Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A) : A solution of $\alpha$-D glucose and a solution of $\beta$-D glucose shows identical optical properties.
Reason (R) : They form identical equilibrium mixture in solution.
In light of the above statements, choose the correct answer from the options given below.

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

Hint:

Mutarotation is the gradual change in optical rotation of a freshly prepared solution of an anomer until it reaches a stable, constant equilibrium value of $+52.7^\circ$ due to the interconversion of $\alpha$ and $\beta$ forms.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
$\alpha$-D-glucose and $\beta$-D-glucose are cyclic diastereomers of glucose that differ in configuration only at the hemiacetal carbon (C-1).
These cyclic isomers are called anomers.
Detailed Explanation:
- Evaluating Assertion (A): Pure, freshly prepared crystalline $\alpha$-D-glucose and $\beta$-D-glucose possess distinct physical and optical properties.
Specifically, a freshly prepared solution of pure $\alpha$-D-glucose has a specific optical rotation of $+112.2^\circ$, while a freshly prepared solution of pure $\beta$-D-glucose has a specific optical rotation of $+18.7^\circ$.
Therefore, their initial optical properties are not identical, making Assertion (A) false.
- Evaluating Reason (R): When dissolved in water, both anomers undergo an interconversion process called mutarotation.
The ring opens to form the straight-chain intermediate and then closes to form either the $\alpha$ or $\beta$ anomer.
Over time, both solutions reach the exact same thermodynamic equilibrium.
This equilibrium mixture consists of approximately $36\%$ $\alpha$-D-glucose, $64\%$ $\beta$-D-glucose, and trace amounts ($<0.1\%$) of the open-chain form, resulting in a stable specific rotation of $+52.7^\circ$.
Thus, both solutions eventually form the identical equilibrium mixture, making Reason (R) true.

Step 2: Final Answer:
Assertion (A) is false but Reason (R) is true. This corresponds to option (D).