The minimum value of `-logK_(b)` will be for the compound

Options

A. o - Nitroaniline
B. p - Nitroaniline
C. p - Chloroaniline
D. p - Methoxyaniline

Answer

To determine the minimum value of \(-\log K_b\) for the given compounds, we need to analyze their basicity. The base dissociation constant (\(K_b\)) is a measure of a base's ability to accept protons, and its logarithmic form, \(pK_b = -\log K_b\), indicates that a lower \(pK_b\) value corresponds to a stronger base (higher \(K_b\)).

### Step-by-Step Solution:

1. **Understanding Basicity**:
- Basicity is defined as the tendency of a compound to donate its lone pair of electrons to an electrophile (often a proton). The stronger the base, the more readily it donates its electrons.

2. **Relationship Between \(K_b\) and \(pK_b\)**:
- The relationship is given by:
\[ pK_b = -\log K_b \]
- A lower \(pK_b\) indicates a higher \(K_b\) and hence a stronger base.

3. **Identifying Electron-Donating and Electron-Withdrawing Groups**:
- Electron-donating groups (EDGs) increase the electron density on the nitrogen atom, making it more basic. Examples include alkyl groups (like -CH₃, -C₂H₅) and methoxy (-OCH₃).
- Electron-withdrawing groups (EWGs) decrease the electron density, making the compound less basic. Examples include nitro (-NO₂) and halogens (like -Cl, -Br).

4. **Analyzing Given Compounds**:
- We have four compounds to analyze:
1. Ortho-nitroaniline
2. Para-nitroaniline
3. Para-chloroaniline
4. Para-methoxyaniline

5. **Comparing Basicity**:
- **Ortho-nitroaniline**: The nitro group is close to the amino group, pulling electron density away, making it less basic.
- **Para-nitroaniline**: The nitro group is further away from the amino group compared to ortho-nitroaniline, making it slightly more basic than the ortho isomer but still less basic than compounds with EDGs.
- **Para-chloroaniline**: The chlorine atom is an electron-withdrawing group, reducing basicity compared to aniline.
- **Para-methoxyaniline**: The methoxy group is an electron-donating group, significantly increasing the electron density on the nitrogen, making it the most basic of the four.

6. **Conclusion**:
- Among the compounds analyzed, para-methoxyaniline has the highest basicity due to the presence of the electron-donating methoxy group. Therefore, it will have the minimum value of \(-\log K_b\) (or the maximum \(K_b\)).

### Final Answer:
The minimum value of \(-\log K_b\) will be for **para-methoxyaniline**.