The most acidic halo substituted phenol among the given compounds is:

Options

A. o-fluorophenol
B. o-chlorophenol
C. o-iodophenol
D. o-bromophenol

Explanation

To determine the most acidic halo-substituted phenol among the given compounds, we need to analyze the stability of the negative charge that forms when the hydrogen ion (H⁺) is removed from the hydroxyl group (–OH) of phenol. The compounds we are considering are ortho-fluorophenol, ortho-chlorophenol, ortho-bromophenol, and ortho-iodophenol.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Ortho-fluorophenol (F)
- Ortho-chlorophenol (Cl)
- Ortho-bromophenol (Br)
- Ortho-iodophenol (I)

2. **Understanding Acidity**:
- The acidity of phenols is influenced by the stability of the phenoxide ion (the ion formed after deprotonation).
- The more stable the phenoxide ion, the more acidic the phenol.

3. **Effect of Halogen Substituents**:
- Halogens can exert two types of effects:
- **-I (Inductive Effect)**: This effect withdraws electron density through the sigma bond and stabilizes the negative charge on the phenoxide ion.
- **+R (Resonance Effect)**: This effect donates electron density and can destabilize the negative charge.

4. **Evaluate the Halogens**:
- Fluorine (F) is highly electronegative and has a strong -I effect, but it also has a +R effect due to resonance which can destabilize the phenoxide ion.
- Chlorine (Cl) has a significant -I effect and a weaker +R effect compared to fluorine.
- Bromine (Br) has a -I effect that is less than chlorine but still stabilizes the negative charge.
- Iodine (I) has the weakest -I effect and is the least electronegative, making it the least effective in stabilizing the negative charge.

5. **Comparing Stability**:
- **Ortho-fluorophenol**: The strong -I effect is countered by the +R effect, making it less acidic.
- **Ortho-chlorophenol**: The -I effect is significant enough to stabilize the negative charge, making it more acidic than fluorine.
- **Ortho-bromophenol**: Less acidic than chlorophenol due to a weaker -I effect.
- **Ortho-iodophenol**: The least acidic due to the weakest -I effect.

6. **Conclusion**:
- The most acidic halo-substituted phenol among the given compounds is **ortho-chlorophenol**.

### Final Answer:
**Ortho-chlorophenol** is the most acidic halo-substituted phenol.

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