See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Concept: Cyclohexane is a saturated cyclic hydrocarbon, while cyclohexene is an unsaturated cyclic hydrocarbon containing a C=C double bond. Simple chemical tests that detect unsaturation (the C=C double bond) can be used to differentiate them. Step 1 - Bromine water test (Br2/H2O): Cyclohexene undergoes electrophilic addition with bromine water, decolorizing the orange-brown Br2 solution to give a colorless product (trans-1,2-dibromocyclohexane). Cyclohexane does not react with bromine water under normal conditions (no UV light), so it does not decolorize Br2/H2O. This test successfully differentiates the two. Step 2 - Baeyer's reagent (dilute alkaline KMnO4, cold): Cyclohexene reacts with cold dilute KMnO4 (Baeyer's reagent) via syn-dihydroxylation, causing the purple/violet color of KMnO4 to decolorize (forming a colorless diol and brown MnO2 precipitate). Cyclohexane does not react under these mild conditions, so the purple color is retained. This test also successfully differentiates the two. Step 3 - Tollen's reagent (ammoniacal AgNO3): Tollen's reagent is used to detect aldehydes (oxidizable carbonyl groups), not C=C double bonds. Neither cyclohexane nor cyclohexene is an aldehyde, so neither would give a silver mirror reaction. Tollen's reagent cannot differentiate them, eliminating option (c). Step 4 - Both (a) and (b): Since both Br2/H2O and Baeyer's reagent independently and reliably differentiate cyclohexane from cyclohexene by detecting the double bond, option (d) 'Both (a) and (b)' is the most complete and correct answer. Why other options fail: Option (c) Tollen's reagent fails because it tests for aldehydes, not alkene unsaturation. Options (a) and (b) alone are each correct but individually incomplete compared to option (d). Therefore, the correct answer is D.