Which of the following is most reactive towards aqueous `HBr`?

Options

A. 1 - Phenyl - 2- propanol
B. 1 - Phenyl - 1 - propanol
C. 3 - Phenyl - 1 - propanol
D. 2 - Phenyl - 1 - propanol

Explanation

To determine which compound is most reactive towards aqueous HBr, we need to analyze the reactivity of the given alcohols based on their structure and the stability of the carbocation that forms during the reaction.

### Step-by-Step Solution:

1. **Understanding the Reaction**: Aqueous HBr dissociates into H⁺ and Br⁻ ions. The H⁺ ion will protonate the hydroxyl (-OH) group of the alcohol, converting it into a better leaving group (water, H₂O), and subsequently leading to the formation of a carbocation.

2. **Identifying the Compounds**: Let's denote the options as follows:
- Option A: 1-Phenyl-2-propanol
- Option B: 1-Propanol
- Option C: 3-Phenyl-1-propanol
- Option D: (not specified in the transcript)

3. **Analyzing Option A (1-Phenyl-2-propanol)**:
- Structure: CH₃-CH(OH)-C₆H₅
- Upon protonation, it forms a carbocation that is secondary and can stabilize due to the adjacent phenyl group.

4. **Analyzing Option B (1-Propanol)**:
- Structure: CH₃-CH₂-CH₂OH
- Upon protonation, it forms a primary carbocation, which is less stable.

5. **Analyzing Option C (3-Phenyl-1-propanol)**:
- Structure: C₆H₅-CH(OH)-CH₂-CH₃
- Upon protonation, it forms a secondary carbocation that is stabilized by the phenyl group, similar to Option A.

6. **Comparing Carbocation Stability**:
- The stability of carbocations generally follows the order: tertiary > secondary > primary > methyl.
- In this case, both Option A and Option C can form secondary carbocations, which are more stable than the primary carbocation formed from Option B.

7. **Conclusion**:
- Since both Option A and Option C form stable secondary carbocations, we need to consider the resonance stabilization provided by the phenyl group.
- The carbocation formed from Option A (1-Phenyl-2-propanol) is more stabilized due to the direct resonance with the phenyl ring compared to Option C (3-Phenyl-1-propanol).
- Therefore, **Option A (1-Phenyl-2-propanol)** is the most reactive towards aqueous HBr.

### Final Answer:
**Option A: 1-Phenyl-2-propanol is the most reactive towards aqueous HBr.**