Before You Begin

Question #43949

Difficulty: 8/10 | 🟢 Free

The Question

The Kiliani-Fischer synthesis of D-glucose using an aqueous \ce{NaCN}/\ce{HCN} buffer (pH $\approx$ 8.5) yields two epimeric heptonic acids, $X$ (major) and $Y$ (minor), after hydrolysis. Under these pH-controlled conditions, the stereochemical outcome deviates from simple kinetic addition. Which of the following statements correctly identifies the major product $X$ and the governing stereochemical rationale?

Options

A.
$X$ is D-glycero-D-gulo-heptonic acid; the reaction is under thermodynamic control, driven by the greater stability of its cyclic 1,5-imino-lactone intermediate which possesses fewer axial substituents compared to the D-ido epimer.
B.
$X$ is D-glycero-D-ido-heptonic acid; the reaction is under thermodynamic control, favoring the open-chain cyanohydrin that minimizes 1,3-syn pentane interactions in its extended zig-zag conformation.
C.
$X$ is D-glycero-D-gulo-heptonic acid; the reaction is under kinetic control, where the Felkin-Anh addition of \ce{CN^-} to the open-chain aldehyde is directed by the C2-hydroxyl group.
D.
$X$ is D-glycero-D-ido-heptonic acid; the stereoselectivity is kinetically determined by the rapid trapping of the reactive conformation generated directly from the ring-opening of $\beta$ -D-glucopyranose.

Check Your Answer

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

Cyanohydrin Stereochemistry|D-Glucose Anomeric Equilibrium|Epimerization vs Anomerization

Why This Question Is Tricky

Common Mistakes

The question tests the 'Hidden Variable' of cyclic intermediate stability overriding open-chain conformational analysis.

Students often memorize the 1,3-syn pentane interaction rule (which successfully predicts D-mannonic acid from D-arabinose) and misapply it to D-glucose.

They fail to account for the fact that in aqueous equilibrium, the cyanohydrins cyclize to 1,5-imino-lactones.

The thermodynamic stability of these pyranose-like forms (determined by the number of axial substituents) dictates the major product, shifting the equilibrium towards the D-gulo epimer despite its less stable open-chain conformation.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Cyanohydrin Stereochemistry
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free