In which of the following amines, the first has lower `pK_(b)` value than the second ?

Options

A. Aniline, m-nitro aniline
B. m-Toluidine, p-toluidine
C. Aniline, p-chloroaniline
D. Aniline, p-aminophenol

Answer

To determine which amine has a lower pK_b value than the other, we need to analyze the basicity of the given amines. The lower the pK_b value, the stronger the base.

### Step-by-Step Solution:

1. **Understanding pK_b and Basicity**:
- pK_b is the negative logarithm of the base dissociation constant (K_b). A lower pK_b value indicates a stronger base.
- The basicity of amines is influenced by the electron-donating or withdrawing effects of substituents on the aromatic ring.

2. **Analyzing the Given Options**:
- We have multiple pairs of amines to compare. Let's denote the pairs as follows:
- Option A: Aniline (C6H5NH2) vs. Meta-nitroaniline (C6H4(NO2)NH2)
- Option B: Meta-aminotoluene vs. Para-aminotoluene
- Option C: Aniline vs. Para-chloroaniline
- Option D: Aniline vs. Para-aminophenol

3. **Evaluating Option A**:
- **Aniline**: The amino group (–NH2) is an electron-donating group, which increases the electron density on nitrogen, enhancing its basicity.
- **Meta-nitroaniline**: The nitro group (–NO2) is a strong electron-withdrawing group, especially in the para and ortho positions. In the meta position, it primarily exerts a –I (inductive) effect, which decreases the electron density on nitrogen, reducing its basicity.
- Conclusion: Aniline is a stronger base than meta-nitroaniline, thus has a higher pK_b value. Therefore, this option does not satisfy the condition.

4. **Evaluating Option B**:
- **Meta-aminotoluene**: The methyl group (–CH3) is an electron-donating group, increasing the basicity.
- **Para-aminotoluene**: The methyl group is also present here, but it is in the para position, which allows for resonance stabilization of the amino group, further increasing basicity.
- Conclusion: Para-aminotoluene is a stronger base than meta-aminotoluene, thus has a lower pK_b value. This option satisfies the condition.

5. **Evaluating Option C**:
- **Aniline**: As previously discussed, it is a strong base.
- **Para-chloroaniline**: The chloro group (–Cl) is a –I (inductive) electron-withdrawing group, which decreases the basicity compared to aniline.
- Conclusion: Aniline is a stronger base than para-chloroaniline, thus has a higher pK_b value. This option does not satisfy the condition.

6. **Evaluating Option D**:
- **Aniline**: As previously discussed, it is a strong base.
- **Para-aminophenol**: The hydroxyl group (–OH) is a –I group but also has a +M (mesomeric) effect, which can stabilize the lone pair on nitrogen. However, the –I effect is stronger, leading to a decrease in basicity.
- Conclusion: Aniline is a stronger base than para-aminophenol, thus has a higher pK_b value. This option does not satisfy the condition.

### Final Answer:
The only pair where the first amine has a lower pK_b value than the second is **Option B: Meta-aminotoluene vs. Para-aminotoluene**.