Q.Three cyclic monosaccharide structures (labelled I, II and III) are drawn as Fischer projections closed by a ring oxygen, each ending in a CH2OH group. Reading the ring carbons from the top (anomeric) carbon downward: in structure I the OH groups are on the right (anomeric), right, left, right; in structure II they are on the left (anomeric), right, left, right; in structure III they are on the left, left, left, right, left. Which of these structures are anomers of one another?
(A) I and II
(B) II and III
(C) I and III
(D) III is an anomer of I and II
(A) I and II
(B) II and III
(C) I and III
(D) III is an anomer of I and II

Glucose Cyclization: Why the Ring Forms

Glucose cyclization is a classic example of an intramolecular reaction — a molecule reacting with itself. Let's build the understanding step by step.

1. The Starting Point: Open-Chain Glucose

Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) in its open-chain form has:

• An aldehyde group ($-\text{CHO}$) at carbon 1 (C1)
• A hydroxyl group ($-\text{OH}$) at carbon 5 (C5)

The aldehyde is electrophilic (electron-deficient at the carbonyl carbon), and the hydroxyl is nucleophilic (electron-rich oxygen with lone pairs).

2. Why Cyclization Happens: Thermodynamic & Kinetic Favorability

The Key Insight

The molecule is flexible — it can bend so that the C5 hydroxyl oxygen approaches the C1 aldehyde carbon. This brings two reactive groups into close proximity.

• Entropy is favorable: One molecule becomes one ring — no loss of translational entropy (unlike two separate molecules reacting).
• Ring strain is manageable: A 5- or 6-membered ring (furanose or pyranose) has minimal angle strain (close to tetrahedral angles).

Result: The reaction is reversible but strongly favors the cyclic form — in aqueous solution, >99% of glucose exists as the ring.

3. The Reaction: Hemiacetal Formation

The nucleophilic oxygen of the C5 hydroxyl attacks the electrophilic carbonyl carbon of C1:

This is a hemiacetal — a carbon bonded to both an $-\text{OH}$ and an $-\text{OR}$ group.

Mechanism (simplified)

1. Protonation of the carbonyl oxygen (acid-catalyzed) makes C1 more electrophilic.
2. Nucleophilic attack by the C5 oxygen.
3. Deprotonation yields the cyclic hemiacetal.

4. The Key Formula(e): Ring Size & Anomeric Carbon

Ring Size Determination

The ring size depends on which hydroxyl attacks:

• C5 hydroxyl → pyranose (6-membered ring: 5 carbons + 1 oxygen)
• C4 hydroxyl → furanose (5-membered ring: 4 carbons + 1 oxygen)

For D-glucose, the C5 attack is overwhelmingly favored, giving the pyranose form.

The Anomeric Carbon

The new chiral center formed at C1 is called the anomeric carbon. Two stereoisomers arise:

• $\alpha$-anomer: $-\text{OH}$ at C1 is trans to the $-\text{CH}_2\text{OH}$ group (axial in the chair conformation)
• $\beta$-anomer: $-\text{OH}$ at C1 is cis to the $-\text{CH}_2\text{OH}$ group (equatorial in the chair conformation)

Why two forms? The attack can occur from either face of the planar carbonyl group — leading to two possible configurations at the new stereocenter.

5. The Equilibrium Constant & Mutarotation

The interconversion between $\alpha$ and $\beta$ anomers is called mutarotation:

At equilibrium (in water at 20°C):

• $\beta$-D-glucopyranose: ~64%
• $\alpha$-D-glucopyranose: ~36%
• Open chain: <0.1% …