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Which of the following compounds gives a positive Tollens test (silver mirror reaction)?

1. A six-membered cyclic hemiacetal with one -OH and one -H on the anomeric carbon adjacent to the ring oxygen
2. A six-membered cyclic acetal with two -OMe groups on the anomeric carbon adjacent to the ring oxygen
3. An alpha-hydroxy ketone of the form R-CO-CH2OH
4. A pyranose ring in which every free hydroxyl group has been methylated

Correct answer: A six-membered cyclic hemiacetal with one -OH and one -H on the anomeric carbon adjacent to the ring oxygen

Solution

Tollens reagent (ammoniacal AgNO3) reduces silver ions to silver metal by oxidising aldehydes. Option A is a cyclic hemiacetal: the ring opens in equilibrium to give a free aldehyde, which then reacts — positive result. Option B is a full acetal (both substituents are -OMe); acetals are stable to base and do not regenerate the aldehyde under Tollens conditions — negative. Option C is an alpha-hydroxy ketone; ketones do not reduce Tollens reagent under normal conditions — negative. Option D is a fully protected pyranose with no free -OH; no hemiacetal equilibrium is possible — negative.

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