Textbook Question
The sugar d-gulose is a sweet-tasting syrup.
a. Draw the Fischer projection for L-gulose.
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20. Carbohydrates
D vs L Enantiomers
3:36 minutesProblem 63a
Textbook Question
How are the following pairs of carbohydrates, shown in a Fischer projection, related to each other? Are they structural isomers, enantiomers, diastereomers, or epimers? Identify each as the D- or L-isomer.
(a) 
Verified step by step guidance1
Step 1: Understand the types of relationships between carbohydrates. Structural isomers have the same molecular formula but different connectivity. Enantiomers are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images. Epimers are diastereomers that differ at only one stereocenter.
Step 2: Analyze the Fischer projections of the given carbohydrates. Look at the arrangement of hydroxyl (-OH) groups and hydrogen atoms around the chiral centers in the molecules.
Step 3: Compare the configurations of all chiral centers in the two carbohydrates. If all chiral centers are opposite, the molecules are enantiomers. If only one chiral center differs, they are epimers. If more than one chiral center differs but not all, they are diastereomers.
Step 4: Determine the d- or l-isomer designation. In Fischer projections, the d-isomer has the hydroxyl group on the right side of the bottom-most chiral center, while the l-isomer has it on the left side.
Step 5: Based on the analysis, classify the relationship between the two carbohydrates and identify their d- or l-isomer designation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Structural Isomers
Structural isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms. This means that the arrangement of atoms in the molecule is different, leading to distinct chemical properties. Understanding structural isomers is crucial for analyzing carbohydrates, as their different structures can significantly affect their biological functions.
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Isomers Concept 1
Stereoisomers
Stereoisomers are compounds that have the same molecular formula and connectivity but differ in the spatial arrangement of their atoms. This category includes enantiomers and diastereomers. Enantiomers are non-superimposable mirror images, while diastereomers are not mirror images and have different physical properties. Recognizing these relationships is essential for understanding carbohydrate behavior in biological systems.
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D- and L- Isomers
D- and L- isomers refer to the configuration of chiral centers in carbohydrates, based on the orientation of the hydroxyl group on the penultimate carbon atom. The 'D' designation indicates that the hydroxyl group is on the right in a Fischer projection, while 'L' indicates it is on the left. This classification is important for identifying the specific type of sugar and its role in metabolism.
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