Which of the phenol derivative is most acidic?

Options

A.
B.
C.
D.

Explanation

To determine which phenol derivative is the most acidic, we need to analyze the effects of substituents on the acidity of phenol. The acidity of phenolic compounds is influenced by the electron-withdrawing or electron-donating nature of substituents attached to the aromatic ring.

### Step-by-Step Solution:

1. **Understanding Acidity in Phenols**:
- The acidity of phenols is primarily due to the stability of the phenoxide ion formed after deprotonation. The more stable the phenoxide ion, the more acidic the phenol.

2. **Identify Substituents**:
- Consider the phenol derivatives in question. Common substituents include halogens (like F, Cl, Br, I) and other groups that can either withdraw or donate electrons.

3. **Electron-Withdrawing vs. Electron-Donating Groups**:
- Electron-withdrawing groups (EWGs) stabilize the negative charge on the phenoxide ion through resonance or inductive effects, thus increasing acidity.
- Electron-donating groups (EDGs) destabilize the phenoxide ion, decreasing acidity.

4. **Effect of Halogens**:
- Among halogens, fluorine (F) is the most electronegative and has a strong -I (inductive) effect, which increases acidity. However, it has a -R (resonance) effect that can also stabilize the phenoxide ion.
- Chlorine (Cl) also has a -I effect but can participate in resonance due to its vacant d-orbitals, which can stabilize the phenoxide ion further.

5. **Comparing Acidity**:
- When comparing fluorine, chlorine, bromine, and iodine:
- Fluorine has a strong -I effect but limited resonance stabilization due to its high electronegativity.
- Chlorine, while less electronegative than fluorine, can stabilize the phenoxide ion through resonance and has a significant -I effect.
- Bromine and iodine are less effective as EWGs due to their larger size and weaker -I effects.

6. **Conclusion**:
- Based on the above analysis, the order of acidity for the phenol derivatives is:
- **Most Acidic: Chlorophenol (due to resonance stabilization)**
- Followed by Fluorophenol, Bromophenol, and Iodophenol.

### Final Answer:
The most acidic phenol derivative is **Chlorophenol**.