Which one of the following is least basic ?

Options

A. Aniline
B. Diethylamine
C. `p-`sulphanilic acid
D. `o-`nitroaniline

Explanation

To determine which compound is the least basic among the given options, we need to analyze the basicity of each compound based on the presence of electron-donating or electron-withdrawing groups and the availability of the lone pair of electrons on the nitrogen atom.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- **Aniline**: C6H5NH2
- **Diethylamine**: (C2H5)2NH
- **Para-sulfonylic acid**: C6H4(NH2)SO3H (4-amino-benzene-sulfonylic acid)
- **Ortho-nitro-aniline**: C6H4(NH2)(NO2) (where NO2 is at the ortho position)

2. **Analyze Basicity**:
- Basicity is influenced by the availability of the lone pair of electrons on the nitrogen atom. The more available the lone pair, the stronger the base.
- Electron-donating groups (like alkyl groups) increase basicity, while electron-withdrawing groups (like nitro and sulfonyl groups) decrease basicity.

3. **Evaluate Each Compound**:
- **Aniline**: The lone pair on nitrogen is available for donation, making it a weak base.
- **Diethylamine**: The two ethyl groups are electron-donating, increasing the availability of the lone pair, making it a stronger base than aniline.
- **Para-sulfonylic acid**: The sulfonyl group (SO3H) is electron-withdrawing, which decreases the availability of the lone pair on nitrogen, but it is still more available than in ortho-nitro-aniline.
- **Ortho-nitro-aniline**: The nitro group (NO2) is a strong electron-withdrawing group, especially at the ortho position, which significantly reduces the availability of the lone pair on nitrogen due to resonance and inductive effects.

4. **Conclusion**:
- Among the four compounds, ortho-nitro-aniline has the least availability of the lone pair on nitrogen due to the strong electron-withdrawing effect of the nitro group at the ortho position.
- Therefore, **the least basic compound is ortho-nitro-aniline**.

### Final Answer:
**Ortho-nitro-aniline is the least basic compound among the given options.**