See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Concept: Tollen's test (silver mirror test) is positive for aldehydes. A cyclic hemiacetal can open to reveal a free aldehyde in equilibrium, making it capable of giving a positive Tollen's test. A full acetal (with two OR groups or one OR and one O in ring) cannot open to give a free aldehyde under the mild basic conditions of Tollen's reagent. Step 1: Identify each compound. (a) This is a cyclic acetal — the ring oxygen plus an exocyclic OCH3 at C2 means C2 bears two oxygen substituents (the ring O and OCH3), making it a full acetal. Full acetals do NOT give positive Tollen's test. (b) This is also a cyclic acetal (or mixed acetal) — the ring oxygen plus an exocyclic OCH3 at C3. This is actually a ketal/acetal type. It does NOT give positive Tollen's test. (c) This is a cyclic hemiacetal — the ring oxygen at position 1 and a free OH at C2. The C2 carbon bears both the ring oxygen and a free hydroxyl group. A cyclic hemiacetal is in equilibrium with the open-chain hydroxy aldehyde form. The open-chain form contains a free aldehyde group, which readily gives a positive Tollen's test. (d) This compound has a tetrahydropyran ring (cyclic ether) with an OH at C4, which is simply a cyclic ether alcohol — not a hemiacetal. The OH is not on the carbon bearing the ring oxygen, so there is no hemiacetal character and no aldehyde in equilibrium. This does NOT give a positive Tollen's test. Step 2: Conclusion — Only compound (c) is a cyclic hemiacetal, which exists in equilibrium with an open-chain aldehyde form, and therefore gives a positive Tollen's test. Therefore, the correct answer is C.