Question

$\alpha$ - D - Glucose + $112^0 \rightarrow + 52.5^0 \leftarrow + 19^0 \beta$D- glucose. For glucose, the above reaction represents

• A. Optical isomerism
• B. Mutarotation
• C. Epimerisation
• D. D and L isomerism

Hint:

Mutarotation = "Muta" (Change) + "Rotation" (Optical).
It only happens in sugars that have a free anomeric carbon (Reducing sugars).
If a sugar is "locked" (like Sucrose), it cannot undergo mutarotation.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question provides a schematic showing that when pure $\alpha$-D-glucose or pure $\beta$-D-glucose are dissolved in water, their specific rotation changes over time until it reaches a stable equilibrium value. This specific phenomenon needs to be identified.
Detailed Explanation:

• Definition of Mutarotation:
Mutarotation is the change in the optical rotation of a solution of a carbohydrate over time as the result of an equilibrium between the $\alpha$ and $\beta$ anomers (ring forms).

• The Glucose Case:
Freshly prepared $\alpha$-D-glucose has a specific rotation of $+112^{\circ}$.
Freshly prepared $\beta$-D-glucose has a specific rotation of $+19^{\circ}$.
When either of these is dissolved in water, the ring opens to form the open-chain aldehyde and then closes again. This process allows the two forms to interconvert.
After some time, an equilibrium mixture is formed consisting of about $36\% \alpha$ and $64\% \beta$ forms.
The specific rotation of this equilibrium mixture is exactly $+52.5^{\circ}$.

• Other Options Analysis:
Optical isomerism: A broad category of isomerism where molecules rotate plane-polarized light differently. Mutarotation is a specific type of this behavior.
Epimerisation: The interconversion of two sugars that differ in configuration at only one chiral center (e.g., Glucose to Galactose).
D and L isomerism: Refers to the configuration of the furthest chiral carbon from the carbonyl group. D-glucose does not spontaneously turn into L-glucose.

• Mechanism:
The reaction is catalyzed by acids or bases. It involves the breaking and reforming of the hemiacetal bond at the anomeric carbon (C1).

Step 3: Final Answer:
The observed change in optical rotation from $+112^{\circ}$ or $+19^{\circ}$ to $+52.5^{\circ}$ is called Mutarotation.