Thionyl chloride is the preferred reagent for converting an alcohol into an alkyl chloride. Which of the following is the reason?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The other products, SO2 and HCl, are gases that escape, so the alkyl chloride is obtained pure.
An alcohol reacts with thionyl chloride to give the alkyl chloride along with sulphur dioxide and hydrogen chloride:
\( \displaystyle \ce{R-OH + SOCl2 -> R-Cl + SO2 ^ + HCl ^} \)
Both by-products are gases. They escape from the reaction mixture, so nothing is left to separate from the alkyl chloride, and the product is pure.
Option (1) The reaction needs anhydrous ZnCl2 as a catalyst, which removes the water formed. — Incorrect. ZnCl2 is the catalyst needed when primary and secondary alcohols react with HCl. It is not part of the thionyl chloride method, and no water is formed there.
Option (2) Thionyl chloride is generated in situ from red phosphorus and chlorine, so it is easy to handle. — Incorrect. Red phosphorus with bromine or iodine gives phosphorus tribromide or triiodide in situ. Thionyl chloride is not made this way.
Option (3) The other products, SO2 and HCl, stay dissolved in the mixture and are washed out with water. — Incorrect. The by-products are right, but they escape as gases; that is the whole advantage of the method.
NCERT (2026-27 edition):
6.4.1 From Alcohols – “The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride. Thionyl chloride is preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides.”