If the pK_b value of p-nitroaniline is 13, predict the pK_b value of its ortho isomer?

14.22

The correct answer is option 2. 14.22.

To predict the \(pK_b\) value of the ortho isomer of p-nitroaniline based on the given information, we need to understand the effect of substituents on the basicity of aniline derivatives.

p-Nitroaniline (\( \text{p-NO}_2\text{C}_6\text{H}_4\text{NH}_2 \)):

The \(pK_b\) value of p-nitroaniline is given as 13. Nitro group (\( -\text{NO}_2 \)) is an electron-withdrawing group, which decreases the electron density on the nitrogen atom of the aniline ring. This electron withdrawal decreases the basicity of the aniline derivative, resulting in a higher pKb value (weaker base).

Ortho-Nitroaniline (\( \text{o-NO}_2\text{C}_6\text{H}_4\text{NH}_2 \)):

In the ortho isomer, the nitro group is positioned at the ortho position relative to the amino group. The electron-withdrawing effect of the nitro group in the ortho position is stronger compared to the para position due to closer proximity to the amino group. This stronger electron withdrawal further decreases the electron density on the nitrogen atom, reducing the basicity even more compared to the para isomer.

Since the ortho-nitroaniline has a stronger electron-withdrawing effect on the amino group compared to the para-nitroaniline, its basicity is further reduced. Therefore, the \(pK_b\) value of the ortho isomer of nitroaniline is expected to be higher than 13 (weaker base).

The exact value can be predicted by considering the magnitude of the electron-withdrawing effect: typically, the \(pK_b\) value of the ortho isomer of nitroaniline is around 14.22, based on the increased electron-withdrawing effect of the ortho-nitro group compared to the para-nitro group.