Question

Name which, if any, of the following are epimers of D-glucose: D-mannose, D-galactose, D-ribose.

Short answer

D-mannose and D-galactose are epimers of D-glucose. D-ribose is not an epimer of D-glucose.

Step-by-step solution

- Understand Epimers

Epimers are stereoisomers that differ in configuration at exactly one chiral carbon.

- Identify Chiral Centers in D-glucose

D-glucose has four chiral centers at carbons 2, 3, 4, and 5. The structural formula of D-glucose is: \[O=C(H)-C(H)(OH)-C(H)(OH)-C(H)(OH)-C(H)(OH)-CH_2OH\]

- Compare D-mannose with D-glucose

D-mannose differs from D-glucose at carbon 2: D-glucose at C2 is \(H-C-OH\) whereas D-mannose at C2 is \(OH-C-H\). Therefore, D-mannose is an epimer of D-glucose at C2.

- Compare D-galactose with D-glucose

D-galactose differs from D-glucose at carbon 4: D-glucose at C4 is \(H-C-OH\) whereas D-galactose at C4 is \(OH-C-H\). Therefore, D-galactose is an epimer of D-glucose at C4.

- Compare D-ribose with D-glucose

D-ribose only has three chiral centers at carbons 2, 3, and 4, making it a pentose, not a hexose like D-glucose. Therefore, D-ribose cannot be an epimer of D-glucose.

Key concepts

Chiral centers

Chiral centers are specific atoms in a molecule, typically carbon atoms, that have four different groups attached to them. This creates a non-superimposable mirror image, similar to how your left and right hands are mirror images but not identical. These atoms play a critical role in the stereochemistry of a molecule.

In the context of carbohydrates like D-glucose, the chiral centers determine the spatial arrangement of the -OH (hydroxyl) and -H (hydrogen) groups attached to each carbon atom. The four chiral centers in D-glucose are found at carbons 2, 3, 4, and 5.

Identifying chiral centers is crucial for understanding the behavior and properties of different sugars, especially when studying isomers and epimers. Recognizing which carbons are chiral helps in distinguishing how sugars differ from each other and how they might react in biological systems.

D-glucose

D-glucose is one of the most important monosaccharides in biology. It is a hexose, meaning it has six carbon atoms, and is classified as an aldose because it has an aldehyde group (-CHO) at carbon 1.

The structure of D-glucose can be written as follows:
\[O=C(H)-C(H)(OH)-C(H)(OH)-C(H)(OH)-C(H)(OH)-CH_2OH\]

D-glucose is known for its four chiral centers at carbons 2, 3, 4, and 5, making it a molecule with multiple stereoisomers. In its most common form, D-glucose has a specific arrangement of hydroxyl (-OH) and hydrogen (-H) groups around these carbon atoms. This particular arrangement distinguishes it from its counterparts like D-mannose and D-galactose.

Stereoisomers

Stereoisomers are molecules that have the same molecular formula but differ in the spatial arrangement of atoms. These differences arise from the presence of chiral centers, leading to various configurations.

In the case of D-glucose, its stereoisomers include epimers such as D-mannose and D-galactose. What makes an epimer special is that it differs from another molecule at only one specific chiral center. For example:

• D-mannose is an epimer of D-glucose at carbon 2, where D-glucose has the arrangement \(H-C-OH\) and D-mannose has \(OH-C-H\).
• D-galactose is an epimer of D-glucose at carbon 4, where D-glucose has \(H-C-OH\) and D-galactose has \(OH-C-H\).

Understanding stereoisomers and epimers is essential for grasping the complexity of carbohydrate chemistry and their roles in biological processes.