Question

Couples. Indicate whether each of the following pairs of sugars consists of anomers, epimers, or an aldose-ketose pair. \(\sqrt{1}\) (a) D-glyceraldehyde and dihydroxyacetone (b) D-glucose and D-mannose (c) D-glucose and D-fructose (d) \(\alpha\) -D-glucose and \(\beta\) -D-glucose (e) D-ribose and D-ribulose (f) D-galactose and D-glucose

Short answer

(a) Aldose-ketose, (b) Epimers, (c) Aldose-ketose, (d) Anomers, (e) Aldose-ketose, (f) Epimers.

Step-by-step solution

Determine Pair (a)

D-glyceraldehyde is an aldose because it has an aldehyde group, while dihydroxyacetone is a ketose as it contains a ketone group. Therefore, this pair is an aldose-ketose pair.

Determine Pair (b)

D-glucose and D-mannose differ in the configuration around the C-2 carbon. Since they only differ in one stereocenter, they are epimers.

Determine Pair (c)

D-glucose is an aldose, having an aldehyde group, while D-fructose is a ketose, having a ketone group. Thus, this pair is an aldose-ketose pair.

Determine Pair (d)

The only difference between \(\alpha\)-D-glucose and \(\beta\)-D-glucose is the configuration around the anomeric carbon. Therefore, they are anomers.

Determine Pair (e)

D-ribose is an aldose, having an aldehyde group, and D-ribulose is a ketose, having a ketone group. This makes them an aldose-ketose pair.

Determine Pair (f)

D-galactose and D-glucose differ in the configuration around the C-4 carbon only. Hence, they are epimers.

Key concepts

Anomers

Anomers are a type of isomer specifically found in cyclic sugars. They differ in the configuration around the anomeric carbon, which is the carbon derived from the carbonyl carbon of the straight-chain form. When a sugar transforms into a ring form, the carbonyl carbon becomes a new stereocenter called the anomeric carbon.

• In the case of glucose, for example, the anomeric carbon is at position C-1. In the alpha (\(\alpha\)) form of glucose, the OH group on the anomeric carbon points in the opposite direction to the CH2OH group from the chair conformation. In the beta (\(\beta\)) form, it points in the same direction.
• This difference in stereochemistry results in two distinct anomers: \(\alpha\)-D-glucose and \(\beta\)-D-glucose.

Anomers are important in carbohydrate chemistry because they influence the sugar's reactivity and properties. The interconversion between these two forms in solution is known as mutarotation.

Epimers

Epimers are sugars that differ only in the configuration around a single stereocenter, excluding the anomeric carbon. This tiny change at just one carbon can result in quite different properties for the sugars.

• For example, D-glucose and D-mannose differ only at carbon 2. D-glucose has the OH group on the right at this position (in its linear form), whereas D-mannose has it on the left.
• Similarly, D-galactose and D-glucose differ at carbon 4. This difference in configuration classifies them as epimers.

Understanding epimers is crucial as they can affect the biological activity and interaction of sugars in metabolic pathways. The slight structural variance can lead to significant functional changes.

Aldose-Ketose Pair

Aldose and ketose sugars are classified based on the type of carbonyl group they contain. An aldose contains an aldehyde group at the end of the molecule, while a ketose has a ketone group, generally on the second carbon of the chain.

• Examples include glucose and fructose. Glucose is an aldose because it bears an aldehyde group, whereas fructose is a ketose with its ketone group on the second carbon.
• Similarly, D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), as well as D-ribose (aldose) and D-ribulose (ketose), are pairs where one contains an aldehyde and the other a ketone.

Identifying whether a sugar is an aldose or ketose is important in understanding its chemical reactivity and role in biological systems, as these groups determine how the sugar participates in reactions.