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Question #45085

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The Question

Consider the anomeric carbon (C1) of the following four derivatives of D-glucopyranose. All four compounds possess the $\alpha$ -configuration, meaning the anomeric substituent is axial in the standard $^4C_1$ chair conformation:

I. $\alpha$ -D-glucopyranose (anomeric substituent = \ce{-OH}) II. Methyl $\alpha$ -D-glucopyranoside (anomeric substituent = \ce{-OCH3}) III. $\alpha$ -D-glucopyranosylmethane (anomeric substituent = \ce{-CH3}) IV. Methyl 1-thio- $\alpha$ -D-glucopyranoside (anomeric substituent = \ce{-SCH3})

Determine the absolute configuration (R/S) at C1 for compounds I, II, III, and IV, respectively.

Options

A.
I: 1R, II: 1R, III: 1S, IV: 1S
B.
I: 1S, II: 1S, III: 1R, IV: 1R
C.
I: 1R, II: 1S, III: 1S, IV: 1R
D.
I: 1S, II: 1R, III: 1R, IV: 1S

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Cahn-Ingold-Prelog (CIP) Priority|Glycosidic Linkage Geometry

Why This Question Is Tricky

Common Mistakes

This question tests a deep stereochemical trap: the decoupling of relative stereochemical descriptors ( $\alpha$ / $\beta$ ) from absolute configurations (R/S) at the anomeric center of carbohydrates.

Students often memorize that ' $\alpha$ -D-glucose is 1R' and blindly apply this to all $\alpha$ -derivatives.

However, the Cahn-Ingold-Prelog (CIP) priorities change depending on the anomeric substituent.

In C-glycosides, the anomeric carbon substituent drops in priority below C2, and in S-glycosides, it rises above the ring oxygen (O5).

These priority shifts cause an inversion of the R/S descriptor despite the 3D spatial geometry (axial substituent) remaining identical.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Cahn-Ingold-Prelog (CIP) Priority
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free