Multiple Choice
How many stereoisomers are possible for a 2-ketohexose, considering D and L isomerism?
28. Carbohydrates
Monosaccharides - D and L Isomerism
2:02 minutesProblem 3
Textbook Question
Indicate whether each of the following structures is D-glyceraldehyde or L-glyceraldehyde, assuming that the horizontal bonds point toward you and the vertical bonds point away from you (Section 4.7):
a. 
b. 
c. 
Verified step by step guidance1
Step 1: Recall the Fischer projection convention. In a Fischer projection, horizontal bonds represent groups pointing toward the viewer, while vertical bonds represent groups pointing away from the viewer.
Step 2: Identify the chiral center in glyceraldehyde. Glyceraldehyde has one chiral center, which is the carbon atom bonded to the hydroxyl group (-OH), the hydrogen atom (H), and the aldehyde group (-CHO).
Step 3: Determine the configuration of the chiral center. Assign priorities to the substituents based on the Cahn-Ingold-Prelog rules: (1) -CHO, (2) -OH, (3) -CH2OH, and (4) H.
Step 4: Analyze the arrangement of the substituents. If the -OH group is on the right side of the Fischer projection, the molecule is d-glyceraldehyde. If the -OH group is on the left side, the molecule is l-glyceraldehyde.
Step 5: Apply the above rules to each structure (a, b, c) provided in the problem. Carefully inspect the position of the -OH group in each Fischer projection to classify them as d-glyceraldehyde or l-glyceraldehyde.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
D and L Configuration
The D and L notation refers to the configuration of chiral centers in sugars. It is based on the orientation of the hydroxyl group (-OH) on the penultimate carbon (the second to last carbon) in the molecule. If the -OH group is on the right side in a Fischer projection, the sugar is designated as D; if it is on the left, it is designated as L.
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Fischer Projection
A Fischer projection is a two-dimensional representation of a three-dimensional organic molecule, particularly useful for depicting carbohydrates. In this representation, vertical lines represent bonds that project away from the viewer, while horizontal lines represent bonds that come toward the viewer. Understanding how to interpret Fischer projections is essential for determining the D or L configuration of sugars.
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Chirality and Stereoisomers
Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image, leading to the existence of stereoisomers. In the case of glyceraldehyde, there are two enantiomers: D-glyceraldehyde and L-glyceraldehyde, which differ in the spatial arrangement of their atoms. Recognizing chirality is crucial for understanding the behavior and reactivity of organic compounds, especially in biological systems.
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