19.8:
Basicity of Aromatic Amines
The basicity of aromatic amines is influenced by the delocalization of the lone pair and the electron-withdrawing inductive effect of the sp2 hybridized carbons of the phenyl ring.
Due to resonance stabilization, the lone pair is less available for protonation. As aniline is more stable than its conjugate acid, the anilinium ion, the energy difference between them is high.
In aliphatic amines, with no resonance stabilization, the localized lone pairs are readily involved in protonation, leading to a lower energy difference between the amine and its conjugate acid. Thus, aliphatic amines are stronger bases than aromatic amines.
The basicity of substituted anilines depends on the nature of the substituent.
Electron-donating groups activate the benzene ring, which increases the basicity of aromatic amines.
Electron-withdrawing groups deactivate the ring, significantly decreasing the basicity of aromatic amines.
Substituting the amine hydrogens with aromatic rings further reduces the basicity of aromatic amines.
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.