Which of the following monosaccharides will give a positive result with Benedict's test (formation of a brick‑red precipitate) at neutral pH?
Answer
D‑glucose
The correct answer is D‑glucose — Therefore, the only sugar among the options that gives a positive Benedict's test at neutral pH is D‑glucose.
Step-by-step solution
- Recall that Benedict's test detects reducing sugars that can donate electrons to Cu2+ ions.
- Identify which sugars are reducing: monosaccharides with a free carbonyl group in open‑chain form.
- D‑glucose has an aldehyde group that readily reduces Cu2+, so it gives a positive test.
- D‑fructose can also reduce but requires alkaline conditions to tautomerise; at neutral pH the reaction is negligible.
- Sucrose is a disaccharide formed by a glycosidic bond between the anomeric carbons, so it lacks a free carbonyl group and does not reduce.
- Cellulose is a polymer of glucose units linked by β‑1,4‑glycosidic bonds; the individual units are not free reducing ends.
- Therefore, the only sugar among the options that gives a positive Benedict's test at neutral pH is D‑glucose.
Common mistake: Choosing sucrose, which is a non‑reducing sugar.
Concept tested: Reducing sugars
Published · How this solution was prepared and checked
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