See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Unsaturation in organic compounds refers to the presence of carbon-carbon double bonds (C=C) or triple bonds (C≡C). A specific chemical test is needed that reacts with these pi bonds to indicate their presence. Step 1: Identify what each reagent tests for. Schiff's reagent (option a) is used to test for aldehydes - it gives a pink/magenta color with aldehydes. This does not specifically test for unsaturation (C=C bonds). Tollens' reagent (option b) is an alkaline solution of silver nitrate (ammoniacal silver nitrate) used to test for aldehydes - it produces a silver mirror with aldehydes. This does not test for unsaturation. Fehling's reagent (option c) is a copper(II) sulfate-based reagent used to test for reducing sugars and aldehydes - it produces a brick-red precipitate. This does not test for unsaturation. Baeyer's reagent (option d) is an alkaline solution of potassium permanganate (KMnO4). It reacts with C=C double bonds and C≡C triple bonds (unsaturated compounds) by oxidizing them, causing the purple/violet color of KMnO4 to decolorize. The discharge of the purple color indicates the presence of unsaturation in the organic compound. Step 2: Why other options fail - Schiff's, Tollens', and Fehling's reagents are all specific tests for aldehydes or reducing sugars, not for carbon-carbon unsaturation. Step 3: Baeyer's reagent (alkaline KMnO4) is the classical test for unsaturation because alkenes and alkynes decolorize the purple solution, confirming the presence of double or triple bonds. Therefore, the correct answer is D.