JEE Main 2025 Jan 28 Shift 2, Chemistry Q4: Mechanism of Some Important Reactions of Alkenes

JEE Main2025Jan 28, Shift 2Chemistry

Identify product [A], [B] and [C] in the following reaction sequence :

$C H_{3} - C \equiv C H P d / C, H_{2} [A] (i) O_{3} (ii) Z n, H_{2} O [B] + [C]$

A
[A] : $C H_{3} - C H = C H_{2}$ , [B] : $C H_{3} C H O$ , [C] : $H C H O$
B
[A] : $C H_{2} = C H_{2}$ , [B] : $C H_{3} - C O - C H_{3}$ (acetone), [C] : $H C H O$
C
[A] : $C H_{3} - C H = C H_{2}$ , [B] : $C H_{3} C H O$ , [C] : $C H_{3} C H_{2} O H$
D
[A] : $C H_{3} C H_{2} C H_{3}$ , [B] : $C H_{3} C H O$ , [C] : $H C H O$

Solution

Step 1: Lindlar-type partial hydrogenation. With $P d / C$ and $H_{2}$ the alkyne propyne is reduced to the alkene, giving [A] = propene, $C H_{3} - C H = C H_{2}$ .

Step 2: Reductive ozonolysis cleaves the $C = C$ double bond and gives two carbonyl fragments. For propene, the carbons on either side of the double bond carry one H and two H respectively, so the products are acetaldehyde and formaldehyde:

[B] = $C H_{3} C H O$ and [C] = $H C H O$ .

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JEE Main 2025 Jan 28 Shift 2, Chemistry Q4: Mechanism of Some Important Reactions of Alkenes

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