Aromatic
-OH compounds are more acidic than aliphatic R-OH alcohols because
of the influence of the delocalized ring electrons. By the same
token, aromatic amines are weaker bases than the aliphatic amines
that we have just discussed. Methyl amine, ,
is similar in strength to ammonia; phenyl amine or aniline, ,
is a weak base with a
of only 4.63.
The reason is outlined to the right. In terms of the resonance structures
that can be drawn, the unprotonated aniline molecule resembles the
phenolate ion, and the protonated anilinium ion is like the phenol
molecule. All four electron pairs around the nitrogen in the anilinium
ion are tied up in sigma bonds to hydrogen or carbon, and none are
free for delocalization into the aromatic ring system. The loss
of a proton in the anilinium molecule leaves a lone pair on the
nitrogen atom that can be shared with the aromatic ring. As in the
phenolate ion, the delocalized system is expanded from six atoms
and six electrons to seven atoms and eight electrons. In both phenol
and aniline, increased delocalization favours the unprotonated
species, which are the phenolate ion and the neutral aniline molecule.
Phenol loses a proton easily to form the phenolate ion, and therefore
is a reasonably stong acid. Aniline only reluctantly accepts a proton
to form the anilinium ion, and hence is a weak base. When an -NH
group is attached to an aliphatic radical it receives no comparable
delocalization stabilization. It is less reluctant to accept a proton
on its nitrogen lone pair, and hence aliphatic amines are stronger
bases.