Question

Sucrose is dextrorotatory but after hydrolysis the mixture show laevorotation, this is because of

• A. Laevorotation of glucose is more than dextrorotation of fructose.
• B. Sucrose is a non-reducing sugar.
• C. Recemic mixture is formed.
• D. Laevorotation of fructose is more than dextrorotation of glucose.

Answer 1

The Correct Option is D

Sucrose is a disaccharide consisting of glucose and fructose. Sucrose itself is dextrorotatory, meaning it rotates plane-polarized light to the right. However, when sucrose undergoes hydrolysis, it breaks down into glucose and fructose. The resulting mixture shows laevorotation (rotation to the left). This occurs because of the individual optical activities of glucose and fructose.

Explanation: 

The key reason the mixture shows laevorotation after hydrolysis is that fructose is more laevorotatory than glucose is dextrorotatory. Thus, after hydrolysis, the laevorotation of fructose outweighs the dextrorotation of glucose, leading to an overall laevorotatory mixture.

Review of the options:

• Option A: "Racemization is formed" - This is incorrect. The reaction does not form a racemic mixture, but rather a mixture of fructose and glucose.
• Option B: "Laevorotation of fructose is more than dextrorotation of glucose" - This is the correct answer. Fructose has a greater laevorotatory effect than glucose's dextrorotatory effect, which causes the overall mixture to exhibit laevorotation after hydrolysis.
• Option C: "Laevorotation of glucose is more than dextrorotation of fructose" - This is incorrect. Glucose does not exhibit stronger laevorotation than fructose.
• Option D: "Sucrose is a non-reducing sugar" - While sucrose is indeed a non-reducing sugar, this fact does not explain the optical rotation observed after hydrolysis.

Conclusion:

The correct answer is Option B: Laevorotation of fructose is more than dextrorotation of glucose.