JEE Main 2021 · 18 March · morning · Single correct · +4 / −1
A non-reducing sugar "A" hydrolyses to give two reducing mono saccharides. Sugar A is
- (a) Fructose
- (b) Galactose
- (c) Glucose
- (d) Sucrose
Answer and solution
Answer (d) Sucrose
Solution
Chemistry ma’am · 3 steps
Two clues here: it must hydrolyse, so it is not a monosaccharide, and it must be non-reducing.
- Hydrolysis clue. A sugar that breaks into two monosaccharides on hydrolysis is a disaccharide. Fructose, galactose and glucose are monosaccharides themselves, so they cannot be hydrolysed further.
- Non-reducing clue. Sucrose is the disaccharide that fits. Its glucose and fructose units are joined through C1 of glucose and C2 of fructose, which are both reducing carbons. With no free aldehyde or ketone group left, sucrose does not reduce Tollens' or Fehling's reagent.
C₁₂H₂₂O₁₁ + H₂O →(H⁺) C₆H₁₂O₆ + C₆H₁₂O₆
- So. The products, glucose and fructose, are free monosaccharides with open reducing groups, so both are reducing sugars. Sugar A is sucrose, option (d).
The idea to keep. A disaccharide is non-reducing only when the bond uses the reducing carbon of both units, as in sucrose. Maltose and lactose keep one free, so they are reducing.
Answer from the NTA final answer key. Solution by PrepWiser.