JEE Main 2021 · 26 August · evening · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and other is labelled as Reason (R).
Assertion (A) : Sucrose is a disaccharide and a non-reducing sugar.
Reason (R) : Sucrose involves glycosidic linkage between C₁ of β-glucose and C₂ of α-fructose.
Choose the most appropriate answer from the options given below :
- (a) Both (A) and (R) are true but (R) is not the true explanation of (A).
- (b) (A) is false but (R) is true.
- (c) (A) is true but (R) is false.
- (d) Both (A) and (R) are true and (R) is the true explanation of (A)
Answer and solution
Answer (c) (A) is true but (R) is false.
Solution
Chemistry ma’am · 4 steps
The link in sucrose is right, but the anomers are swapped: it is α-glucose with β-fructose.
- Check the Assertion. Sucrose hydrolyses to one glucose and one fructose, so it is a disaccharide. It does not reduce Tollens' reagent or Fehling's solution, so it is a non-reducing sugar. (A) is true.
- Why non-reducing. The glycosidic bond joins C1 of glucose to C2 of fructose. These are the anomeric carbons of both units, so neither ring can open to give a free -CHO or >C=O group.
- Check the Reason. In sucrose the link is between C1 of α-D-glucose and C2 of β-D-fructose. The Reason says β-glucose and α-fructose, which swaps both anomers. So (R) is false.
- So. (A) is true and (R) is false, which is option (c).
The idea to keep. Sucrose is α-D-glucose C1 joined to β-D-fructose C2. Both anomeric carbons are used up in the bond, so there is no free reducing group. That is why it is non-reducing.
Answer from the NTA final answer key. Solution by PrepWiser.