Textbook Question
What is the configuration of each of the asymmetric centers in the Fischer projection of
b. D-galactose?
5. Chirality
R and S of Fischer Projections
5:37 minutesProblem 23
Textbook Question
Which configuration (R or S) does the bottom asymmetric carbon have for the D series of sugars? Which configuration for the L series?
Verified step by step guidance1
Identify the bottom asymmetric carbon in the Fischer projection of the sugar.
Assign priorities to the four substituents attached to the bottom asymmetric carbon based on the Cahn-Ingold-Prelog priority rules.
Orient the molecule so that the lowest priority group is pointing away from you.
Determine the order of the remaining three groups: if the sequence is clockwise, the configuration is R; if counterclockwise, it is S.
For D sugars, the bottom asymmetric carbon typically has the R configuration, while for L sugars, it typically has the S configuration.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chirality and Asymmetric Carbons
Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image, often due to the presence of an asymmetric carbon atom. An asymmetric carbon is a carbon atom bonded to four different groups, leading to two possible configurations, known as enantiomers. Understanding chirality is crucial for determining the R or S configuration of sugars.
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Cahn-Ingold-Prelog Priority Rules
The Cahn-Ingold-Prelog (CIP) priority rules are a set of guidelines used to assign priorities to the substituents attached to an asymmetric carbon. According to these rules, the substituents are ranked based on atomic number, with higher atomic numbers receiving higher priority. This ranking is essential for determining whether the configuration is R (rectus, right) or S (sinister, left) when viewing the molecule from a specific orientation.
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D and L Configuration in Sugars
The D and L notation in sugars refers to the configuration of the asymmetric carbon farthest from the aldehyde or ketone group. In D sugars, this carbon has the same configuration as D-glyceraldehyde, while in L sugars, it has the opposite configuration. This distinction is important for understanding the stereochemistry of carbohydrates and their biological roles.
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