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JEE Main 2025 Jan 24 Shift 1, Chemistry Q16: Mechanism of Some Important Reactions of Alkenes

JEE Main2025Jan 24, Shift 1Chemistry
Q.

Aman has been asked to synthesise the molecule

He thought of preparing the molecule using an aldol condensation reaction. He found a few cyclic alkenes in his laboratory. He thought of performing ozonolysis reaction on alkene to produce a dicarbonyl compound followed by aldol reaction to prepare "x". Predict the suitable alkene that can lead to the formation of "x".

  1. A
  2. B
  3. C
  4. D
Solution

Work the synthesis backwards from the target. The product is acetyl-cyclopentane (a five-membered ring bearing a COCH group), made by an intramolecular aldol condensation. The aldol precursor must be an open-chain dicarbonyl whose two carbonyls, after enolisation and ring closure, leave a five-membered ring carrying an acetyl side chain.

The required dicarbonyl is a 1,6-diketo/keto-aldehyde with seven carbons in total: CH-CO-(CH)-CHO. An intramolecular aldol between the methyl ketone enolate and the aldehyde, followed by dehydration, would give the cyclopentene; reduction of the C=C (or recognising the aldol product itself) ultimately gives 1-acetylcyclopentane.

Ozonolysis (O, then Zn/HO) cleaves a C=C and places C=O on each former alkene carbon. To get the seven-carbon , cleave a six-membered ring whose double bond is between (i) a carbon bearing a methyl substituent and (ii) an unsubstituted CH. That alkene is 1-methylcyclohex-1-ene.

Cleavage of 1-methylcyclohex-1-ene gives the keto-aldehyde directly; intramolecular aldol then yields the acetylcyclopentane target.

Option (1).

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