Consider the following statements about the conversion of carboxylic acids into acyl chlorides.
(A) The hydroxyl group of carboxylic acids is easily replaced by a chlorine atom on treating with PCl5, PCl3 or SOCl2.
(B) Thionyl chloride is preferred for this conversion because the other two products are gaseous and escape the reaction mixture.
(C) The reaction of a carboxylic acid with PCl5 gives the acyl chloride along with H3PO3.
(D) One molecule of PCl3 converts three molecules of a carboxylic acid into the acyl chloride.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B) and (D) only
The –OH of a carboxyl group behaves like the –OH of an alcohol: phosphorus chlorides and thionyl chloride replace it by chlorine and give the acyl chloride. The three reagents differ in their by-products.
(A) The hydroxyl group of carboxylic acids is easily replaced by a chlorine atom on treating with PCl5, PCl3 or SOCl2. — True. All three give R–COCl.
(B) Thionyl chloride is preferred for this conversion because the other two products are gaseous and escape the reaction mixture. — True. SO2 and HCl leave as gases, so the acyl chloride is easier to purify:
\( \displaystyle \ce{RCOOH + SOCl2 -> RCOCl + SO2 ^ + HCl ^} \)
(C) The reaction of a carboxylic acid with PCl5 gives the acyl chloride along with H3PO3. — False. With PCl5 the other products are POCl3 and HCl; H3PO3 is the by-product of PCl3:
\( \displaystyle \ce{RCOOH + PCl5 -> RCOCl + POCl3 + HCl} \)
(D) One molecule of PCl3 converts three molecules of a carboxylic acid into the acyl chloride. — True. PCl3 has three chlorine atoms, and each replaces one –OH:
\( \displaystyle \ce{3RCOOH + PCl3 -> 3RCOCl + H3PO3} \)
NCERT (2026-27 edition):
8.9.2 Reactions Involving Cleavage of C–OH Bond – “3. Reactions with PCl5, PCl3 and SOCl2
The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl5, PCl3 or SOCl2. Thionyl chloride (SOCl2) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.”
\( \displaystyle \ce{RCOOH + PCl5 -> RCOCl + POCl3 + HCl} \)
\( \displaystyle \ce{3RCOOH + PCl3 -> 3RCOCl + H3PO3} \)
\( \displaystyle \ce{RCOOH + SOCl2 -> RCOCl + SO2 + HCl} \)