The conversion of propyne into acetone requires three reagents. Which of the following is not one of them?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Zinc dust
Propyne is converted into acetone by hydration: water adds across the triple bond in the presence of dilute \( \ce{H2SO4} \) and \( \ce{HgSO4} \).
\( \displaystyle \ce{CH3-C#CH + H2O} \xrightarrow[\ce{HgSO4}]{\ce{H2SO4}} \ce{CH3COCH3} \)
The –OH adds to the middle carbon, so the C=O group ends up on C-2 and the product is propanone (acetone).
Option (1) Water — One of the three reagents. It is the molecule that is added across the triple bond.
Option (2) Zinc dust — Not one of them. Zinc dust with water is used after ozonolysis of alkenes, not in the hydration of an alkyne.
Option (3) H2SO4 — One of the three reagents. It provides the acid medium for the addition of water.
Option (4) HgSO4 — One of the three reagents. It catalyses the addition of water to the triple bond.
NCERT (2026-27 edition):
8.2.1 Preparation of Aldehydes and Ketones – “(ii) By hydration of alkynes: Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. All other alkynes give ketones in this reaction (Unit 9, Class XI).”