Question

Which of the following are reducing disaccharides?
A. Galactose
B. Maltose
C. Lactose
D. Trehalose

• A. A and C Only
• B. B and C Only
• C. A and D Only
• D. B and D Only

Hint:

Reducing Disaccharides = Maltose, Lactose, Cellobiose (have free anomeric $C1$).
Non-Reducing Disaccharides = Sucrose, Trehalose (both anomeric carbons bound in linkage).
Galactose = Reducing MONOSACCHARIDE.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the carbohydrates that qualify specifically as reducing disaccharides.

Step 2: Key Formula or Approach:
A sugar is reducing if it possesses a free hemiacetal or hemiketal anomeric carbon atom capable of opening into an open-chain aldehyde/ketone to reduce Fehling's or Benedict's reagents. To be a *reducing disaccharide*, it must be composed of two monosaccharide units where at least one anomeric carbon remains free (unbound in the glycosidic linkage).

Step 3: Detailed Explation:

• A. Galactose: A reducing sugar, but it is a monosaccharide, not a disaccharide. Thus, A is excluded by the definition in the question.

• B. Maltose: Disaccharide composed of two $\text{D}$-glucose units joined by an $\alpha(1 \to 4)$ glycosidic bond. The second glucose unit retains a free hemiacetal anomeric carbon ($C1$), making maltose a reducing disaccharide. Thus, B is correct.

• C. Lactose: Disaccharide composed of $\text{D}$-galactose and $\text{D}$-glucose joined by a $\beta(1 \to 4)$ glycosidic bond. The glucose residue retains a free anomeric carbon ($C1$), making lactose a reducing disaccharide. Thus, C is correct.

• D. Trehalose: Disaccharide composed of two glucose units linked via their anomeric carbons $\alpha(1 \leftrightarrow 1)\alpha$. Because both anomeric carbons are involved in the glycosidic bond, trehalose is a non-reducing disaccharide. Thus, D is incorrect.

Step 4: Fil Answer:
Maltose and Lactose are reducing disaccharides, corresponding to Option (B).