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 determine in which conversion phosphorus pentachloride (PCl5) is used as a reagent, we need to analyze the reactions provided and identify where PCl5 acts as a source of chloride ions for nucleophilic substitution.

### Step-by-Step Solution:

1. **Understanding the Role of PCl5**:
- Phosphorus pentachloride (PCl5) is a reagent that provides chloride ions (Cl-) for nucleophilic substitution reactions. It is important to recognize that PCl5 is not used in electrophilic addition reactions, where chlorine is added as an electrophile.

2. **Identifying the Reaction Types**:
- We need to look at the options provided and classify them as either electrophilic addition reactions or nucleophilic substitution reactions.
- Electrophilic addition involves the addition of atoms across a double bond, while nucleophilic substitution involves the replacement of one functional group with another.

3. **Analyzing the Given Reactions**:
- For each reaction, determine if PCl5 is involved in a nucleophilic substitution:
- **Reaction A**: If it involves adding Cl- to a double bond, it is an electrophilic addition.
- **Reaction B**: Similar to Reaction A, if it involves adding Cl- to a double bond, it is also an electrophilic addition.
- **Reaction C**: If it involves the conversion of an alcohol (like C2H5OH) to an alkyl chloride (like C2H5Cl) through the substitution of OH- with Cl-, it indicates a nucleophilic substitution.

4. **Conclusion**:
- The only reaction where phosphorus pentachloride is used as a reagent for nucleophilic substitution is in Reaction C, where ethanol (C2H5OH) is converted to ethyl chloride (C2H5Cl) by replacing the hydroxyl group (OH-) with a chloride ion (Cl-).

### Final Answer:
The correct answer is **C: C2H5OH to C2H5Cl**.