Question

Statement I: The structure of Maltose is given below: Maltose is a non-reducing sugar.

Statement II: The structure of Lactose is given below: Lactose is a reducing sugar.

In the light of the above statements, choose the correct answer:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Before deciding if a disaccharide is reducing, work out which carbon numbers the glycosidic bond actually joins in each sugar. If the bond leaves even one anomeric carbon free on either unit, the sugar can still open into an aldehyde form and act as a reducing sugar.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A sugar is "reducing" if it has a free hemiacetal group (an anomeric carbon not involved in a glycosidic bond). This allows the ring to open into an aldehyde form that can reduce Tollen's or Fehling's reagents.

Step 2: Key Formula or Approach:
1. Maltose: Glucose + Glucose ($\alpha$-1,4 bond). 2. Lactose: Galactose + Glucose ($\beta$-1,4 bond). 3. Sucrose: Glucose + Fructose (1,2 bond).

Step 3: Detailed Explanation:
1. Maltose analysis: In maltose, the C1 of the first glucose is bonded to the C4 of the second glucose. The C1 of the second glucose is free. Therefore, maltose is a reducing sugar. Statement I is false. 2. Lactose analysis: In lactose, the C1 of galactose is bonded to the C4 of glucose. The C1 of the glucose unit is free. Therefore, lactose is a reducing sugar. Statement II is true.

Step 4: Final Answer:
Statement I is false, but Statement II is true.

Answer 2

Concept:

• A sugar is reducing only if at least one anomeric carbon (the carbon that was the carbonyl carbon in the open-chain form) is left free after the glycosidic bond is formed, since a free anomeric carbon can open back to an aldehyde/ketone and reduce Fehling's or Tollens' reagent.
• In a disaccharide, the glycosidic bond uses up the anomeric carbon of one unit. If it links to a non-anomeric carbon (like C4 or C6) of the second unit, that second unit's own anomeric carbon stays free, and the sugar is reducing. If it links to the anomeric carbon of both units (as in a 1,2-bond), no free anomeric carbon is left, and the sugar is non-reducing.

Step 1: Identify the linkage in Maltose
Maltose is built from two glucose units joined by an $\alpha(1\rightarrow4)$ glycosidic bond. The bond forms between C1 (anomeric carbon) of the first glucose and C4 of the second glucose.

Step 2: Check for a free anomeric carbon in Maltose
C4 is not an anomeric carbon, so only the first glucose loses its anomeric carbon to the bond. The second glucose still has its C1 free. A free anomeric carbon means the ring can open to expose a reactive aldehyde group.

Step 3: Conclude for Maltose
Since a free anomeric carbon is present, Maltose reduces Fehling's and Tollens' reagents, so Maltose is a reducing sugar. This makes Statement I ("Maltose is a non-reducing sugar") false.

Step 4: Identify the linkage in Lactose
Lactose is built from galactose and glucose joined by a $\beta(1\rightarrow4)$ glycosidic bond, formed between C1 (anomeric carbon) of galactose and C4 of glucose.

Step 5: Check for a free anomeric carbon in Lactose and conclude
C4 is again not anomeric, so the glucose unit keeps its C1 free. A free anomeric carbon is present, so Lactose is a reducing sugar, and Statement II is true.

Final Answer: Statement I is false, but Statement II is true.