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Question #44802

Difficulty: 8/10 | 🟢 Free

The Question

The acid-catalyzed hydrolysis of sucrose to D-glucose and D-fructose at 298 K is monitored by polarimetry. The apparent pseudo-first-order rate constant is calculated as kapp=1tln⁡(r0−r∞rt−r∞)k_{app} = \frac{1}{t} \ln \left( \frac{r_0 - r_\infty}{r_t - r_\infty} \right). In a very dilute acid, the mutarotation rate of the products is comparable to the hydrolysis rate, causing kappk_{app} to deviate from the true hydrolysis rate constant ktruek_{true} during the early stages of the reaction. Given the specific rotations [α]D25[\alpha]_D^{25}: Sucrose = +66.5∘+66.5^\circ, α\alpha-D-Glucopyranose = +112.2∘+112.2^\circ, β\beta-D-Glucopyranose = +18.7∘+18.7^\circ, equilibrium D-Glucose = +52.7∘+52.7^\circ. Assuming the hydrolysis proceeds via the most stable oxocarbenium ion intermediate, identify the specific rotation of the D-glucose anomer initially released, and the relationship between kappk_{app} and ktruek_{true} at early times (t>0t > 0).

Options

  • A.

    +112.2∘ and kapp<ktrue+112.2^\circ \text{ and } k_{app} < k_{true}

  • B.

    +18.7∘ and kapp>ktrue+18.7^\circ \text{ and } k_{app} > k_{true}

  • C.

    +112.2∘ and kapp>ktrue+112.2^\circ \text{ and } k_{app} > k_{true}

  • D.

    +52.7∘ and kapp=ktrue+52.7^\circ \text{ and } k_{app} = k_{true}

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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

Pseudo-First Order Kinetics|D-Glucose Anomeric Equilibrium|Reducing Sugar Criteria

Why This Question Is Tricky

Common Mistakes

Tests the intersection of carbohydrate reaction mechanisms (oxocarbenium ion stability determining the cleavage site) and chemical kinetics (the assumption of instantaneous mutarotation in polarimetric rate determination).

Students often rote-learn the inversion of cane sugar without realizing that the transient intermediates (α\alpha-D-glucose) have different optical properties, causing a kinetic trap if mutarotation is not infinitely fast.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterChemical Kinetics - Class 12
ConceptPseudo-First Order Kinetics
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree