MCQ Detailed View

Alkyl halides can be prepared from alcohols by replacing the hydroxyl group (–OH) with a halogen atom. Several reagents can be used for this substitution, but not all are equally efficient.

1. 
   

   SOCl₂ (Thionyl chloride) in pyridine:
   This is considered the best method for preparing alkyl chlorides. The reaction proceeds smoothly, and the byproducts (SO₂ and HCl) are gases, which escape from the reaction mixture. This leaves behind only the desired alkyl chloride, giving a pure product with excellent yield. Pyridine is added to absorb the released HCl and to promote the reaction.

   
   

   Equation:
   R–OH + SOCl₂ → R–Cl + SO₂↑ + HCl↑

   
   


2. 
   

   PCl₃ (Phosphorus trichloride):
   Also converts alcohols to alkyl chlorides, but it produces phosphorous acid (H₃PO₃) as a byproduct. The reaction works but is less efficient compared to SOCl₂.

   
   


3. 
   

   PCl₅ (Phosphorus pentachloride):
   Converts alcohols into alkyl chlorides with the formation of POCl₃ and HCl. This method is effective but less clean and often gives lower purity products.

   
   


4. 
   

   Zn / HCl:
   This reagent is used for reducing purposes, not for converting alcohols into alkyl halides.

   
   

Because of its clean reaction, gaseous byproducts, and high yield, the SOCl₂ / pyridine method is considered the best for the preparation of alkyl halides from alcohols.