In which one of the following conversions phosphorus pentachloride is used as a reagent?

Options

A. `H_(2)C = CH_(2) rarr CH_(3)CH_(2)`Cl
B. HC `equiv CH rarr CH_(2)` = CHCl
C. `CH_(3) CH_(2)` OH `rarr CH_(3) CH_(2)` Cl
D. None of these

Answer

To solve the question regarding the use of phosphorus pentachloride (PCl5) as a reagent, we will analyze the given options and determine which conversion involves PCl5.

### Step-by-Step Solution:

1. **Understanding the Role of PCl5**:
Phosphorus pentachloride is commonly used to convert alcohols into alkyl chlorides. It reacts with alcohols to replace the hydroxyl (-OH) group with a chlorine atom (-Cl).

2. **Analyzing the Options**:
We need to evaluate each option to see if it involves the conversion of an alcohol or similar compound using PCl5.

- **Option A**: This option does not involve an alcohol or a suitable substrate for PCl5. Therefore, PCl5 is not used here.

- **Option B**: Similar to option A, this option does not involve a reaction with an alcohol or a compound that would react with PCl5. Hence, PCl5 is not applicable.

- **Option C**: This option involves the reaction of ethanol (C2H5OH) with PCl5. The reaction proceeds as follows:
\[ C_2H_5OH + PCl_5 \rightarrow C_2H_5Cl + HCl + POCl_3 \]
In this reaction, the hydroxyl group (-OH) of ethanol is replaced by a chlorine atom (-Cl), resulting in the formation of ethyl chloride (C2H5Cl).

3. **Conclusion**:
After analyzing all the options, we find that phosphorus pentachloride is used as a reagent in **Option C**, where it reacts with ethanol to form ethyl chloride.

### Final Answer:
Phosphorus pentachloride is used as a reagent in **Option C**.