The correct order of reactivity of following alcohols with HBr is
(I) 1-Phenylpropan-1-ol
(II) 1-Phenylpropan-2-ol
(III) 3-Phenylpropan-1-ol

Options

A. `I gt II gt III`
B. `III gt II gt I`
C. `I gt III gt II`
D. `III gt I gt II`

Explanation

To determine the correct order of reactivity of the given alcohols with HBr, we need to analyze the structures of the alcohols and the stability of the resulting carbocations formed during the reaction.

### Step-by-Step Solution:

1. **Identify the Structures of the Alcohols**:
- (I) 1-Phenylpropan-1-ol: This has a phenyl group (C6H5) attached to the first carbon of a propane chain, with the hydroxyl (OH) group also on the first carbon.
- (II) 1-Phenylpropan-2-ol: The phenyl group is attached to the first carbon, and the hydroxyl group is on the second carbon.
- (III) 3-Phenylpropan-1-ol: The phenyl group is on the third carbon, and the hydroxyl group is on the first carbon.

2. **Understand the Mechanism of Reaction with HBr**:
- When alcohols react with HBr, the hydroxyl group (OH) is protonated to form water (H2O), which is a good leaving group. This leads to the formation of a carbocation.

3. **Determine the Type of Carbocations Formed**:
- For (I) 1-Phenylpropan-1-ol: The carbocation formed is a primary carbocation, which is less stable.
- For (II) 1-Phenylpropan-2-ol: The carbocation formed is a secondary carbocation, which is more stable than a primary carbocation.
- For (III) 3-Phenylpropan-1-ol: The carbocation formed is also a primary carbocation, but it has a phenyl group that can stabilize the positive charge through resonance.

4. **Analyze Carbocation Stability**:
- Carbocation stability order: Tertiary > Secondary > Primary.
- The presence of resonance in (I) and (III) due to the phenyl group can stabilize the positive charge, but (II) has a secondary carbocation which is inherently more stable than a primary one.

5. **Rank the Reactivity Based on Carbocation Stability**:
- (II) 1-Phenylpropan-2-ol will be the most reactive due to the formation of a stable secondary carbocation.
- (I) 1-Phenylpropan-1-ol will be next due to resonance stabilization of the primary carbocation.
- (III) 3-Phenylpropan-1-ol will be the least reactive as it forms a primary carbocation with less stability.

6. **Final Order of Reactivity**:
- The correct order of reactivity with HBr is: **(II) > (I) > (III)**.