituents on the aryl ring of aromatic amines increases the basicity of the amine by increasing electron density, and hence the availability of lone pair on the nitrogen. On the other hand, electron-withdrawing functional groups on the aryl ring of amines decrease the basicity to a significant extent. For example, as depicted in Figure 1, the para –NO2 group decreases the basicity of amines by a factor of 3800 due to extensive delocalization of the lone pair of electrons in the amino group.
Figure 1. The electron delocalization in p-nitroaniline.
An additional factor responsible for the reduced basicity of arylamines compared to aliphatic amines is the electron-withdrawing inductive effect of the sp2-hybridized carbons of the aromatic ring as compared to the sp3-hybridized carbons of alkylamines. As shown in Figure 2, the substitution of arylamines with additional aryl rings significantly decreases the basicity of the arylamines. For this reason, diphenylamine and triphenylamine are less basic than aniline by a factor of 6300 and 108, respectively.
Figure 2. The effect of increasing substitution of aromatic rings on the basicity.