The correct answer is Option (3) → Respond to Schiff's test
Glucose is an aldehyde in its open-chain form and exhibits typical aldehydic reactions, but the response to certain tests varies:
- Responds to Tollen's Test: Correct. Glucose reduces ammoniacal silver nitrate to give a silver mirror (positive Tollen's test).
- On reduction with HI: Correct. Glucose, on prolonged heating with concentrated HI and red phosphorus, undergoes reduction of the carbonyl group and replacement of all oxygen atoms, yielding n-hexane (CH₃-CH₂-CH₂-CH₂-CH₂-CH₃).
- Respond to Fehling's test: Correct. Glucose reduces Fehling's solution to give a brick-red precipitate of Cu₂O.
- Respond to Schiff's test: Incorrect. Schiff's test (using Schiff's reagent: fuchsin dye decolorized with SO₂) is primarily positive for aliphatic aldehydes, especially those that react rapidly. Glucose does not give a positive Schiff's test (no magenta/pink color restoration) because its aldehydic group is in equilibrium with the cyclic hemiacetal form, and only a very small proportion exists as the free aldehyde, making the reaction too slow or negligible under standard conditions. Aromatic aldehydes and some small aliphatic aldehydes respond readily, but aldoses like glucose do not.
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