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Question #44952

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The Question

Consider the behavior of D-glucopyranose ( $\textbf{G}$ ) and the nucleoside uridine ( $\textbf{U}$ ) in an aqueous buffer at $298\text{ K}$ and $\text{pH } 7.0$ . While $\textbf{G}$ exhibits a time-dependent change in specific rotation (mutarotation), $\textbf{U}$ maintains a constant specific rotation. Which of the following statements correctly explains the fundamental stereoelectronic reason for the inability of $\textbf{U}$ to undergo mutarotation?

Options

A.
The N-glycosidic linkage in $\textbf{U}$ is an N,O-acetal where the uracil moiety exists predominantly in the diketo (lactam) form. The N1 lone pair is delocalized via amide resonance, drastically reducing its basicity and leaving group ability, thereby preventing ring-opening to an acyclic intermediate at pH 7.0.
B.
The uracil ring in $\textbf{U}$ exists predominantly in the enol (lactim) form, which provides a strong $-I$ effect that thermodynamically destabilizes the oxocarbenium ion required for mutarotation.
C.
Mutarotation in $\textbf{U}$ is kinetically feasible via an open-chain intermediate, but the equilibrium is completely shifted to the $\beta$ -anomer due to severe 1,3-diaxial interactions present in the $\alpha$ -anomer.
D.
The N-glycosidic bond in $\textbf{U}$ undergoes rapid reversible cleavage to an oxocarbenium ion, but the keto-enol tautomerization of the uracil base directs nucleophilic attack exclusively to the $\beta$ -face via neighboring group participation.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

D-Glucose Anomeric Equilibrium|Nitrogenous Base Keto-Enol Tautomerism|Nucleoside vs Nucleotide

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to differentiate between the reactivity of hemiacetals (which undergo mutarotation) and N,O-acetals (which are locked).

The 'Hidden Variable' is the keto-enol (lactam-lactim) tautomerism of the nitrogenous base.

Students often memorize that nucleosides don't mutarotate but fail to understand the stereoelectronic reason: the predominant lactam form involves the N1 lone pair in amide resonance, rendering it a terrible leaving group and preventing the ring-opening mechanism entirely at neutral pH.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	D-Glucose Anomeric Equilibrium
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free