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Question #44006

Difficulty: 8/10 | 🟢 Free

The Question

D-Glucose is treated with an excess of methylmagnesium bromide ( $\ce{CH3MgBr}$ ) in anhydrous THF at 298 K. The reaction mixture is subsequently quenched with an excess of heavy water ( $\ce{D2O}$ ). Let $x$ be the number of moles of methane gas evolved per mole of D-Glucose during the first step. Identify $x$ and the major organic product ( $P$ ) isolated after the quench.

Options

A.
$x = 5$ ; $P$ is an anomeric mixture of D-glucopyranose-1,2,3,4,6- $O$ - $d_5$
B.
$x = 5$ ; $P$ is 1-C-methyl-D-glucitol-1,2,3,4,5,6- $O$ - $d_6$
C.
$x = 6$ ; $P$ is 1-C-methyl-D-glucitol-1,2,3,4,5,6- $O$ - $d_6$
D.
$x = 4$ ; $P$ is methyl D-glucopyranoside-2,3,4,6- $O$ - $d_4$

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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

Grignard Reaction Pathways|Kiliani-Fischer Chain Extension|D-Glucose Anomeric Equilibrium

Why This Question Is Tricky

Common Mistakes

This question tests the kinetic supremacy of acid-base reactions over nucleophilic addition.

Students often blindly apply the 'Grignard + Carbonyl -> Alcohol' template, forgetting that unprotected carbohydrates are polyols.

The hidden trap is that the Grignard reagent is consumed by the 5 acidic hydroxyl groups, forming a deactivated poly-alkoxide that prevents any Kiliani-Fischer type chain extension.

Option B traps students who correctly identify the 5 active hydrogens but incorrectly assume nucleophilic addition still occurs.

Quick Facts About This Question

Question Information

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