Carbohydrates
Both maltose and lactose are disaccharides that give a positive Fehling's test and undergo mutarotation, unlike sucrose. What structural reason makes these two sugars reducing while sucrose is not?
Select the correct option:
Solution
One anomeric carbon remains free in maltose and lactose but not in sucrose
Comparing reducing and non-reducing disaccharides hinges on counting how many anomeric carbons remain free. Maltose is two glucose units joined by an alpha-1,4 glycosidic bond, and lactose is galactose linked to glucose by a beta-1,4 bond; in both cases only one of the two anomeric carbons is used in the linkage, leaving the other free. That free anomeric carbon can open into an aldehyde form, so it reduces Fehling's and Tollens' reagents and shows mutarotation, marking these as reducing sugars. In sucrose, the glucose C1 and fructose C2 anomeric carbons are both consumed by the glycosidic bond, leaving no free hemiacetal, so it is non-reducing. The claim about fructose is irrelevant because maltose and lactose contain no fructose. The alpha-versus-beta distinction does not determine reducing ability, since both alpha and beta linked sugars here are reducing. They are genuine disaccharides, not disguised monosaccharides. This follows the NCERT treatment of disaccharides. As a consistency check, the presence of exactly one free anomeric carbon predicts a single reducing end per molecule, which agrees with the observed positive tests.
🔒 Solution Hidden from View
Submit your answer to unlock the detailed step-by-step solution.
More carbohydrates Practice Questions
Both D-glucose and D-fructose give the same osazone when treated with excess phenylhydrazine, despit...
Both D-glucose and D-fructose give the same osazone when treated with excess phenylhydrazine, despit...
On treatment with concentrated nitric acid, D-glucose is oxidised at both ends of its carbon chain t...
On treatment with concentrated nitric acid, D-glucose is oxidised at both ends of its carbon chain t...
A carbohydrate sample cannot be hydrolysed into any simpler sugar and contains a single polyhydroxy ...
A carbohydrate sample cannot be hydrolysed into any simpler sugar and contains a single polyhydroxy ...
Sucrose does not reduce Tollens' reagent or Fehling's solution even though it is a disaccharide buil...
Sucrose does not reduce Tollens' reagent or Fehling's solution even though it is a disaccharide buil...
Open-chain D-glucose reacts with hydroxylamine to give an oxime and adds one molecule of hydrogen cy...
Open-chain D-glucose reacts with hydroxylamine to give an oxime and adds one molecule of hydrogen cy...
About This Question
- Subject
- chemistry
- Chapter
- biomolecules
- Topic
- carbohydrates
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
One anomeric carbon remains free in maltose and lactose but not in sucrose
Comparing reducing and non-reducing disaccharides hinges on counting how many anomeric carbons remain free. Maltose is two glucose units joined by an alpha-1,4 glycosidic bond, and lactose is galactose linked to glucose by a beta-1,4 bond; in both cases only one of the two anomeric carbons is used in the linkage, leaving the other free. That free anomeric carbon can open into an aldehyde form, so it reduces Fehling's and Tollens' reagents and shows mutarotation, marking these as reducing sugars. In sucrose, the glucose C1 and fructose C2 anomeric carbons are both consumed by the glycosidic bond, leaving no free hemiacetal, so it is non-reducing. The claim about fructose is irrelevant because maltose and lactose contain no fructose. The alpha-versus-beta distinction does not determine reducing ability, since both alpha and beta linked sugars here are reducing. They are genuine disaccharides, not disguised monosaccharides. This follows the NCERT treatment of disaccharides. As a consistency check, the presence of exactly one free anomeric carbon predicts a single reducing end per molecule, which agrees with the observed positive tests.
This medium difficulty chemistry question is from the chapter biomolecules, covering the topic of carbohydrates. It appeared in the 2025 exam.
Looking for more practice? Explore all chemistry questions or browse biomolecules questions on RankGuru.