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?
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Ch. 23 - Carbohydrates and Nucleic Acids
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 23, Problem 6a,b
a. Draw D-allose, the C3 epimer of glucose.
b. Draw D-talose, the C2 epimer of d-galactose
Verified step by step guidance1
Step 1: Understand the concept of epimers. Epimers are diastereomers that differ in the configuration of only one stereogenic center. For example, the C3 epimer of glucose means that the configuration at the third carbon (C3) is inverted compared to D-glucose, while the rest of the stereocenters remain the same.
Step 2: Recall the structure of D-glucose. D-glucose is an aldohexose with the following stereochemistry: C2 (R), C3 (S), C4 (R), and C5 (R). To draw D-allose, invert the configuration at C3 while keeping the configurations at C2, C4, and C5 the same.
Step 3: Draw D-allose. Start with the Fischer projection of D-glucose, then invert the hydroxyl group at C3. In D-glucose, the hydroxyl group at C3 is on the left; in D-allose, it will be on the right.
Step 4: Recall the structure of D-galactose. D-galactose is also an aldohexose with the following stereochemistry: C2 (R), C3 (S), C4 (S), and C5 (R). To draw D-talose, invert the configuration at C2 while keeping the configurations at C3, C4, and C5 the same.
Step 5: Draw D-talose. Start with the Fischer projection of D-galactose, then invert the hydroxyl group at C2. In D-galactose, the hydroxyl group at C2 is on the right; in D-talose, it will be on the left.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Epimers
Epimers are a specific type of diastereomer that differ in configuration at only one stereogenic center. In the context of carbohydrates, epimers are important for understanding the structural variations between sugars, such as D-allose and D-talose, which are epimers of glucose and D-galactose, respectively. Recognizing the position of the differing hydroxyl group is crucial for accurately drawing these sugars.
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Identifying Types of Stereoisomers
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 that go back into the plane, while horizontal lines represent bonds that come out of the plane. Understanding how to interpret and draw Fischer projections is essential for accurately representing the structures of D-allose and D-talose.
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D and L Configuration
The D and L configuration refers to the orientation of the hydroxyl group on the chiral carbon farthest from the carbonyl group in a sugar molecule. In D-sugars, this hydroxyl group is on the right in Fischer projections, while in L-sugars, it is on the left. This classification is fundamental for distinguishing between different sugars and is critical when drawing D-allose and D-talose.
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Related Practice
Textbook Question
How many asymmetric carbons and stereoisomers are there for an aldohexose? For a ketohexose?
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Textbook Question
Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection.
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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) 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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