Which of the following monosaccharides will give a brick‑red precipitate in Benedict’s test when the test is performed in alkaline medium? (a) D-fructose (b) D-sorbitol (c) D-mannose (d) D-glucitol
Answer
(c) D-mannose
The correct answer is (c) D-mannose — Therefore, only D‑mannose will give the brick‑red precipitate in the test.
Step-by-step solution
- Benedict’s test detects reducing sugars, i.e., sugars that can reduce Cu²⁺ to Cu⁺, producing a brick‑red precipitate.
- Reducing sugars possess an open‑chain aldehyde or free hemiacetal group.
- D‑fructose is a ketose and non‑reducing; D‑sorbitol and D‑glucitol are sugar alcohols and non‑reducing.
- D‑mannose, an aldohexose, has a free aldehyde group and is a reducing sugar.
- Therefore, only D‑mannose will give the brick‑red precipitate in the test.
Common mistake: Selecting a ketose or sugar alcohol as a reducing sugar.
Concept tested: Benedict’s test for reducing sugars
Published · How this solution was prepared and checked
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