Question

Identify the following as diastereomers, enantiomers, and/ or anomers. (a) \(\alpha\) -D-fructose and \(\beta\) -D-fructose (b) D-galactose and L-galactose (c) \(\mathrm{L}\) -allose and \(\mathrm{D}\) -glucose (both aldohexoses)

Short answer

(a) Anomers, (b) Enantiomers, (c) Diastereomers.

Step-by-step solution

Understand the Definitions

Diastereomers are stereoisomers that are not mirror images of each other. Enantiomers are non-superimposable mirror images. Anomers are isomers differing at a new asymmetric carbon formed on ring closure.

Analyze Alpha and Beta D-Fructose

Alpha (α) and Beta (β) D-fructose differ in the configuration at the anomeric carbon (C-2 in fructose), making them anomers. Since they are not mirror images, they are not enantiomers.

Analyze D-Galactose and L-Galactose

D-galactose and L-galactose are mirror images across all chiral centers, making them enantiomers. However, they are not anomers because there is no difference at an anomeric carbon.

Analyze L-Allose and D-Glucose

L-allose and D-glucose are both aldohexoses but have different configurations at multiple carbon atoms, making them diastereomers as they are not mirror images nor do they differ at an anomeric carbon.

Key concepts

Diastereomers

Diastereomers are a fascinating class of stereoisomers. Unlike enantiomers, diastereomers are not mirror images of each other. They occur in compounds that have multiple chiral centers, where at least one center has a different configuration, while other centers may or may not be the same.

Key characteristics of diastereomers include:

• Differing physical properties, such as boiling points, melting points, and solubilities, making them easier to separate than enantiomers.
• Diastereomers can have different chemical properties in reactions and may interact differently with other chiral molecules.
• They can exist in compounds with any number of chiral centers, giving rise to a variety of possible isomers.

In the exercise, L-allose and D-glucose exemplify diastereomers because they contain different configurations at several carbon atoms but do not mirror each other.

Enantiomers

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. Like your left and right hands, they are symmetric but not identical when superimposed.

Key attributes of enantiomers include:

• Identical physical properties such as melting and boiling points (when measured in achiral environments).
• Different interactions with polarized light; they will rotate plane-polarized light in equal amounts but in opposite directions, a property termed "optical activity."
• Distinct interactions with other chiral molecules, which can lead to dramatically different biological activity.

D-galactose and L-galactose are perfect examples of enantiomers. They have opposite configurations at every chiral center, making them mirror images but not identical.

Anomers

Anomers are a unique type of stereoisomers found in carbohydrates. They are formed when a sugar molecule cyclizes, resulting in a new chiral center at the carbon that was a carbonyl carbon, known as the anomeric carbon.

Essential features of anomers include:

• The distinction between the anomers usually lies at the anomeric carbon, which can have two possible configurations: alpha (α) or beta (β).
• They are critical in determining the properties and reactivity of sugars.
• Anomers are a subset of epimers, differing only at the anomeric carbon.

In the original exercise, α-D-fructose and β-D-fructose are identified as anomers, differing specifically at the configuration of the anomeric carbon (C-2 for fructose), but they do not act as mirror images.