Question

Glucose and mannose are: A. anomers. B. epimers. C. enantiomers. D. non superimposable mirror images.

Short answer

Glucose and mannose are epimers.

Step-by-step solution

Define the Terms

- **Anomers**: These are isomers that differ in configuration only at the anomeric carbon atom. - **Epimers**: These are monosaccharides that differ in configuration at only one chiral carbon other than the anomeric carbon. - **Enantiomers**: These are non-superimposable mirror images of each other. - **Non-superimposable Mirror Images**: This is another term for enantiomers, where two molecules are mirror images and cannot be superimposed.

Analyze Glucose and Mannose Structures

Glucose and mannose are both aldohexoses, meaning they have the same molecular formula and the same basic structure, but differ at a specific stereocenter. Specifically, glucose and mannose differ at the C-2 position from each other.

Determine Type of Isomer

Since glucose and mannose differ only at the C-2 carbon, they do not fit the description of anomers, which differ at the anomeric carbon, nor enantiomers, which need to be mirror images. Instead, they fit the definition of epimers, differing in configuration at only one chiral carbon.

Conclusion

Given that glucose and mannose differ at one chiral center other than the anomeric carbon, they are classified as epimers.

Key concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. They are the basic building blocks for more complex carbohydrates like disaccharides and polysaccharides. Monosaccharides consist of a single sugar unit and have the general molecular formula \( C_nH_{2n}O_n \).
They can be categorized based on the number of carbon atoms they contain:

• **Trioses**: 3 carbon atoms
• **Tetroses**: 4 carbon atoms
• **Pentoses**: 5 carbon atoms
• **Hexoses**: 6 carbon atoms

Glucose and mannose are examples of aldoses, which are monosaccharides with an aldehyde group. They are classified as aldohexoses because they each contain six carbons and an aldehyde group. Monosaccharides can also be ketoses if they contain a ketone group.

Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same chemical formula but a different arrangement of atoms in space, leading to different properties. There are several types of isomerism in chemistry:

• **Structural Isomers**: These have the same molecular formula but differ in the connectivity of atoms.
• **Stereoisomers**: Their atoms are connected in the same order but differ in spatial orientation.

Among stereoisomers, two important types are enantiomers and epimers:
An **epimer** is a type of stereoisomer where the compounds differ in configuration at only one specific chiral carbon. For example, glucose and mannose are epimers differing at the C-2 carbon. An **enantiomer** refers to non-superimposable mirror images like your left and right hands. Understanding isomerism is crucial in fields like pharmacology, where the arrangement of atoms can drastically change the function and effectiveness of a compound.

Chiral Carbon

A chiral carbon is an essential concept in understanding isomerism. It is a carbon atom bonded to four different atoms or groups, making it a center for asymmetry.
This asymmetry is a key factor that allows for the diversity of isomers, including epimers and enantiomers.
Chiral carbons are like the stars of stereochemistry because they dictate how molecules can be arranged in space.
Here’s how to recognize a chiral carbon:

• Does it have four different groups attached?
• Can its mirror image not be superimposed on it?

For glucose and mannose, the difference at the C-2 position demonstrates how just one chiral carbon can lead to different sugar isomers that have unique properties.

Anomeric Carbon

The anomeric carbon is a special type of chiral center that is found in cyclic forms of sugars. It is derived from the carbonyl carbon (either aldehyde or ketone) when the sugar molecule forms a ring.
In glucose, for example, the C-1 carbon becomes the anomeric carbon during cyclization.
Here’s how you can identify the anomeric carbon:

• It is adjacent to the oxygen in the sugar’s pyranose or furanose ring structure.
• In aldohexoses like glucose, it was originally the carbon-1.

The configuration of the hydroxyl group (-OH) attached to the anomeric carbon can vary, giving rise to alpha and beta anomers. This is why it plays a crucial role in defining sugar characteristics, like how sugars behave in solution, how they interact with enzymes, and their sweetness levels. However, it's important to note that epimers like glucose and mannose would not differ at the anomeric carbon, as epimers differ at carbons other than the anomeric one.