Which of the following reagents can be used to prepare and alkyl halide from an alcohol `(ROH)`:-

Options

A. `NaCl`
B. `HCl +ZnCl_(2)`
C. `SOCl_(2)`
D. `PCl_(5)`

Answer

To determine which reagents can be used to prepare an alkyl halide from an alcohol (ROH), we will analyze each option provided.

### Step-by-Step Solution:

1. **Identify the Reaction**: The general reaction to convert an alcohol (ROH) to an alkyl halide involves replacing the hydroxyl group (OH) with a halogen (X).

2. **Evaluate the First Option: NaCl**:
- When ROH reacts with NaCl, no reaction occurs.
- Reason: Chloride ion (Cl-) is a better leaving group than hydroxide ion (OH-), but NaCl does not provide a suitable condition for substitution. Therefore, NaCl cannot be used to prepare an alkyl halide from an alcohol.

3. **Evaluate the Second Option: HCl and ZnCl2 (Lucas Reagent)**:
- ROH reacts with HCl in the presence of ZnCl2.
- This combination is known as Lucas Reagent, which facilitates the conversion of alcohols to alkyl halides.
- Result: Alkyl halide is formed. This is a valid method.

4. **Evaluate the Third Option: SOCl2 (Thionyl Chloride)**:
- ROH reacts with SOCl2.
- This reaction results in the formation of an alkyl halide and the byproduct SO2, which is released as a gas.
- Result: Pure alkyl halide is obtained. This is an efficient method.

5. **Evaluate the Fourth Option: PCl5 (Phosphorus Pentachloride)**:
- ROH reacts with PCl5.
- The reaction produces alkyl halide along with byproducts POCl3 and HCl.
- Result: Alkyl halide is formed. This is also a valid method.

### Conclusion:
The reagents that can be used to prepare an alkyl halide from an alcohol (ROH) are:
- HCl and ZnCl2 (Lucas Reagent)
- SOCl2 (Thionyl Chloride)
- PCl5 (Phosphorus Pentachloride)

### Summary of Valid Reagents:
- **HCl + ZnCl2**
- **SOCl2**
- **PCl5**