JEE Main · 2021 · 18 Mar 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

Dilute alkali removes an $\alpha$-hydrogen from one molecule of cyclopentanone, and the enolate so formed adds to the carbonyl carbon of a second molecule. The aldol product is the $\beta$-hydroxy ketone 2-(1-hydroxycyclopentyl)cyclopentan-1-one, with the hydroxyl group on the carbon of the second ring that was the carbonyl carbon. Acid and heat then eliminate this hydroxyl together with the hydrogen on the $\alpha$-carbon that joins the two rings, and the new double bond ends up conjugated with the remaining carbonyl group. The products are 2-(1-hydroxycyclopentyl)cyclopentan-1-one and, after dehydration, 2-cyclopentylidenecyclopentan-1-one.