Question

-OH group at which carbon center of glucose form a pyranose ring structure?

• A. C6
• B. C4
• C. C3
• D. C5

Hint:

Remember ring formation in carbohydrates:
- Glucose pyranose ring: C1 aldehyde + C5 hydroxyl \(\rightarrow\) 6-membered ring.
- Fructose furanose ring: C2 ketone + C5 hydroxyl \(\rightarrow\) 5-membered ring.

Answer 1

The Correct Option is D

Concept:
In aqueous solution, open-chain D-(+)-glucose undergoes intramolecular cyclization between a hydroxyl group and the aldehyde carbon at C1 to form a six-membered cyclic hemiacetal known as a pyranose ring.

Step 1: Numbering the Open-Chain Glucose Skeleton:
In the Fischer projection of D-glucose:
- C1 is the terminal aldehyde carbon (\(-\text{CH}=\text{O}\)),
- C2, C3, C4, and C5 are chiral secondary alcohol carbons carrying \(-\text{OH}\) groups,
- C6 is the primary alcohol carbon (\(-\text{CH}_2\text{OH}\)).

Step 2: Thermodynamic Stability of Ring Sizes:
Six-membered rings are thermodynamically favored because they minimize angle and torsional strain.
Nucleophilic attack of the hydroxyl oxygen at C5 onto the planar aldehyde carbonyl at C1 forms a six-membered ring containing five carbon atoms and one oxygen atom:
\[ \text{Ring members} = \text{C1}-\text{C2}-\text{C3}-\text{C4}-\text{C5}-\text{O} \] This six-membered heterocyclic ring is named a pyranose ring due to its structural resemblance to pyran.

Step 3: Verification of Other Hydroxyl Groups:
- Attack by the \(-\text{OH}\) group at C4 would form a five-membered furanose ring, which is less stable and less prevalent in aldohexoses than the pyranose form.
- The \(-\text{OH}\) group at C6 is outside the ring and remains as an exocyclic \(-\text{CH}_2\text{OH}\) substituent on the C5 carbon.
Therefore, the pyranose ring of glucose is formed specifically by the hydroxyl group at C5.
Final Answer:
The \(\text{-OH}\) group involved in forming the pyranose ring is at C5, corresponding to option (D).