Chemistry Reference
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carboxylic acid, electron density is reduced adjacent to the acid functionality, effec-
tively rendering the aromatic ring electron withdrawing ortho to the nitro group. An
electron-withdrawing group attached to a carboxylic acid stabilizes the anion result-
ing from deprotonation, thus increasing its acidity and lowering its pK
a
. In actuality,
the pK
a
of o-nitrobenzoic acid is 2.16, thus supporting the conclusion of this
problem.
b. p-NO
2
The structure of p-nitrobenzoic acid is shown below with partial charges assigned to
the ring system. Because the electron-withdrawing nitro group is located para to the
carboxylic acid, electron density is reduced adjacent to the acid functionality, effec-
tively rendering the aromatic ring electron withdrawing para to the nitro group. An
electron-withdrawing group attached to a carboxylic acid stabilizes the anion result-
ing from deprotonation, thus increasing its acidity and lowering its pK
a
. In actuality,
the pK
a
of p-nitrobenzoic acid is 3.41, thus supporting the conclusion of this
problem.
c. m-NO
2
The structure of m-nitrobenzoic acid is shown below with partial charges assigned to
the ring system. Because the electron-withdrawing nitro group is located meta to the
carboxylic acid, electron density is increased adjacent to the acid functionality,
effectively rendering the aromatic ring electron donating meta to the nitro group.
An electron-donating group attached to a carboxylic acid destabilizes the anion
resulting from deprotonation, thus decreasing its acidity and raising its pK
a
. In actu-
ality, the pK
a
of m-nitrobenzoic acid is 3.47, reflecting the electron-withdrawing
nature of the nitrophenyl group. While this value does not strictly support the con-
clusion of this problem, the trend, compared to Problems 5(a) and 5(b), indicates
