Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 5

Chapter 23, Problem 5

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 guidance

Step 1: Identify the bottom asymmetric carbon in both D-Glucose and L-Glucose. This is the carbon atom directly attached to the CH₂OH group.

Step 2: Assign priorities to the substituents attached to the bottom asymmetric carbon based on the Cahn-Ingold-Prelog priority rules. The substituents are: CH₂OH (highest priority), OH, H, and the rest of the chain.

Step 3: Determine the spatial arrangement of the substituents around the bottom asymmetric carbon for D-Glucose. Visualize or use a model to determine whether the configuration follows a clockwise (R) or counterclockwise (S) rotation.

Step 4: Repeat the process for L-Glucose. Note that the substituents on the bottom asymmetric carbon are mirrored compared to D-Glucose, which will affect the configuration.

Step 5: Conclude the configurations for both D-Glucose and L-Glucose based on the analysis. For D-Glucose, the bottom asymmetric carbon typically has an R configuration, while for L-Glucose, it has an 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 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 organic chemistry, chiral centers, typically carbon atoms with four different substituents, give rise to two configurations: R (rectus) and S (sinister). Understanding chirality is crucial for determining the specific configuration of sugars like D-Glucose and L-Glucose.

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional organic molecules, particularly useful for depicting the stereochemistry of sugars. In these projections, vertical lines represent bonds going back into the plane, while horizontal lines represent bonds coming out. This format allows for easy identification of chiral centers and their configurations, essential for distinguishing between D and L forms of sugars.

D and L Configuration

The D and L notation for sugars is based on the orientation of the hydroxyl group (-OH) on the penultimate carbon (the second to last carbon) in the Fischer projection. If the -OH group is on the right side, the sugar is classified as D (from dexter, meaning right), while if it is on the left, it is classified as L (from laevus, meaning left). This classification is fundamental in carbohydrate chemistry and helps in understanding the biological roles of different sugar forms.

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Representations of L-Configuration

Related Practice

Textbook Question

Draw the mirror images of glucose and fructose. Are glucose and fructose chiral? Do you expect them to be optically active?

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Textbook Question

How many asymmetric carbons and stereoisomers are there for an aldohexose? For a ketohexose?

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Textbook Question

Draw the Haworth projection for the cyclic structure of D-mannose by laying down the Fischer projection.

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Textbook Question

c. Draw d-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.

d. Draw the C4 “epimer” of D-xylose. Notice that this “epimer” is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.

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Textbook Question

a. Draw D-allose, the C3 epimer of glucose.

b. Draw D-talose, the C2 epimer of d-galactose

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Textbook Question

(a) There is only one ketotriose, called dihydroxyacetone. Draw its structure.

(b) There is only one aldotriose, called glyceraldehyde. Draw the two enantiomers of glyceraldehyde.

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