Ethanoic acid reacts with thionyl chloride (SOCl2) to give ethanoyl chloride. Which of the following are the other two products of this reaction?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → SO2 and HCl
Thionyl chloride replaces the –OH of the carboxyl group by chlorine. The sulphur and oxygen leave as sulphur dioxide, and the H of –OH leaves with a chlorine atom as hydrogen chloride:
\( \displaystyle \ce{CH3COOH + SOCl2 -> CH3COCl + SO2 ^ + HCl ^} \)
Both by-products are gases and escape from the mixture, which is why SOCl2 is preferred for making acyl chlorides: the product is easier to purify.
Why the other options are incorrect
Option (2) POCl3 and HCl — Incorrect. These are formed when the acid reacts with PCl5: \( \ce{RCOOH + PCl5 -> RCOCl + POCl3 + HCl} \).
Option (3) SO2 and Cl2 — Incorrect. No chlorine gas is set free: one chlorine atom goes into the acyl chloride and the other into HCl.
Option (4) H3PO3 and HCl — Incorrect. H3PO3 is formed with PCl3, and then without HCl: \( \ce{3RCOOH + PCl3 -> 3RCOCl + H3PO3} \).
NCERT (2026-27 edition):
8.9.2 Reactions Involving Cleavage of C–OH Bond – “Thionyl chloride (SOCl2) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.”
\( \displaystyle \ce{RCOOH + SOCl2 -> RCOCl + SO2 + HCl} \)