Chemistry Reference
In-Depth Information
from deprotonation, thus increasing its acidity and lowering its pK
a
. In actuality, the
pK
a
of p-nitroaniline is 1.0, thus supporting the conclusion of this problem.
c. m-NO
2
The structure of m-nitroaniline is shown below with partial charges assigned to the
ring system. Because the electron-withdrawing nitro group is located meta to
the amine, electron density is increased adjacent to the amine functionality, effec-
tively rendering the aromatic ring electron donating meta to the nitro group. An
electron-donating group attached to an amine destabilizes the anion resulting from
deprotonation, thus decreasing its acidity and raising its pK
a
. In actuality, the pK
a
of
m-nitroaniline is 2.47, thus supporting the conclusion of this problem reflecting the
electron-withdrawing nature of the nitrophenyl group. While this value does not strictly
support the conclusion of this problem, the trend, compared to Problems 4(a) and 4(b),
indicates that the m-NO
2
has less of an effect on acidity than o-NO
2
and p-NO
2
.Infact,
NO
2
groups are so electron-withdrawing that they render the phenyl ring electron-
withdrawing in its entirety.
d. p-NH
2
The structure of p-aminoaniline is shown below with partial charges assigned to
the ring system. Because the electron-donating amino group is located para to the
amine, electron density is increased adjacent to the amine functionality, effectively
rendering the aromatic ring electron donating para to the amino group. An
electron-donating group attached to an amine destabilizes the anion resulting from
deprotonation, thus decreasing its acidity and raising its pK
a
. In actuality, the pK
a
of p-phenylenediamine is 6.2, thus supporting the conclusion of this problem.
