Q.Consider compound (A): a cyclic (pyranose) form of glucose, drawn as a Fischer-type projection closed into a ring by an oxygen that joins the top carbon (the former carbonyl / anomeric carbon) to the ring carbon bearing the terminal -CH2OAc group. The key feature is that the anomeric (top) carbon carries an acetylated oxygen, -OAc, rather than a free hemiacetal -OH; the terminal group is -CH2OAc and the remaining ring carbons carry -OAc or -OH substituents. Why does compound (A) not react with hydroxylamine (NH2OH) to form an oxime?

Ring-Chain Tautomerism: Understanding the "Why" Behind the Equilibrium

What Is Ring-Chain Tautomerism?

Ring-chain tautomerism is a special type of dynamic equilibrium where a molecule exists in two interconvertible forms:

• Open-chain form (usually with a carbonyl group and a nucleophilic group like –OH, –NH₂, –SH)
• Cyclic (ring) form (formed by intramolecular attack of the nucleophile on the carbonyl carbon)

The key idea: the same molecule can close into a ring or open into a chain, and the position of equilibrium depends on ring stability and steric/electronic factors.

The Core Formula: Ring Size Preference

The 5- and 6-Membered Ring Rule

Statement: For most ring-chain tautomeric systems, 5- and 6-membered rings are strongly favoured over smaller (3,4) or larger (7+) rings.

Why does this hold? — The Derivation of Preference

1. Angle Strain (Baeyer Strain Theory)

When a ring forms, bond angles deviate from the ideal tetrahedral angle ($109.5^\circ$ for sp³ carbons).

• 3-membered ring: Bond angles ≈ $60^\circ$ → severe angle strain ($\Delta H \approx +115 \text{ kJ/mol}$)
• 4-membered ring: Bond angles ≈ $90^\circ$ → significant strain ($\Delta H \approx +110 \text{ kJ/mol}$)
• 5-membered ring: Bond angles ≈ $108^\circ$ → very little strain (nearly ideal)
• 6-membered ring: Bond angles ≈ $109.5^\circ$ (chair conformation) → strain-free

Result: The ring form is only stable when angle strain is minimal — hence 5 and 6 are favoured.

2. Entropy Factor (Ring-Closing Probability)

The probability of the two ends of the chain meeting to form a ring depends on chain length.

• For a chain of $n$ atoms, the effective concentration of the reactive ends is proportional to $1/n^{3/2}$ (from random-walk statistics).
• Shorter chains (n=3,4) have higher effective concentration → easier to close, BUT the ring is too strained.
• Longer chains (n≥7) have lower effective concentration → harder to close, AND the ring has transannular strain (steric repulsion across the ring).

Optimum: n=5 or 6 gives the best balance — low strain + reasonable closing probability.

The Equilibrium Constant Expression

For a general ring-chain tautomerism:

The equilibrium constant $K$ is:

Why does $K$ depend on ring size?

From thermodynamics:

And:

• $\Delta H^\circ$ is dominated by strain energy (angle strain + torsional strain + transannular strain)
• $\Delta S^\circ$ is negative for ring closure (one molecule → one molecule, but loss of conformational freedom)

For 5- and 6-membered rings:

• $\Delta H^\circ$ is small (low strain) → favourable
• $\Delta S^\circ$ is moderately negative → slightly unfavourable
• Net: $\Delta G^\circ$ is negative → $K > 1$ (ring form dominates)

For 3- and 4-membered rings:

• $\Delta H^\circ$ is large positive (high strain) → very unfavourable
• $\Delta S^\circ$ is still negative
• Net: $\Delta G^\circ$ is positive → $K \ll 1$ (open-chain dominates)

The "Anomeric Effect" in Ring-Chain Tautomerism (Special Case)

In sugar chemistry, ring-chain tautomerism shows an additional effect: …