Which of the following is the strongest base in water?

Options

A. (a)
B. (b)
C. (c)
D. (d)

Answer

To determine which of the given compounds is the strongest base in water, we need to analyze the ability of each compound to accept a proton (H⁺) and the stability of the resulting conjugate acid. The stronger the base, the more stable its conjugate acid will be after accepting a proton.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question provides four compounds: Pyrrole, Aniline, Pyridine, and Piperidine. We will analyze each one to determine its basicity in water.

2. **Analyze Pyrrole**:
- When pyrrole accepts a proton, it forms a conjugate acid with a positive charge on nitrogen.
- The resulting structure becomes anti-aromatic due to the loss of aromaticity (4 π electrons instead of 6).
- Anti-aromatic compounds are less stable, making pyrrole a weak base.
- **Conclusion**: Pyrrole is not a strong base.

3. **Analyze Aniline**:
- Aniline accepts a proton to form anilinium ion (NH₃⁺).
- The phenyl group is electron-withdrawing, which increases the positive charge density on nitrogen.
- This increased positive charge makes the anilinium ion less stable.
- **Conclusion**: Aniline is not a strong base.

4. **Analyze Pyridine**:
- Pyridine accepts a proton to form a pyridinium ion.
- The sp² hybridized carbon in pyridine is electronegative and withdraws electron density from nitrogen.
- This localization of positive charge makes the conjugate acid less stable.
- **Conclusion**: Pyridine is not a strong base.

5. **Analyze Piperidine**:
- Piperidine accepts a proton to form a conjugate acid with a positive charge on nitrogen.
- The carbon atoms in piperidine are sp³ hybridized, which are less electronegative and can donate electron density.
- This donation decreases the positive charge density on nitrogen, increasing the stability of the conjugate acid.
- **Conclusion**: Piperidine is the strongest base among the options.

6. **Final Conclusion**:
- Based on the stability of the conjugate acids formed, piperidine is the strongest base in water.

### Answer:
The strongest base in water among the given options is **Piperidine (D)**.