Chapter 24.SE, Problem 52AP

Interpretation Introduction

Interpretation:

Drawing resonance structures why p-nitroaniline is less basic than m-nitroaniline by a factor of 30 is to be explained. Given that the pKa values of the corresponding ammonium ions are 1.0 and 2.5 respectively.

Concept introduction:

Amines are basic as they can donate the lone pair if electrons on the N atom. The basic strength of amines depends on the availability of lone pair of electrons on nitrogen. Generally aryl amines are less basic than alkyl amines. An electron withdrawing substituent present on the aromatic ring reduces the base strength while an electron releasing substituent present on the aromatic ring enhances the base strength.

To account::

By drawing resonance structures why p-nitroaniline is less basic than m-nitroaniline by a factor of 30. Given that the pKa values of the corresponding ammonium ions are 1.0 and 2.5 respectively.

Answer:

p-Nitroaniline is less basic than m-nitroaniline by a factor of 30 since in p-nitroaniline the nitro group is in conjugation with the amino group and hence resonance delocalization as shown is possible.

In m-nitroaniline such delocalization to nitro group is not possible and only inductive effect operates. Thus m-nitroaniline can donate electron faster than p-nitroaniline. Hence p-nitroaniline is weaker than m-nitroaniline.

Explanation:

The electron withdrawing inductive effect of the nitro group reduces the availability of lone pair of electrons on N of the amino group. But in p-nitroaniline in addition to inductive effect electron delocalization of the lone pair of the amino group in to the nitro group also is possible. In m-nitroaniline such a delocalization to the nitro group is not possible. Thus in comparison, the lone pair on N of the amino group is less available for donation in p-nitroaniline than those in m-nitroaniline. Hence p-nitroaniline is less basic than m-nitroaniline.

Conclusion:

p-Nitroaniline is less basic than m-nitroaniline by a factor of 30 since in p-nitroaniline the nitro group is in conjugation with the amino group and hence resonance delocalization as shown is possible.

In m-nitroaniline such delocalization to nitro group is not possible. Thus m-nitroaniline can donate electron faster than p-nitroaniline. Hence p-nitroaniline is weaker than m-nitroaniline.