Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Sucrose is a disaccharide and a non-reducing sugar.
Reason (R) : Sucrose involves glycosidic linkage between C\(_1\) of \(\beta\)-glucose and C\(_2\) of \(\alpha\)-fructose.
Choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are true and (R) is the true explanation of (A).
• B. Both (A) and (R) are true but (R) is not the true explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Remember the composition of important disaccharides:
\textbf{Sucrose:} \(\alpha\)-D-Glucose + \(\beta\)-D-Fructose (linkage: \(\alpha\)-1, \(\beta\)-2) \(\rightarrow\) Non-reducing.
\textbf{Maltose:} \(\alpha\)-D-Glucose + \(\alpha\)-D-Glucose (linkage: \(\alpha\)-1,4) \(\rightarrow\) Reducing.
\textbf{Lactose:} \(\beta\)-D-Galactose + \(\beta\)-D-Glucose (linkage: \(\beta\)-1,4) \(\rightarrow\) Reducing.
A sugar is non-reducing only if the anomeric carbons of ALL its units are involved in glycosidic bonding.

Answer 1

The Correct Option is C

Step 1: Analyzing Assertion (A)
Assertion (A) states that Sucrose is a disaccharide and a non-reducing sugar.
Disaccharide: Sucrose is formed by the condensation of two monosaccharide units, glucose and fructose. Therefore, it is a disaccharide.
Non-reducing sugar: A sugar is reducing if it has a free hemiacetal or hemiketal group. In sucrose, the anomeric carbon of glucose (C1) and the anomeric carbon of fructose (C2) are involved in the glycosidic bond. Since both anomeric carbons are locked in the bond, there are no free hemiacetal or hemiketal groups. Thus, sucrose cannot open its ring structure to form a free aldehyde or ketone group and does not reduce Tollens' or Fehling's reagent. Hence, it is a non-reducing sugar.
Therefore, Assertion (A) is true.
Step 2: Analyzing Reason (R)
Reason (R) describes the glycosidic linkage in sucrose. It states that the linkage is between C\(_1\) of \(\beta\)-glucose and C\(_2\) of \(\alpha\)-fructose.
The actual structure of sucrose consists of a glycosidic linkage between the C\(_1\) of \(\alpha\)-D-glucose and the C\(_2\) of \(\beta\)-D-fructose. The statement in Reason (R) has the anomers of both glucose and fructose incorrect. It should be \(\alpha\)-glucose and \(\beta\)-fructose.
Therefore, Reason (R) is false.
Step 3: Final Answer
Since Assertion (A) is true and Reason (R) is false, the correct option is (C).