Aldehydes, Ketones and Carboxylic Acids

Concept:Natural rubber (cis-1,4-polyisoprene) on complete ozonolysis gives a keto-aldehyde which undergoes intramolecular aldol condensation under basic conditions to form a cyclopentenone derivative.Explanation:Each isoprene unit in natural rubber has one double bond.Complete ozonolysis with $\mathrm{O_3}$ and reductive workup ($\mathrm{Zn/H_2O}$) cleaves every $\mathrm{C=C}$.The major product from each unit is 4-oxopentanal (levulinic aldehyde), $\mathrm{CH_3COCH_2CH_2CHO}$ (compound $\mathbf{X}$).This molecule has a methyl ketone ($\mathrm{-COCH_3}$) which gives a positive iodoform test, and an aldehyde ($\mathrm{-CHO}$) which gives a positive Tollen’s test.When $\mathbf{X}$ is heated with aqueous $\mathrm{NaOH}$, the base generates an enolate from the methyl ketone.This enolate attacks the aldehyde carbon intramolecularly, forming a five‑membered ring with a hydroxyl group.Dehydration (loss of water) then produces an $\alpha,\beta$-unsaturated ketone.The final product $\mathbf{Y}$ is 3‑methylcyclopent‑2‑enone.Answer:The correct structure of $\mathbf{Y}$ is shown in option A.