NH 2
Aniline
−NH 2 group is attached
directly to the benzene ring
Physical properties of aniline
Aniline is a polar compound, and can form intermolecular hydrogen
bonding between two aniline molecules. Aniline has higher b.p. (184
C)
than nonpolar compounds of the same molecular weight. It also forms
hydrogen bonds with water. This hydrogen bonding accounts for the
solubility of aniline in water (3.7 g/100 g water).
N
H
H
N
H
H
O H
H
N
H
H
Intermolecular hydrogen bonding
in aniline
Intermolecular hydrogen bonding
of aniline with water
..
:
:
Basicity of aniline Aniline, like all other amines, is a basic compound
(K b ¼ 4.2 Â 10
À10 ). Anilinium ion has a pK a ¼ 4.63, whereas methylammonium ion has a pK a ¼ 10.66. Arylamines, e.g. aniline, are less basic than
alkylamines, because the nitrogen lone pair electrons are delocalized by
interaction with the aromatic ring p electron system and are less available
for bonding to H
þ . Arylamines are stabilized relative to alkylamines
because of the five resonance structures as shown below. Resonance
stabilization is lost on protonation, because only two resonance structures
are possible for the arylammonium ion.
NH 2
NH 2
NH 2
NH 2
NH 2
:
:
+
+
+
Resonance contributors of aniline
_
..
_
..
_
..
NH 3
NH 3
+
+
Arylammonium ion
4.6 AROMATIC COMPOUNDS AND THEIR DERIVATIVES
135