Chapter 22, Problem 30P

Interpretation Introduction

Interpretation:

The stereochemistry of $\alpha -\text{D}-\text{idopyranose}$ with respect to equatorial and axial chair conformations is to be explained.

Concept introduction:

舧 Chair conformations: It is the most stable conformation, which accurately shows the spatial arrangement of atoms.

舧 Equatorial bonds are parallel to the average plane of the ring, whereas axial bonds are perpendicular to the average plane of the ring.

舧 The conformation having bonds at the equatorial positions are more stable than those with bonds at the axial position.

舧 On flipping the cyclohexane ring, axial bonds become equatorial bonds and equatorial bonds becomes axial bond.

舧 Bulkier group acquires equatorial positions to form stable conformer due to steric factors.

舧 The most stable configuration of aldopyranoses is when the $-{\text{CH}}_{\text{2}}\text{OH}$ group exists in equatorial conformation.

舧 Stereochemistry: The equatorial orientation refers to the spatial arrangement of $-{\text{CH}}_{\text{2}}\text{OH}$ group at $\text{C-2}$ in the same plane, and the axial orientation refers to the spatial arrangement of $-{\text{CH}}_{\text{2}}\text{OH}$ group at $\text{C-2}$ along the axis.

舧 The stability of the atom can be estimated at the values of the anomeric effect of the $\text{O:OH}$ bonds.

舧 The anomeric effect is lowest for sugars with equatorial orientation, which results in lower energetic state, and consequently this type of orientation confers higher stability.

舧 The anomeric effect is highest for sugars with axial orientation, which results in higher energetic state, and consequently this type of orientation confers lower stability.