Chemistry Reference
In-Depth Information
NH
2
Benzene
Aniline
Figure 7.6
The structures of benzene and aniline.
The k
max
of benzene is 204 nm, whereas the k
max
of aniline is 230 nm.
This is due to the lone pair of electrons on the NH
2
interacting with the ring
electrons to increase the electron density throughout the ring, particularly
at the
ortho
and
para
positions of the ring, as shown in Figure 7.7.
+
NH
2
+
NH
2
+
NH
2
NH
2
-
-
-
Figure 7.7
The
M effect of aniline.
This
mesomeric
(or M) effect is seen when aniline is placed in a solu-
tion of pH 8-14, i.e. when the basic aniline is unionised. When aniline is
placed in a solution of pH
7, the k
max
returns to virtually the value
obtained for benzene (203 nm). What is happening is that aniline in acidic
solution reacts to form the anilinium salt. The lone pair of electrons on the
nitrogen is now involved in bond formation to an H
ion and can no longer
function as an auxochrome. The structure of aniline hydrochloride is shown
in Figure 7.8.
+
H
3
Cl
-
N
k
= 203 nm
max
Figure 7.8
The structure of aniline hydrochloride and its
k
max
value.
