Q. 28 Which one of the following conformations of glucose is most stable?
(A) Boat
(B) Half Chair
(C) Chair
(D) Planar
Correct Answer
The correct answer is (C) Chair.
Glucose, in its pyranose form, adopts a six-membered ring akin to cyclohexane. The chair form minimizes angle, torsional, and steric strain, making it the predominant structure in solution.
Option Analysis
Chair Conformation
All hydroxyl groups occupy equatorial positions in the β-D-glucopyranose chair (⁴C₁), avoiding 1,3-diaxial interactions. This results in zero ring strain, far lower energy than alternatives (about 4-15 kcal/mol below boat forms).
Boat Conformation
Boat forms introduce flagpole interactions (steric strain between C1 and C4 hydrogens) and eclipsed C-H bonds causing torsional strain (around 7 kcal/mol total strain). Stable boat minima exist but remain 4-15 kcal/mol higher than chair.
Half Chair Conformation
Half chair features partial planarity with high torsional and angle strain from distorted bond angles and eclipsed bonds. It ranks lowest in stability among puckered forms, serving mainly as a transition state.
Planar Conformation
A fully planar ring maximizes angle strain (ideal tetrahedral angles deviate to 120°) and torsional strain from all eclipsed bonds. No biological relevance; highly unstable compared to puckered chairs.
Stability Order
| Conformation | Relative Strain (kcal/mol) | Key Strain Factors |
|---|---|---|
| Chair | 0 | Minimal |
| Boat | ~7 | Flagpole, torsional |
| Half Chair | Highest | Angle, torsional |
| Planar | Extreme | Angle, full eclipsing |
This hierarchy aligns with cyclohexane analogs, confirmed by DFT calculations on glucose.
