Question

Draw chair conformations for \(\alpha\)-D-mannopyranose and \(\beta\)-D-mannopyranose. Label the anomeric carbon in each.

Short answer

Question: Draw and label the chair conformations for α-D-mannopyranose and β-D-mannopyranose, and identify the position of the anomeric carbon in each structure. Answer: α-D-mannopyranose: C1 - axial (up), C2 - equatorial (right), C3 - axial (up), C4 - axial (up); Anomeric carbon is C1. β-D-mannopyranose: C1 - equatorial (right), C2 - equatorial (right), C3 - axial (up), C4 - axial (up); Anomeric carbon is C1.

Step-by-step solution

Draw the basic chair conformation structure

Draw a hexagonal ring structure in the shape of a chair. The chair conformation is a six-membered ring structure with alternating axial and equatorial positions, where the axial positions (up or down) are perpendicular to the plane of the ring, and the equatorial positions (left or right) are in the same plane as the ring.

Identify the anomeric carbon and its configuration

In α-D-mannopyranose and β-D-mannopyranose, the anomeric carbon is the first carbon (C1). The main difference between the α and β anomers lies in the orientation of the hydroxyl group attached to the anomeric carbon. In α-D-mannopyranose, the hydroxyl group is in the axial position (up), while in β-D-mannopyranose, it is in the equatorial position (right).

Draw hydroxyl groups for the other carbons in the ring

For both α-D-mannopyranose and β-D-mannopyranose, the hydroxyl groups are attached to carbons 2, 3, and 4. For D-mannopyranose: - Carbon 2 (C2) has the hydroxyl group in the equatorial position (right) - Carbon 3 (C3) has the hydroxyl group in the axial position (up) - Carbon 4 (C4) has the hydroxyl group in the axial position (up)

Label the anomeric carbon in each chair conformation

Now that we have identified the anomeric carbon (C1) and have the correct hydroxyl group orientation, label the anomeric carbon in both α-D-mannopyranose and β-D-mannopyranose. The chair conformations for α-D-mannopyranose and β-D-mannopyranose are as follows: α-D-mannopyranose: C1 - axial (up), C2 - equatorial (right), C3 - axial (up), C4 - axial (up) β-D-mannopyranose: C1 - equatorial (right), C2 - equatorial (right), C3 - axial (up), C4 - axial (up) Remember to label the anomeric carbon (C1) in each structure.

Key concepts

Anomeric Carbon

In carbohydrates, particularly in sugar molecules that form cyclic structures, there's a special carbon atom known as the anomeric carbon. This carbon is central to understanding sugar chemistry because it's the carbon atom that becomes a new chiral center upon cyclization.

During cyclization, the carbonyl group (either aldehyde or ketone) of the sugar reacts with a hydroxyl group to form a new stereocenter, thus creating two distinct configurations: alpha (α) and beta (β). In chair conformations for cyclic sugars, like α-D-mannopyranose and β-D-mannopyranose, the position of the hydroxyl group on the anomeric carbon determines its configuration. If the hydroxyl group is on the same side as the oxygen in the hemiacetal ring, the sugar is in the β configuration. Conversely, if the hydroxyl group is on the opposite side, it’s in the α configuration.

While drawing these structures, it's important to establish which carbon is the anomeric one. In mannopyranose, it's the first carbon atom (C1), which was initially part of the carbonyl group before ring formation. Labeling this carbon clearly in chair conformations helps to identify the sugar and understand its properties and reactions, like how it will interact with other molecules.

α-D-Mannopyranose

α-D-Mannopyranose is an anomer of the sugar mannose in which the hydroxyl group attached to the anomeric carbon is positioned axially, pointing upwards when drawn in a standard chair conformation. To visualize this, imagine sitting in a recliner; the axial positions are like the armrests sticking straight up or straight down from the seat, while equatorial positions are akin to footrests extending outwards, more or less level with the seat.

In the case of α-D-mannopyranose, not only is the hydroxyl group on the anomeric carbon axial, but we also see the specific placement of hydroxyl groups on other carbons. For example, at carbon 2 (C2), the hydroxyl group is equatorial, meaning it extends out from the ring, positioned for minimal steric hindrance, which often equates to a more stable structure. However, the hydroxyl groups on carbons 3 (C3) and 4 (C4) are both axial, like C1’s hydroxyl group. Understanding these configurations is crucial for predicting the physical properties and reactivity of the sugar.

β-D-Mannopyranose

As the counterpart to α-D-mannopyranose, β-D-mannopyranose is constructed with a small but significant difference. In this anomer, the hydroxyl group on the anomeric carbon (C1) is equatorial and extends outwards in the same plane as the ring, similar to the 'footrests' analogy used before. This placement generally provides a more stable conformation due to less steric clash with adjacent substituents.

Importantly, the other hydroxyl group orientations remain consistent with those in α-D-mannopyranose, with carbon 2 (C2) bearing an equatorial hydroxyl group for stability. However, the hydroxyl groups on carbon 3 (C3) and carbon 4 (C4) are still axial, creating an upward point from the plane of the ring. Both α and β configurations coexist in equilibrium in solution, but their individual stability and reactivity depend largely on the orientation of the hydroxyl group at the anomeric center. These subtleties can influence how these sugars interact in biological systems and how they can be utilized in chemical syntheses.