Textbook Question
Figure 23-2 shows that the degradation of D-glucose gives D-arabinose, an aldopentose. Arabinose is most stable in its furanose form. Draw D-arabinofuranose.
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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 10b
Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.
Verified step by step guidance1
Understand the problem: The question asks for the structure of D-ribofuranose, which is the furanose (five-membered ring) form of D-ribose. Ribose is a monosaccharide with the molecular formula C₅H₁₀O₅, and it is the C2 epimer of arabinose, meaning the configuration at the second carbon is inverted compared to arabinose.
Recall the structure of D-ribose: D-ribose is an aldopentose, meaning it has five carbon atoms, one of which is part of an aldehyde group (-CHO). The remaining carbons are part of hydroxyl (-OH) and hydrogen (-H) groups, arranged in a specific stereochemistry.
Convert D-ribose to its furanose form: In aqueous solution, D-ribose cyclizes to form a furanose ring. This occurs when the hydroxyl group on C4 attacks the aldehyde group on C1, forming a hemiacetal. This reaction creates a five-membered ring structure (furanose).
Determine the stereochemistry: In D-ribofuranose, the stereochemistry of the chiral carbons (C2, C3, and C4) must match that of D-ribose. Specifically, the hydroxyl group on C2 is on the right (down in the Haworth projection), the hydroxyl group on C3 is on the left (up in the Haworth projection), and the hydroxyl group on C4 is on the right (down in the Haworth projection).
Draw the structure: Represent the furanose ring as a five-membered ring with oxygen as one of the vertices. Place the hydroxyl and hydrogen groups on the appropriate carbons based on the stereochemistry described. Ensure that the anomeric carbon (C1) has either an α or β configuration, depending on whether the hydroxyl group is trans or cis to the CH₂OH group on C4.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Epimer
An epimer is a type of stereoisomer that differs in configuration at only one specific carbon atom. In the case of ribose and arabinose, they are both pentoses but differ at the C2 position. Understanding epimers is crucial for recognizing how small changes in molecular structure can lead to different properties and functions in carbohydrates.
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Identifying Types of Stereoisomers
Furanose Form
The furanose form refers to a five-membered ring structure that certain sugars, like ribose, can adopt. This conformation is formed when the carbonyl group reacts with a hydroxyl group on the same molecule, leading to a more stable cyclic structure. Recognizing the furanose form is essential for understanding the stability and reactivity of sugars in biological systems.
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06:23Monosaccharides - Forming Cyclic Hemiacetals
D- and L- Configuration
The D- and L- configuration of sugars is based on the orientation of the hydroxyl group on the chiral carbon farthest from the carbonyl group. In D-sugars, this hydroxyl group is on the right in a Fischer projection, while in L-sugars, it is on the left. This distinction is important for identifying the specific sugar and its biological role, as well as for drawing accurate structural representations.
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Related Practice
Textbook Question
When D-glucose is reduced with sodium borohydride, optically active glucitol results. When optically active D-galactose is reduced, however, the product is optically inactive. Explain this loss of optical activity.
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Textbook Question
Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranose is +150.7°, and that of the β anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at equilibrium.
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Textbook Question
The carbonyl group in D-galactose may be isomerized from C1 to C2 by brief treatment with dilute base (by the enediol rearrangement). The product is the C4 epimer of fructose. Draw the furanose structure of the product.
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Textbook Question
Talose is the C4 epimer of mannose. Draw the chair conformation of D-talopyranose.
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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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