Which of the following reacts fastest with conc. HCl ?

Options

A. `(CH_(3))_(3)C-OH`
B.
C. `H_(2)C=CH-CH_(2)OH`
D.

Answer

To determine which compound reacts fastest with concentrated HCl, we need to analyze the stability of the carbocations formed from each compound when they react with HCl. The stability of carbocations is a crucial factor in determining the rate of reaction.

### Step-by-Step Solution:

1. **Identify the Compounds**: We have four compounds to consider:
- Tertiary alcohol: \( \text{(CH}_3\text{)}_3\text{COH} \)
- Benzyl alcohol: \( \text{C}_6\text{H}_5\text{CH}_2\text{OH} \)
- Allyl alcohol: \( \text{CH}_2=\text{CH}\text{CH}_2\text{OH} \)
- 2-Phenyl ethanol: \( \text{C}_6\text{H}_5\text{CH}_2\text{CH}_2\text{OH} \)

2. **Reaction with HCl**: When these alcohols react with concentrated HCl, they form oxonium ions (protonated alcohols) which then lose water to form carbocations:
- Tertiary alcohol forms a tertiary carbocation.
- Benzyl alcohol forms a benzyl carbocation.
- Allyl alcohol forms an allyl carbocation.
- 2-Phenyl ethanol forms a primary carbocation.

3. **Stability of Carbocations**:
- **Tertiary Carbocation**: Very stable due to hyperconjugation and inductive effects from three alkyl groups.
- **Benzyl Carbocation**: Very stable due to resonance stabilization from the adjacent benzene ring.
- **Allyl Carbocation**: Moderately stable due to resonance but less than benzyl.
- **Primary Carbocation**: Least stable as it has no alkyl groups to stabilize the positive charge.

4. **Comparison of Stability**:
- The order of stability of the carbocations formed is:
- Benzyl carbocation > Tertiary carbocation > Allyl carbocation > Primary carbocation.

5. **Conclusion**: Since the benzyl carbocation is the most stable, the compound that reacts fastest with concentrated HCl is the benzyl alcohol \( \text{C}_6\text{H}_5\text{CH}_2\text{OH} \).

### Final Answer:
The compound that reacts fastest with concentrated HCl is **Benzyl alcohol**.