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Question #45047

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The Question

D-Glucose undergoes mutarotation in aqueous solution at $298\text{ K}$ , reaching an equilibrium between its $\alpha$ and $\beta$ pyranose forms with a standard free energy change $\Delta G^\circ_{\alpha \rightarrow \beta} = -1.55\text{ kJ/mol}$ . Upon addition of excess phenylhydrazine (\ce{PhNHNH2}) and heating, D-glucose forms an osazone via the Weygand mechanism. This mechanism involves a series of hybridization changes at C1 and C2, driven by the thermodynamic stability of the intermediates, analogous to the Amadori rearrangement observed when D-glucose reacts with an amino acid zwitterion (e.g., glycine at pH 7.4) in the Maillard reaction.

Given $R = 8.314\text{ J K}^{-1}\text{mol}^{-1}$ and $e^{0.625} \approx 1.87$ .

Determine the correct sequence of (C1, C2) hybridizations for the following species in the osazone formation pathway: I. Cyclic Hemiacetal II. Phenylhydrazone III. Ene-hydrazine IV. Keto-hydrazine (Amadori intermediate)

Also, calculate the mole percentage of the $\alpha$ -anomer in the initial mutarotation equilibrium.

Options

A.
$(sp^3, sp^3) \rightarrow (sp^2, sp^3) \rightarrow (sp^2, sp^2) \rightarrow (sp^3, sp^2) \text{ ; } 34.8\%$
B.
$(sp^3, sp^3) \rightarrow (sp^2, sp^3) \rightarrow (sp^2, sp^2) \rightarrow (sp^2, sp^2) \text{ ; } 65.2\%$
C.
$(sp^3, sp^3) \rightarrow (sp^3, sp^2) \rightarrow (sp^2, sp^2) \rightarrow (sp^3, sp^2) \text{ ; } 34.8\%$
D.
$(sp^3, sp^3) \rightarrow (sp^2, sp^3) \rightarrow (sp^3, sp^2) \rightarrow (sp^3, sp^2) \text{ ; } 65.2\%$

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How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Hybridization and Geometry|Glycosidic Linkage Geometry|Osazone Formation Logic

Why This Question Is Tricky

Common Mistakes

Tests the student's ability to integrate thermodynamic equilibrium calculations with detailed mechanistic stereoelectronics.

Students often memorize the 1:3 stoichiometry of osazone formation but overlook the hidden Amadori-type rearrangement (Weygand mechanism) that dictates the hybridization shifts at C1 and C2.

Furthermore, a common thermodynamic trap is misassigning the direction of the mutarotation equilibrium; students frequently calculate the major product incorrectly by applying the equilibrium constant to $[\alpha]/[\beta]$ instead of $[\beta]/[\alpha]$ , failing to account for the superior stability of the $\beta$ -chair conformation where all substituents are equatorial.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Hybridization and Geometry
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free