Question:
Which among the following is/are reducing sugar?
(i) Glucose (ii) Galactose (iii) Ribose (iv) Fructose
Which among the following is/are reducing sugar?
(i) Glucose (ii) Galactose (iii) Ribose (iv) Fructose
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A sugar is reducing if it has a free aldehyde or can form one via keto-enol tautomerism. Both aldoses and many ketoses are reducing sugars.
Updated On: Jun 19, 2026
- (iii) only
- (i), (ii), (iv) only
- (i), (iii), (iv) only
- (i), (ii), (iii) and (iv)
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The Correct Option is D
Solution and Explanation
Step 1: Definition of reducing sugar.
A reducing sugar is a carbohydrate that contains a free aldehyde group or a free ketone group capable of reducing mild oxidizing agents such as Fehling’s solution or Tollens’ reagent.
Step 2: Analysis of given sugars.
- Glucose: An aldohexose with a free aldehyde group in its open-chain form; it reduces Fehling’s solution.
- Galactose: Another aldohexose; also a reducing sugar.
- Ribose: An aldopentose; contains a free aldehyde group and is reducing.
- Fructose: A ketohexose; although it has a ketone group, it can isomerize to an aldehyde form under basic conditions and act as a reducing sugar.
Step 3: Conclusion based on chemical tests.
All four sugars can reduce mild oxidizing agents, hence they are all classified as reducing sugars.
Step 4: Final conclusion.
\[ \boxed{\text{(i), (ii), (iii) and (iv) are all reducing sugars}} \]
A reducing sugar is a carbohydrate that contains a free aldehyde group or a free ketone group capable of reducing mild oxidizing agents such as Fehling’s solution or Tollens’ reagent.
Step 2: Analysis of given sugars.
- Glucose: An aldohexose with a free aldehyde group in its open-chain form; it reduces Fehling’s solution.
- Galactose: Another aldohexose; also a reducing sugar.
- Ribose: An aldopentose; contains a free aldehyde group and is reducing.
- Fructose: A ketohexose; although it has a ketone group, it can isomerize to an aldehyde form under basic conditions and act as a reducing sugar.
Step 3: Conclusion based on chemical tests.
All four sugars can reduce mild oxidizing agents, hence they are all classified as reducing sugars.
Step 4: Final conclusion.
\[ \boxed{\text{(i), (ii), (iii) and (iv) are all reducing sugars}} \]
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