Answer: SOCl2.
- A HCl
- B PCl5
- C SOCl2
- D ZnCl2/HCl
Correct answer: C. SOCl2
Explanation: Thionyl chloride (SOCl₂) converts alcohols to alkyl chlorides with overall retention of configuration (through a cyclic intermediate) and releases only gaseous by-products (SO₂ and HCl), giving a pure product.
SN2 proceeds in a single step with backside attack and inversion of configuration, while SN1 forms a planar carbocation intermediate first, leading to racemisation.
Concept context
Study carbon-halogen compounds: how they are made, how they react via SN1/SN2 and elimination, their stereochemistry, and their environmental impact.