Option A: HCl + ZnCl
‌2 (Lucas Reagent)
Reaction:
ROH+HCl| ZnCl2 |
| ‌───────▶‌ |
RCl+H2OByproduct: Water
(H2O). Water needs to be separated from the alkyl chloride, which can be challenging, especially for lower molecular weight alkyl chlorides that may be soluble in water or form azeotropes. This makes obtaining pure alkyl chloride directly difficult.
Option B:
PCl5 (Phosphorus Pentachloride)
Reaction:
ROH+PCl5⟶RCl+POCl3+HClByproducts: Phosphorus oxychloride
(POCl3) and hydrogen chloride
(HCl).POCl3 is a liquid, and HCl is a gas. The separation of these byproducts from the desired alkyl chloride may require careful distillation or other purification steps, affecting the direct purity.
Option C: SOCl
‌2 (Thionyl Chloride)
Reaction:
ROH+SOCl2| ‌ Pyridine ‌ |
| ‌───────▶‌ |
RCl+SO2↑+HCl↑Byproducts: Sulfur dioxide
(SO2) and hydrogen chloride
(HCl). Both of these are gases. They readily escape from the reaction mixture as they are formed. This leaves behind a relatively pure alkyl chloride, making the purification process very simple and efficient. This is often considered the best method for preparing pure alkyl halides because the gaseous byproducts automatically separate from the desired product.
Option D:
PCl3 (Phosphorus Trichloride)
Reaction:
3ROH+PCl3⟶3RCl+H3PO3Byproduct: Phosphorous acid
(H3PO3). This is a non-volatile solid or viscous liquid. It needs to be separated from the alkyl chloride, typically by distillation or extraction, which adds
purification steps.
Conclusion:
Among the given reagents,
SOCl2 (thionyl chloride) is the preferred reagent for the preparation of pure alkyl chloride from alcohol because the byproducts
(SO2. and HCl
) are gases that escape from the reaction mixture, leaving behind a pure alkyl chloride.