Chemistry Reference
In-Depth Information
4. As electron-donating and electron-withdrawing substituents will affect the acidity of
organic molecules, so will they affect the basicity. How will the following substituents
affect (raise, lower, or no change) the pK
a
of aniline (aminobenzene)? Explain using
partial charges to illustrate inductive effects. Remember,orefers to ortho positions,
m refers to meta positions, and p refers to the para position. In addressing these pro-
blems, assume that the acidity of the amine is influenced solely by the partial charges
induced by additional ring substituents.
Note: It is important to realize that, in addition to inductive effects, there are other
factors that influence acidity and pK
a
values. Therefore, while this problem asks for
expectations regarding how inductive effects influence pK
a
values, in actuality, the
measured values may be different than anticipated.
The pK
a
of aniline is 4.63
a. o-NO
2
The structure of o-nitroaniline is shown below with partial charges assigned to the
ring system. Because the electron-withdrawing nitro group is located ortho to
the amine, electron density is reduced adjacent to the amine functionality, effectively
rendering the aromatic ring electron withdrawing ortho to the nitro group. An
electron-withdrawing group attached to an amine stabilizes the anion resulting
from deprotonation, thus increasing its acidity and lowering its pK
a
. In actuality,
the pK
a
of o-nitroaniline is
2
0.26, thus supporting the conclusion of this problem.
b. p-NO
2
The structure of p-nitroaniline is shown below with partial charges assigned to the
ring system. Because the electron-withdrawing nitro group is located para to the
amine, electron density is reduced adjacent to the amine functionality, effectively
rendering the aromatic ring electron withdrawing para to the nitro group. An
electron-withdrawing group attached to an amine stabilizes the anion resulting
