`underset("Specific rotation" = +50^(@))(Ph-CH(OH)CH_(3)) overset(SOCl_(2))(rarr) underset(Cl)underset(|)(PhCH) - CH_(3)`
Which of the following acts as a leaving group?

Options

A. `overset(theta)(O)H`
B. `Cl^(theta)`
C. `SO_(2)`
D. `E2`

Explanation

To determine which group acts as a leaving group in the reaction involving the conversion of an alcohol to a chloride using thionyl chloride (SOCl₂), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
- The starting compound is a secondary alcohol (Ph-CH(OH)CH₃) and the product is a benzylic chloride (Ph-CHCl-CH₃).

2. **Understand the Role of SOCl₂**:
- Thionyl chloride (SOCl₂) is commonly used to convert alcohols to alkyl chlorides. In this reaction, SOCl₂ reacts with the alcohol to facilitate the substitution reaction.

3. **Mechanism of Reaction**:
- The reaction proceeds via a nucleophilic substitution mechanism (S_N2).
- The alcohol (Ph-CH(OH)CH₃) first reacts with SOCl₂, where the sulfur atom in SOCl₂ acts as an electrophile.

4. **Formation of an Intermediate**:
- The oxygen of the alcohol attacks the sulfur atom, forming an intermediate where the oxygen has a positive charge (due to forming three bonds) and the sulfur becomes a single bond with the oxygen.
- This intermediate contains a chloride ion (Cl⁻) and a positively charged oxygen.

5. **Leaving Group Identification**:
- In the next step, the positively charged oxygen can donate its electrons back to the sulfur atom, allowing one of the chloride ions to leave as a chloride ion (Cl⁻).
- The leaving group in this context is the species that departs during the reaction.

6. **Final Product Formation**:
- After the chloride ion leaves, the remaining structure undergoes further reaction where the chlorine attacks the carbon, leading to the formation of the benzylic chloride (Ph-CHCl-CH₃).
- The by-products of this reaction include sulfur dioxide (SO₂) and hydrogen chloride (HCl).

7. **Conclusion**:
- The leaving group in this reaction is sulfur dioxide (SO₂), which is formed when the alcohol reacts with SOCl₂.

### Final Answer:
The leaving group in this reaction is **SO₂** (sulfur dioxide).