Question

Functional group responsible for reducing property of lactose is

• A. Hydroxyl group on glucose
• B. Aldehyde group on galactose
• C. Keto group on glucose
• D. Aldehyde group on glucose

Hint:

Lactose Chemistry: Reducing power comes from the FREE ALDEHYDE GROUP ON GLUCOSE (Anomeric C-1). (Galactose C-1 is locked in the glycosidic bond).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Carbohydrate chemistry of lactose disaccharide: lactose ($eta$-D-galactopyranosyl-(1$ ightarrow$4)-D-glucose) possesses a free, unbonded hemiacetal aldehyde group on the anomeric C-1 carbon of the glucose moiety.
Key Formula or Approach:
\[ \text{Lactose Structure: } \beta\text{-D-Galactopyranosyl-(1}\rightarrow\text{4)-D-Glucopyranose} \quad \implies \quad \mathbf{Free \text{ } C-1 \text{ Aldehyde Group on Glucose}} \]

Step 2: Detailed Explanation:
In carbohydrate chemistry and dairy biochemistry:
- Lactose is a reducing disaccharide consisting of $\beta$-D-galactopyranose linked to D-glucopyranose via a $\beta$-(1$\rightarrow$4) glycosidic bond.
1. In the Galactose unit, the anomeric C-1 carbon is locked in the glycosidic linkage.
2. In the Glucose unit, the anomeric C-1 carbon is free and exists in chemical equilibrium between cyclic pyranose ring forms and the open-chain aldehyde form.
- This free potential Aldehyde group on the C-1 carbon of Glucose (D) can mutarotate, enolize, and reduce alkaline copper reagents (Fehling's solution, Benedict's, Lane-Eynon method) to cuprous oxide ($\text{Cu}_2\text{O}$), imparting the reducing property of lactose.

Step 3: Final Answer:
Therefore, the functional group is Aldehyde group on glucose, matching option (D).