Which is the weakest base among the followings ?

Options

A.
B.
C.
D.

Answer

To determine which compound is the weakest base among the given options, we need to analyze the electron-donating or electron-withdrawing effects of the substituents attached to the nitrogen atom in each compound.

### Step-by-Step Solution:

1. **Understanding Basicity**:
- A base is defined as a species that can donate electrons. The strength of a base is determined by its ability to donate these electrons.
- Weak bases are poor electron donors.

2. **Identifying Substituents**:
- Look at the substituents attached to the nitrogen in each compound. Electron-donating groups (EDGs) enhance basicity, while electron-withdrawing groups (EWGs) reduce basicity.

3. **Analyzing the First Compound**:
- The first compound has a nitrogen atom with a lone pair of electrons and a methyl (CH3) group attached.
- The CH3 group is an electron-donating group, which increases the basicity of the nitrogen. Thus, this compound is a strong base.

4. **Analyzing the Second Compound**:
- The second compound also contains a nitrogen atom with a lone pair and a methyl group at the meta position.
- The presence of the CH3 group still provides an electron-donating effect, making this compound a good base as well.

5. **Analyzing the Third Compound**:
- The third compound has a nitrogen with a lone pair and a methyl group at the meta position.
- Similar to the previous compounds, the methyl group acts as an EDG, enhancing the basicity. Therefore, this compound is also a good base.

6. **Analyzing the Fourth Compound**:
- The fourth compound has two methyl groups adjacent to the nitrogen atom.
- Although methyl groups are EDGs, their proximity creates steric hindrance, which hinders the nitrogen's ability to donate its lone pair of electrons effectively.
- As a result, this compound is unable to act as a strong base and is considered the weakest base among the four.

7. **Conclusion**:
- After analyzing all four compounds, the fourth compound, which has two adjacent methyl groups causing steric hindrance, is identified as the weakest base.

### Final Answer:
The weakest base among the given compounds is **the fourth compound**.

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