What are the products when acetic acid reacts with PCl3?

The correct answer is Option (1) → Acetyl chloride + H₃PO₃

The –OH of a carboxylic acid is replaced by a chlorine atom with PCl₃, giving an acyl chloride. Three molecules of acid react with one PCl₃, and the three OH groups end up on phosphorus as phosphorous acid, H₃PO₃:

    \( \displaystyle \ce{3CH3COOH + PCl3 -> 3CH3COCl + H3PO3} \)

Why the other options are incorrect

Option (2) Acetyl chloride + POCl₃ — Incorrect. POCl₃ (with HCl) is the by-product with PCl₅, not with PCl₃.

Option (3) Acetic anhydride + H₃PO₃ — Incorrect. Acetic anhydride is made by heating the acid with a dehydrating agent such as P₂O₅ or H₂SO₄; PCl₃ gives the acid chloride.

Option (4) Acetic anhydride + POCl₃ — Incorrect. Neither product forms: the organic product is acetyl chloride, and POCl₃ belongs to the PCl₅ reaction.

NCERT (2026-27 edition):
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.”