Which of the following reagent produces pure alkyl halides when heated with alcohols?

Options

A. `PCl_5`
B. `PCl_3`
C. `SOCl_2`
D. dry HCl

Explanation

To determine which reagent produces pure alkyl halides when heated with alcohols, we need to analyze the reactions of various reagents with alcohols. Let's consider the reagents mentioned: PCl5, PCl3, and SOCl2.

### Step-by-Step Solution:

1. **Identify the Reagents**: The reagents in question are PCl5 (Phosphorus pentachloride), PCl3 (Phosphorus trichloride), and SOCl2 (Thionyl chloride).

2. **Reaction with PCl5**:
- When an alcohol (e.g., CH3CH2CH2OH) reacts with PCl5, the reaction produces an alkyl halide (e.g., CH3CH2CH2Cl) along with side products like H3PO3 (phosphorous acid).
- **Conclusion**: The presence of H3PO3 means that the alkyl halide produced is not pure.

3. **Reaction with PCl3**:
- Similarly, when alcohol reacts with PCl3, it also produces alkyl halides but generates side products like POCl3 (phosphoryl chloride) and HCl (hydrochloric acid).
- **Conclusion**: The formation of POCl3 indicates that the alkyl halide is not pure.

4. **Reaction with SOCl2**:
- When alcohol reacts with SOCl2, the reaction yields an alkyl halide (e.g., CH3CH2CH2Cl) along with gaseous products such as SO2 (sulfur dioxide) and HCl (hydrochloric acid).
- **Conclusion**: Since SO2 and HCl are gases, they escape from the reaction mixture, leading to a higher entropy and resulting in the formation of pure alkyl halides.

5. **Final Answer**: Among the reagents mentioned, **SOCl2** is the reagent that produces pure alkyl halides when heated with alcohols.