Why is thionyl chloride (SOCl2) the preferred chemical reagent over phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl3) for converting carboxylic acids into acyl chlorides?

Answer and explanation

Correct answer: C. Byproducts (SO2 and HCl) are volatile gases that escape to leave pure acyl chloride

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- With : ;
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  1. Step 1: Carboxylic acids react with halogenating agents to form acyl chlorides:
  2. Step 2: In the thionyl chloride reaction, both byproducts, sulfur dioxide () and hydrogen chloride (), are gases that bubble out of solution.
  3. Step 3: This leaves the acyl chloride product virtually pure without requiring tedious fractional distillation, making the synthetically preferred reagent.
Answer
The byproducts formed in the reaction (SO2 and HCl) are volatile gases that escape from the reaction mixture, leaving the acyl chloride in pure form
Why not the others: 'Thionyl chloride produces a solid precipitate...' is false because both byproducts are gaseous; 'PCl5 explodes spontaneously...' is exaggerated; reacts smoothly but yields liquid which is difficult to separate; 'Thionyl chloride oxidizes into peroxy acid...' contradicts its role as a chlorinating agent.

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