Which one of the following amines is the weakest base?

Options

A.
B.
C.
D.

Answer

To determine which of the given amines is the weakest base, we need to analyze the basicity of each amine based on the presence of the lone pair on the nitrogen atom and the effects of substituents on the nitrogen atom.

### Step-by-Step Solution:

1. **Identify the Amines**: Let's denote the four amines as A, B, C, and D. We need to compare their basicity.

2. **Understand Basicity in Amines**: Basicity in amines is primarily determined by the availability of the lone pair of electrons on the nitrogen atom. The more available the lone pair is for protonation, the stronger the base.

3. **Localized vs. Delocalized Lone Pairs**:
- A localized lone pair is more basic than a delocalized lone pair.
- If the lone pair on nitrogen is involved in resonance with an aromatic ring (like in aniline), it becomes less available for bonding with protons, thus decreasing basicity.

4. **Analyze Each Amine**:
- **Amine B**: If this amine has a lone pair that is localized and not involved in resonance, it will be a strong base.
- **Amine A**: If this amine has a lone pair that is delocalized with a benzene ring, it will be less basic than Amine B.
- **Amine C**: If this amine also has a lone pair that is delocalized with a benzene ring, it will have similar basicity to Amine A.
- **Amine D**: If this amine has an alkyl group (like CH3) attached to the nitrogen, the +I (inductive) effect of the alkyl group will increase the electron density on nitrogen, making it more basic than the others.

5. **Compare Basicity**:
- From the analysis, Amine D is the most basic due to the +I effect.
- Amine B is more basic than Amine A and C due to the localized lone pair.
- Amine A and Amine C are less basic because their lone pairs are delocalized with the benzene ring.

6. **Conclusion**: The weakest base among the given amines is Amine A (or C, depending on their specific structures), as it has the lone pair delocalized with the benzene ring, making it less available for protonation.

### Final Answer:
The weakest base among the given amines is **Amine A**.