Which of the following have maximum value of `pK_(b)` in Aqueous phase-

Options

A. N-Methyl aniline
B. N,N-di ethyl ethanamine
C. Benzenamine
D. Phenyl methanamine

Answer

To determine which of the given compounds has the maximum value of \( pK_b \) in the aqueous phase, we need to analyze the basicity of each compound. The \( pK_b \) value is inversely related to the base strength; a higher \( pK_b \) indicates a weaker base. Here’s a step-by-step solution:

### Step 1: Understand the Concept of Basicity
- Basicity is the ability of a compound to accept protons (H⁺ ions). The stronger the base, the more readily it accepts protons, resulting in a lower \( pK_b \) value.
- Conversely, a weaker base will have a higher \( pK_b \) value.

### Step 2: Identify the Compounds
The compounds given are:
1. N-methyl aniline
2. Diethylamine
3. Aniline (Benzamine)
4. Phenylmethanamine

### Step 3: Analyze Each Compound
1. **N-methyl aniline**: The methyl group is an electron-donating group, which increases the electron density on the nitrogen, enhancing its basicity.

2. **Diethylamine**: Similar to N-methyl aniline, the ethyl groups are also electron-donating, making this compound a strong base.

3. **Aniline (Benzamine)**: The amino group (-NH₂) is attached to a benzene ring. The lone pair on nitrogen can participate in resonance with the benzene ring, reducing its availability to accept protons and thus decreasing its basicity.

4. **Phenylmethanamine**: This compound has a methanamine group (-NH₂) connected to a phenyl group. The resonance effect of the phenyl group will also decrease the basicity of the nitrogen atom.

### Step 4: Compare Basicity
- **N-methyl aniline** and **diethylamine** are stronger bases due to the electron-donating effects of their alkyl groups.
- **Aniline** has reduced basicity because of resonance stabilization, which lowers the availability of the lone pair on nitrogen.
- **Phenylmethanamine** is similar to aniline in terms of resonance effects, but the basicity is slightly lower due to the additional methylene group.

### Step 5: Determine the Weakest Base
Among the four compounds, **aniline** has the weakest basicity due to the resonance effect that stabilizes the lone pair on nitrogen, making it less available for protonation. Therefore, it will have the highest \( pK_b \) value.

### Conclusion
The compound with the maximum value of \( pK_b \) in the aqueous phase is **Aniline (Benzamine)**.