Read the passage carefully and answer the Questions.
Aldehydes and ketones are important organic compounds prepared mainly by oxidation of alcohols. Primary alcohols give aldehyde and secondary alcohols yield ketones. Aldehydes can also be formed by partial reduction of acid chlorides, reduction of esters or by Gattermann-Koch reaction. Ketones are prepared via Friedel-Craft's acylation, hydration of alkyne and reaction of Grignard's reagent with nitriles. Chemically, aldehydes and ketones are reactive in nature due to the carbonyl group and undergo nucleophilic addition reactions. Aldehydes are more reactive than ketones and can be oxidized to carboxylic acids, while both can be reduced to alcohols or hydrocarbons and exhibit condensation reaction.
Hydration of propyne in the presence of \( H_2SO_4 \) and \( HgSO_4 \) gives
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Propan-2-one
Alkynes add a molecule of water across the triple bond when warmed with dilute sulphuric acid and mercury(II) sulphate (the \( \ce{Hg^2+} \) ion is the catalyst). The addition follows Markovnikov’s rule: the hydrogen goes to the carbon that already carries more hydrogen atoms, and the –OH goes to the other carbon. The first product is an enol (an –OH on a doubly bonded carbon), which is unstable and at once rearranges (tautomerises) into the carbonyl compound.
In propyne, \( \ce{CH3-C#CH} \), the terminal carbon (C-1) carries the hydrogen, so the –OH goes to C-2. The enol, prop-1-en-2-ol, then changes into a ketone:
\( \displaystyle \ce{CH3-C#CH + H2O} \xrightarrow{\ce{HgSO4},\ \text{dil. }\ce{H2SO4}} \ce{CH3-C(OH)=CH2} \rightarrow \ce{CH3-CO-CH3} \)
So the product is propan-2-one (acetone).
Why the other options are incorrect
Option (1) Propan-1-al — Incorrect. An aldehyde would need the –OH on the terminal carbon, against Markovnikov’s rule. Propanal is obtained from propyne only by the anti-Markovnikov route of hydroboration–oxidation.
Option (3) Propan-1-ol — Incorrect. Hydration of an alkyne gives a carbonyl compound, because the enol tautomerises; an alcohol is obtained by hydration of an alkene. Propan-1-ol comes from propene by hydroboration–oxidation.
Option (4) Propan-2-ol — Incorrect. This is the product of acid-catalysed hydration of propene, \( \ce{CH3-CH=CH2} \), an alkene, not of propyne.
NCERT (2026-27 edition):
From Hydrocarbons – …
