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Question #44951

Difficulty: 8/10 | 🟢 Free

The Question

D-Glucose exists in a catalytic solvent system at 300 K300\text{ K} as an equilibrium mixture of α\alpha- and β\beta-anomers. The standard free energy change for the mutarotation equilibrium (α⇌β\alpha \rightleftharpoons \beta) is ΔG∘=−1.725 kJ mol−1\Delta G^\circ = -1.725\text{ kJ mol}^{-1}.

Excess acetic anhydride is added to completely acetylate the glucose. Under these specific conditions, the rate of mutarotation is significantly faster than the rate of esterification, placing the reaction under Curtin-Hammett control.

The absolute free energy of the transition state (G‡G^\ddagger) for the rate-determining step of the α\alpha-anomer acetylation is 4.475 kJ mol−14.475\text{ kJ mol}^{-1} lower than that of the β\beta-anomer acetylation.

Calculate the mole percentage of α\alpha-D-glucopyranose pentaacetate in the final product mixture.

(Given: RT=2.5 kJ mol−1RT = 2.5\text{ kJ mol}^{-1} at 300 K300\text{ K}, ln⁡2=0.69\ln 2 = 0.69, ln⁡6=1.79\ln 6 = 1.79)

Options

  • A.

    85.7%85.7\%

  • B.

    75.0%75.0\%

  • C.

    33.3%33.3\%

  • D.

    14.3%14.3\%

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

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

Concepts Behind This Question

D-Glucose Anomeric Equilibrium|Esterification Reversibility|Nucleophilic Acyl Substitution

Why This Question Is Tricky

Common Mistakes

This question tests the Curtin-Hammett principle.

A common trap is assuming the major product derives from the major equilibrium isomer, or incorrectly calculating the product ratio by multiplying the equilibrium constant by the ratio of activation energies while confusing absolute transition state energy (G‡G^\ddagger) with activation energy (ΔG‡\Delta G^\ddagger).

The ground state equilibrium constant (ΔG∘\Delta G^\circ) is a distractor; under Curtin-Hammett conditions, the product ratio is determined entirely by the absolute free energy difference of the transition states.

Students who mistakenly use the 4.475 kJ mol−14.475\text{ kJ mol}^{-1} value as the difference in activation energies (ΔΔG‡\Delta \Delta G^\ddagger) will incorrectly calculate a product ratio of 3:1, leading to the 75.0% distractor.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterBiomolecules - Class 12
ConceptD-Glucose Anomeric Equilibrium
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree