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 11
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.
Verified step by step guidance1
Identify the starting molecule, D-galactose, which is an aldohexose (a six-carbon sugar with an aldehyde group at C1). Understand that the enediol rearrangement involves the migration of the carbonyl group from C1 to C2 under basic conditions.
Recognize that the enediol rearrangement proceeds through the formation of an enediol intermediate. The base abstracts a proton from the alpha-carbon (C2), forming a resonance-stabilized enediol structure where the double bond is between C1 and C2, and hydroxyl groups are attached to both carbons.
After the enediol intermediate is formed, the carbonyl group reforms, but this time at C2 instead of C1. This converts the sugar from an aldose (aldehyde-containing sugar) to a ketose (ketone-containing sugar). The resulting ketose is D-fructose.
Determine the C4 epimer of D-fructose. An epimer is a stereoisomer that differs in configuration at only one stereogenic center. The C4 epimer of D-fructose is D-psicose, which has the opposite stereochemistry at C4 compared to D-fructose.
Draw the furanose structure of D-psicose. In the furanose form, the sugar adopts a five-membered ring structure. The ketone group at C2 reacts with the hydroxyl group at C5 to form a cyclic hemiketal. Ensure the stereochemistry at C4 reflects the epimeric difference from D-fructose.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Carbonyl Group and Isomerization
The carbonyl group (C=O) is a functional group found in aldehydes and ketones, playing a crucial role in organic reactions. Isomerization refers to the process where a compound is transformed into another compound with the same molecular formula but a different structure. In the context of D-galactose, the carbonyl group can shift from the C1 position to the C2 position, leading to the formation of an enediol intermediate, which is essential for the rearrangement.
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Monosaccharides - D and L Isomerism
Enediol Rearrangement
The enediol rearrangement is a chemical reaction where a carbonyl compound is converted into an enediol, which is a compound containing both a double bond and a hydroxyl group. This rearrangement allows for the migration of the carbonyl group, facilitating the formation of different isomers. In this case, the enediol form of D-galactose allows for the isomerization to produce a C4 epimer of fructose.
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Furanose Structure
Furanose refers to a five-membered ring structure formed by the cyclization of sugars, particularly those containing a carbonyl group. In the case of the C4 epimer of fructose, the furanose form is generated when the hydroxyl group on C2 reacts with the carbonyl carbon, resulting in a stable cyclic structure. Understanding the furanose structure is essential for visualizing the final product of the isomerization process described in the question.
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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.
Textbook Question
Draw and name the products of bromine water oxidation of
(a) D-mannose
(b) D-galactose
(c) D-fructose
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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
Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.
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