Which one of the following amines is the weakest base?

Options

A.
B.
C.
D.

Answer

To determine which amine is the weakest base among the given options, we need to analyze the basicity of each amine based on the electron-donating or electron-withdrawing effects of substituents attached to the nitrogen atom. Here’s a step-by-step solution:

### Step 1: Understanding Basicity of Amines
Amines are basic due to the presence of a lone pair of electrons on the nitrogen atom. The basicity of amines can be influenced by the presence of substituents:
- **Electron-donating groups** (EDGs) increase the electron density on nitrogen, enhancing basicity.
- **Electron-withdrawing groups** (EWGs) decrease the electron density on nitrogen, reducing basicity.

### Step 2: Analyze Each Option
Let’s evaluate each option provided in the question:

1. **Aniline (C6H5NH2)**:
- The lone pair on nitrogen is delocalized into the aromatic ring due to resonance. This delocalization decreases the electron density on nitrogen, making it less basic.

2. **Benzylamine (C6H5CH2NH2)**:
- The lone pair on nitrogen is not involved in resonance with the benzene ring, thus it retains its electron density. This makes benzylamine a stronger base compared to aniline.

3. **Methylamine (CH3NH2)**:
- The methyl group (CH3) is an electron-donating group. It increases the electron density on nitrogen, making methylamine a stronger base.

4. **Dimethylamine (CH3)2NH**:
- Similar to methylamine, the presence of two methyl groups further increases the electron density on nitrogen, making dimethylamine even more basic.

### Step 3: Conclusion
Based on the analysis:
- **Aniline** is the weakest base due to the resonance effect that delocalizes the lone pair of electrons on nitrogen, reducing its availability to accept protons.

### Final Answer:
**Aniline is the weakest base among the given amines.**

---