Which of the following cannot be a base?

Options

A.
B. `CH_(3)-underset(O)underset(||)C-NH_(2)`
C.
D.

Answer

To determine which of the given compounds cannot act as a base, we need to analyze each option based on their ability to donate a pair of electrons. Bases are typically defined as substances that can accept protons (H⁺) or donate electron pairs.

### Step-by-Step Solution:

1. **Identify the Options**: We need to analyze the compounds provided in the options (A, B, C, and D) to see which one cannot act as a base.

2. **Analyze Option A (Phenol)**:
- Phenol has a hydroxyl group (-OH) attached to a benzene ring. The oxygen atom in the hydroxyl group has a lone pair of electrons.
- Although the lone pair participates in resonance with the aromatic system, it can still donate an electron pair to a proton (H⁺).
- **Conclusion**: Phenol can act as a base.

3. **Analyze Option B (Amine)**:
- This compound contains a nitrogen atom with lone pairs of electrons.
- The lone pairs on nitrogen can be used to bond with a proton (H⁺), allowing it to act as a base.
- **Conclusion**: The amine can act as a base.

4. **Analyze Option C (Negative Ion)**:
- This compound has a negative charge on an oxygen atom, indicating that it has extra electrons.
- The negative charge can readily donate an electron pair to a proton (H⁺), making it a strong base.
- **Conclusion**: This compound can act as a base.

5. **Analyze Option D (Quaternary Ammonium Ion)**:
- This compound has a nitrogen atom bonded to four substituents, resulting in a positive charge on the nitrogen.
- Since the nitrogen is already positively charged, it has no available lone pairs to donate; it cannot accept a proton as it has already reached its maximum bonding capacity.
- **Conclusion**: This compound cannot act as a base.

### Final Answer:
The compound that cannot be a base is **Option D**.