Which of the following are reducing sugar?
A. Sucrose
B. Maltose
C. Lactose
D. Mannose
E. Glucose
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B, C, D, E only
A reducing sugar reduces Fehling’s solution and Tollens’ reagent because it has a free aldehyde or ketone group (a free hemiacetal carbon in the ring form). All monosaccharides, aldoses and ketoses, are reducing. A disaccharide is reducing only if the glycosidic bond leaves one of these groups free.
(A) Sucrose — Non-reducing sugar. The glycosidic bond joins C1 of glucose to C2 of fructose, so both reducing groups are used up.
(B) Maltose — Reducing sugar. C1 of one glucose is linked to C4 of the other; C1 of the second glucose is free.
(C) Lactose — Reducing sugar. C1 of galactose is linked to C4 of glucose; C1 of the glucose unit is free.
(D) Mannose — Reducing sugar. An aldohexose (a monosaccharide), so it is reducing.
(E) Glucose — Reducing sugar. An aldohexose with a free –CHO group in its open-chain form.
NCERT (2026-27 edition):
Classification of Carbohydrates – “… The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars.”
Disaccharides – “… In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which these functional groups are free, are called reducing sugars, for example, maltose and lactose.”