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JEE Main 2025 Apr 4 Shift 1, Chemistry Q4: Properties and Important Reactions of Aldehydes & Ketones

JEE Main2025Apr 4, Shift 1Chemistry
Q.

Aldol condensation is a popular and classical method to prepare -unsaturated carbonyl compounds. This reaction can be both intermolecular and intramolecular. Predict which one of the following is not a product of intramolecular aldol condensation ?

  1. A

    1

  2. B

    2

  3. C

    3

  4. D

    4

Solution

An intramolecular aldol condensation requires two carbonyl groups in the same molecule, separated by enough carbons (typically a 1,5- or 1,6-dicarbonyl) so that the enolate of one can attack the other and form a 5- or 6-membered ring with an internal -unsaturated enone after dehydration.

Options (1), (2), and (3) all arise from intramolecular aldol condensations of suitable diketo or keto-aldehyde precursors. Each product contains an endocyclic C=C conjugated with the ring C=O — the hallmark of an aldol condensation product.

Option (4), in contrast, bears an exocyclic =CH on the carbon to the carbonyl. Such an exocyclic methylene comes from a crossed (intermolecular) aldol condensation of a cyclic ketone with formaldehyde (HCHO), since formaldehyde has no -hydrogens of its own. It is not an intramolecular aldol product.

The correct answer is option (4).

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