Without referring to the chapter, draw the chair conformations of
(b) α-D-allopyranose (the C3 epimer of glucose).

Verified step by step guidance

1. Understand the structure of α-D-allopyranose: It is a six-membered cyclic sugar (pyranose form) and the C3 epimer of glucose. This means the hydroxyl group (-OH) on the third carbon (C3) is inverted compared to α-D-glucopyranose.
2. Recall the rules for drawing chair conformations: In a chair conformation, substituents on the ring alternate between axial (perpendicular to the ring plane) and equatorial (parallel to the ring plane) positions. The α-anomer indicates that the anomeric hydroxyl group (-OH on C1) is in the axial position (down) for D-sugars.
3. Assign the stereochemistry for each carbon: Start with C1 (anomeric carbon) and place the -OH group in the axial position (down). For C2, place the -OH group in the equatorial position (up). For C3, since it is the epimer of glucose, place the -OH group in the axial position (down). Continue assigning the stereochemistry for C4 and C5 based on the D-configuration of the sugar.
4. Draw the chair conformation: Begin by sketching the six-membered chair structure. Place the substituents (hydroxyl groups and hydrogens) on each carbon according to the stereochemistry determined in the previous step. Ensure that the axial and equatorial positions alternate correctly.
5. Double-check the structure: Verify that the hydroxyl groups and hydrogens are placed correctly based on the α-anomer and the C3 epimer of glucose. Confirm that the chair conformation is drawn accurately with proper stereochemistry.

Key Concepts

Chair Conformation

Chair conformation is a three-dimensional representation of cyclohexane and its derivatives, which minimizes steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, resembling a chair, allowing for more stable interactions between substituents. Understanding chair conformations is crucial for predicting the stability and reactivity of cyclic compounds.

Epimers

Epimers are a specific type of diastereomer that differ in configuration at only one stereogenic center. In the case of a-D-allopyranose, it is an epimer of glucose at the C3 position, meaning that while they share the same molecular formula and most stereocenters, they differ in the orientation of the hydroxyl group at that specific carbon. Recognizing epimers is essential for understanding carbohydrate chemistry and their biological roles.

Pyranose Structure

Pyranose refers to a six-membered ring structure formed by the cyclization of monosaccharides, typically involving five carbon atoms and one oxygen atom. In the case of a-D-allopyranose, the ring structure is stabilized by the formation of an acetal linkage between the anomeric carbon and a hydroxyl group. Familiarity with pyranose structures is vital for analyzing the properties and reactions of sugars in organic chemistry.