Multiple Choice
Which of the following depicts a D-sugar?
28. Carbohydrates
Monosaccharides - D and L Isomerism
5:43 minutesProblem 67
Textbook Question
Starting with a Fischer projection of d-glucose (and other sugars), switch only the stereocenter that gave it the designation of d. What new sugar have you made? [Hint: The answer is not l-glucose.]
Verified step by step guidance1
Identify the stereocenter in the Fischer projection of d-glucose that determines its 'd' designation. This is typically the chiral center farthest from the aldehyde group (C-5 in glucose).
Understand that the 'd' or 'l' designation is based on the orientation of the hydroxyl group (-OH) on the chiral center farthest from the carbonyl group. In d-glucose, this hydroxyl group is on the right side in the Fischer projection.
Switch the configuration of this specific stereocenter (C-5) from 'd' to its opposite configuration. This means changing the position of the hydroxyl group from the right to the left in the Fischer projection.
Recognize that changing the configuration of this stereocenter does not result in l-glucose, but rather an epimer of glucose. An epimer is a sugar that differs from another sugar in the configuration at only one chiral center.
Determine the name of the new sugar formed by this change. Since only the C-5 stereocenter is altered, the new sugar is d-mannose, which is an epimer of d-glucose at C-2.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Fischer Projection
A Fischer projection is a two-dimensional representation of a three-dimensional organic molecule by projection. It is particularly useful for depicting carbohydrates and amino acids. In this projection, the carbon chain is drawn vertically, with the most oxidized carbon at the top, and horizontal lines represent bonds coming out of the plane towards the viewer.
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Stereochemistry
Stereochemistry involves the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In sugars, stereochemistry is crucial as it determines the sugar's identity and properties. The 'd' or 'l' designation in sugars refers to the configuration of the chiral center farthest from the carbonyl group, which is critical in determining the sugar's stereochemical identity.
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Epimers
Epimers are sugars that differ only in the configuration around one specific carbon atom. In the context of the question, changing the stereocenter that gives d-glucose its 'd' designation results in an epimer of glucose. This change does not produce l-glucose but rather another sugar, such as d-mannose or d-galactose, depending on which stereocenter is altered.
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