Which of the following is the least basic amine?

p-Nitroaniline

The correct answer is option 2. p-Nitroaniline.

To determine which amine is the least basic among the given options, we need to consider the electron-withdrawing and electron-donating effects of substituents on the aromatic ring. These effects influence the availability of the lone pair of electrons on the nitrogen atom for protonation, thereby affecting the basicity of the amine.

Effects of Substituents on Basicity

Electron-Withdrawing Groups (EWGs):

Groups like -NO\(_2\) (nitro) and -CN (cyano) withdraw electron density through resonance and/or inductive effects. This decreases the electron density on the nitrogen atom, making it less available for protonation and thus decreasing the basicity.

Electron-Donating Groups (EDGs):

Groups like -NH\(_2\) (amino) and -OCH\(_3\) (methoxy) donate electron density through resonance or inductive effects. This increases the electron density on the nitrogen atom, making it more available for protonation and thus increasing the basicity.

Analyzing the Given Amines

1. p-Aminobenzonitrile:

Structure: \( \text{C}_6\text{H}_4\text{NH}_2\text{C} \equiv \text{N} \) (with -CN group at the para position)

The -CN group is a strong electron-withdrawing group through both resonance and inductive effects. This significantly reduces the basicity of the amine.

2. p-Nitroaniline:

Structure: \( \text{C}_6\text{H}_4\text{NH}_2\text{NO}_2 \) (with -NO\(_2\) group at the para position)

The -NO\(_2\) group is a strong electron-withdrawing group through resonance and inductive effects. It also significantly reduces the basicity of the amine.

3. p-Chloroaniline:

Structure: \( \text{C}_6\text{H}_4\text{NH}_2\text{Cl} \) (with -Cl group at the para position)

The -Cl group is a weak electron-withdrawing group through inductive effects. It slightly reduces the basicity of the amine but not as much as strong EWGs like -NO\(_2\) or -CN.

4. p-Bromoaniline:

Structure: \( \text{C}_6\text{H}_4\text{NH}_2\text{Br} \) (with -Br group at the para position)

The -Br group, like -Cl, is a weak electron-withdrawing group through inductive effects. It slightly reduces the basicity of the amine but not as much as strong EWGs.

Conclusion

Among the given options, p-Nitroaniline and p-Aminobenzonitrile have the strongest electron-withdrawing groups, making them the least basic. However, the p-Nitroaniline with the -NO\(_2\) group is generally more strongly electron-withdrawing than the -CN group in p-Aminobenzonitrile.

Therefore, the least basic amine among the options is: p-Nitroaniline.