In which of the following compounds is hydroxylic proton the most acidic ?

Options

A.
B.
C.
D.

Answer

To determine which compound has the most acidic hydroxylic proton, we need to analyze the influence of substituents on the hydroxyl group (-OH) in each compound. The acidity of a hydroxylic proton is influenced by the electron-withdrawing or electron-donating effects of nearby substituents.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We need to look at the structures of the given compounds and identify the position of the hydroxylic (-OH) groups and any substituents attached to the carbon chain.

2. **Understand Hydroxylic Proton Acidity**:
The acidity of a hydroxylic proton is influenced by the electronegativity of substituents and their distance from the hydroxyl group. Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the conjugate base after deprotonation.

3. **Analyze Each Compound**:
- **First Compound**: Contains fluorine (F) as a substituent. Fluorine is a strong EWG due to its high electronegativity and shows a strong -I (inductive) effect.
- **Second Compound**: Contains iodine (I) as a substituent. Iodine is less electronegative than fluorine and has a weaker -I effect, making the hydroxylic proton less acidic compared to the first compound.
- **Third Compound**: Has fluorine on the second carbon and another substituent on the third carbon. The -I effect decreases with distance, so the acidity is less than the first compound.
- **Fourth Compound**: Has fluorine attached to the first carbon. This proximity to the hydroxyl group increases the -I effect, making the hydroxylic proton more acidic compared to the other compounds.

4. **Compare Acidity**:
- The first compound has a strong -I effect due to fluorine.
- The second compound has a weaker -I effect due to iodine.
- The third compound has fluorine but at a greater distance, reducing its effect.
- The fourth compound has fluorine closest to the hydroxyl group, maximizing the -I effect.

5. **Conclusion**:
Based on the analysis, the fourth compound has the most acidic hydroxylic proton due to the strong electron-withdrawing effect of fluorine being closest to the hydroxyl group.

### Final Answer:
The compound with the most acidic hydroxylic proton is the **fourth compound**.