Question

\(\alpha-\mathrm{D}-(+)\)-glucose and \(\beta-\mathrm{D}-(+)\)-glucose are (a) Conformers (b) Epimers (c) Anomers (d) Enantiomers

Short answer

(c) Anomers.

Step-by-step solution

Understand the Nature of Glucose Isomers

Glucose can exist in different forms depending on the arrangement of atoms. These forms can be categorized based on structural differences at specific positions in the molecule.

Define Each Type of Isomer

- **Conformers**: Different spatial orientations of the atoms due to rotation around single bonds. - **Epimers**: Stereoisomers that differ in configuration at one specific carbon atom. - **Anomers**: Special type of epimers that differ in configuration at the anomeric carbon (the carbonyl carbon after ring formation). - **Enantiomers**: Mirror images of each other, non-superimposable.

Identify the Anomeric Carbon

In glucose, the anomeric carbon is the first carbon in the ring structure (C1). The two forms, alpha (α) and beta (β), differ at this carbon.

Compare
- and  - Forms

In
-D-(+)-glucose, the OH group on the anomeric carbon is trans to the CH2OH group, whereas in  -D-(+)-glucose, the OH group is cis to the CH2OH group.

Determine the Isomer Type

Since
- and  - differ at the anomeric carbon, they are classified as **Anomers**. They are not conformers, epimers (in the general sense), or enantiomers.

Key concepts

Anomers

Anomers are a unique type of isomer found in carbohydrates like glucose. When glucose molecules form a ring structure, a new chiral center is created at the carbon atom that becomes the anomeric carbon. Typically, this anomeric carbon was once the carbonyl carbon before the ring formation.

Anomers arise due to the orientation of the hydroxyl group attached to the anomeric carbon. In the case of glucose, this results in two different isomers: alpha (α) and beta (β). In (α-D-(+)-glucose ext{)}, the hydroxyl group is on the opposite side (trans) to the CH₂OH group. In (β-D-(+)-glucose ext{)}, it is on the same side (cis) as the CH₂OH group.

• Alpha anomer: OH group is trans to the CH₂OH group.
• Beta anomer: OH group is cis to the CH₂OH group.

These small differences define the major structural characteristic of anomers, usually affecting the molecule's physical properties and reactivity.

Epimers

Epimers are a type of stereoisomer found in molecules like sugars. These isomers differ from each other in the configuration at just one specific chiral carbon atom, not including the anomeric carbon, which distinguishes epimers from anomers. This singular change can lead to significant differences in biological function and chemical behavior.

For example, D-glucose and D-galactose are epimers at the fourth carbon atom (C4). This distinction impacts how sugars can be utilized or metabolized in biological systems. While both are sugars, the slight structural change at C4 differentiates them in terms of function and recognition by enzymes.

• Only one chiral center differs between epimers.
• Epimers are not mirror images of each other.
• The differences in configuration can drastically change the properties of the sugars.

Epimers are important in the study of stereochemistry because they showcase how a small change at a single carbon can impact the overall behavior of the molecule.

Enantiomers

Enantiomers are a fundamental concept in stereochemistry, characterized as molecules that are mirror images of each other but cannot be superimposed, much like left and right hands. These are also known as optical isomers, as they can rotate plane-polarized light in different directions.

Each enantiomer will have a chiral center, making them important in many biological processes. For example, consider L-glucose and D-glucose. These are enantiomers because they are mirror images across all chiral centers. Their non-superimposable nature means that they interact differently with other chiral molecules.

• Enantiomers exist in pairs.
• They have identical physical properties, except for the rotation of polarized light.
• They can impart drastically different biological activities.

Understanding enantiomers is crucial in fields like pharmacology, where the biological effects of each enantiomer can be drastically different.