Which of the following is the weakest base ?

Options

A. `(CH_3)_2NH`
B. `(CH_3)_3N`
C. `C_6H_5NH_2`
D. `(CH_3)_4N^(o+)`

Explanation

To determine which of the following compounds is the weakest base, we need to analyze the basicity of different amines based on their structure and the presence of lone pairs on nitrogen.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We have several nitrogen-containing compounds, specifically amines, which can be classified into primary, secondary, tertiary, and quaternary amines.

2. **Understand Basicity**:
Basicity in amines is primarily due to the presence of a lone pair of electrons on the nitrogen atom. The ability of the nitrogen to donate this lone pair makes it a Lewis base.

3. **Effect of Alkyl Groups**:
The basicity of amines increases with the presence of electron-donating groups, such as alkyl groups. These groups enhance the availability of the lone pair on nitrogen, making it more basic.

4. **Analyze Each Compound**:
- **Tertiary Amine**: This compound has three alkyl groups attached to nitrogen, which significantly increases its basicity due to the electron-donating effect of the alkyl groups.
- **Secondary Amine**: This compound has two alkyl groups, which also increases its basicity, but not as much as the tertiary amine.
- **Aniline (C6H5NH2)**: Aniline has a lone pair on nitrogen, but this lone pair is involved in resonance with the aromatic ring, which decreases its basicity.
- **Quaternary Amine**: This compound has four alkyl groups attached to nitrogen, but it does not have a lone pair available for donation, making it non-basic.

5. **Rank the Basicity**:
- The tertiary amine is the most basic due to the presence of three electron-donating alkyl groups.
- The secondary amine is next, with two alkyl groups.
- Aniline follows, as its lone pair is involved in resonance, reducing its basicity.
- The quaternary amine is the weakest base since it lacks a lone pair to donate.

6. **Conclusion**:
Based on the analysis, the weakest base among the given options is the **quaternary amine** (option D), as it does not have a lone pair of electrons available for donation.

### Final Answer:
The weakest base is **option D: Quaternary Amine**.