Which is the following phenol has lowest `pK_(a)` ?

Options

A.
B.
C.
D.

Answer

To determine which phenol has the lowest pKa, we need to analyze the acidity of each phenol based on the presence of electron-withdrawing or electron-donating groups. The lower the pKa, the more acidic the compound is.

### Step-by-Step Solution:

1. **Understanding pKa and Acidity**:
- pKa is a measure of acidity; lower pKa values indicate stronger acids. Acidity is related to the tendency of a compound to release H⁺ ions.

2. **Identifying the Compounds**:
- We have four phenolic compounds to analyze. Let's denote them as:
- Option 1: Phenol (C₆H₅OH)
- Option 2: p-Cresol (C₆H₄(CH₃)OH)
- Option 3: Ortho-fluorophenol (C₆H₄(OH)(F))
- Option 4: Ortho-chlorophenol (C₆H₄(OH)(Cl))

3. **Analyzing Each Compound**:
- **Option 1 (Phenol)**: No electron-withdrawing or donating groups. It has moderate acidity.
- **Option 2 (p-Cresol)**: Contains a methyl group (CH₃), which is an electron-donating group. This decreases acidity because it makes it harder to release H⁺.
- **Option 3 (Ortho-fluorophenol)**: Fluorine is an electron-withdrawing group, which increases acidity. However, it can form intramolecular hydrogen bonds with the hydroxyl group, making it less likely to release H⁺.
- **Option 4 (Ortho-chlorophenol)**: Chlorine is also an electron-withdrawing group. It stabilizes the phenoxide ion formed after the release of H⁺ and does not form intramolecular hydrogen bonds, making it easier for H⁺ to be released.

4. **Comparing Acidity**:
- Since p-Cresol is eliminated due to the electron-donating effect, we compare ortho-fluorophenol and ortho-chlorophenol.
- Ortho-fluorophenol has intramolecular hydrogen bonding, which hinders H⁺ release.
- Ortho-chlorophenol, on the other hand, stabilizes the phenoxide ion without the hindrance of intramolecular hydrogen bonding.

5. **Conclusion**:
- Therefore, ortho-chlorophenol (Option 4) has the lowest pKa value among the given phenols, making it the most acidic.

### Final Answer:
The phenol with the lowest pKa is **Option 4: Ortho-chlorophenol**.