Assertion (A): In the preparation of alkyl chlorides from alcohols, thionyl chloride gives an impure product, because its by-products remain mixed with the alkyl chloride.
Reason (R): The by-products of the reaction of an alcohol with thionyl chloride are the gases sulphur dioxide and hydrogen chloride.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is false but (R) is true.
An alcohol can be converted into an alkyl chloride with concentrated HCl, phosphorus halides or thionyl chloride. With thionyl chloride, the other two products are gases:
\( \displaystyle \ce{R-OH + SOCl2 -> R-Cl + SO2 ^ + HCl ^} \)
Assertion (A): In the preparation of alkyl chlorides from alcohols, thionyl chloride gives an impure product, because its by-products remain mixed with the alkyl chloride. — False. The by-products are gases that escape from the reaction mixture, so thionyl chloride gives a pure alkyl chloride.
This is why it is the preferred reagent.
Reason (R): The by-products of the reaction of an alcohol with thionyl chloride are the gases sulphur dioxide and hydrogen chloride. — True. The alkyl chloride is formed along with SO2 and HCl, both gases, as the equation shows.
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.”