Textbook Question
Draw the mechanism for the interconversion of α-D-glucose and β-D-glucose in dilute HCl.
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Ch. 20 - The Organic Chemistry of Carbohydrates
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
Chapter 21, Problem 46
A hexose was obtained after (+)-glyceraldehyde underwent three successive Kiliani–Fischer syntheses. Identify the hexose from the following experimental information: oxidation with nitric acid forms an optically active aldaric acid; a Wohl degradation followed by oxidation with nitric acid forms an optically inactive aldaric acid; and a second Wohl degradation forms erythrose.
Verified step by step guidance1
Step 1: Understand the Kiliani–Fischer synthesis. This reaction extends the carbon chain of an aldose by one carbon atom, creating a pair of epimers at the new stereocenter. Since ( + )-glyceraldehyde is the starting material, it undergoes three successive Kiliani–Fischer syntheses, resulting in a hexose with multiple stereocenters.
Step 2: Analyze the oxidation with nitric acid. Oxidation of the hexose with nitric acid converts both the aldehyde group and the terminal primary alcohol group into carboxylic acids, forming an aldaric acid. The fact that the aldaric acid is optically active indicates that the hexose is not symmetric, meaning it has a chiral center arrangement that does not result in a meso compound.
Step 3: Examine the first Wohl degradation. The Wohl degradation shortens the carbon chain of the hexose by one carbon atom, converting it into a pentose. Oxidation of this pentose with nitric acid forms an optically inactive aldaric acid. This indicates that the pentose is symmetric, meaning it is a meso compound. This symmetry provides a clue about the stereochemistry of the hexose.
Step 4: Analyze the second Wohl degradation. A second Wohl degradation shortens the pentose into a tetrose, specifically erythrose. Erythrose is a known compound with a specific stereochemistry, and this information helps backtrack the stereochemical configuration of the original hexose.
Step 5: Combine all the information. Based on the experimental data (optically active aldaric acid from the hexose, optically inactive aldaric acid from the pentose, and erythrose from the tetrose), deduce the stereochemistry of the hexose. The hexose must be D-mannose, as it fits all the given experimental observations and stereochemical requirements.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Kiliani–Fischer Synthesis
The Kiliani–Fischer synthesis is a method for elongating carbon chains in aldoses, allowing the formation of new hexoses from smaller sugars. This reaction involves the addition of a cyanide ion to an aldehyde, followed by hydrolysis to yield a new aldose. Understanding this process is crucial for determining the structure of the hexose produced from glyceraldehyde.
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Monosaccharides - Kiliani-Fischer
Aldaric Acids
Aldaric acids are dicarboxylic acids derived from aldoses through oxidation of both the aldehyde and hydroxyl groups. The formation of optically active and inactive aldaric acids in the question indicates the stereochemical properties of the original hexose. Recognizing the relationship between the hexose and its corresponding aldaric acid is essential for identifying the sugar.
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Wohl Degradation
Wohl degradation is a chemical reaction that allows for the conversion of aldoses into smaller sugars, typically by cleaving the sugar into two parts. This process is significant in the context of the question, as it provides insights into the structure of the original hexose based on the products formed after degradation. The ability to trace back the transformations helps in identifying the hexose from the experimental data.
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Related Practice
Textbook Question
Propose a mechanism for the formation of d-allose from d-glucose in a basic solution.
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Textbook Question
The disaccharide lactulose consists of a D-galactopyranose subunit and a D-fructofuranose subunit joined by a β-1,4′-glycosidic linkage. After treatment of lactulose with 1. excess CH3I/Ag2O, 2. HCl/H2O, the d-galactopyranose subunit was found to have one nonmethylated OH group, whereas the D-fructofuranose subunit had two. Draw the structure of ⍺-lactulose.
Textbook Question
Treatment with sodium borohydride converts aldose A to an optically inactive alditol. Wohl degradation of A forms B, whose alditol is optically inactive. Wohl degradation of B forms D-glyceraldehyde. Identify A and B.
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Textbook Question
A D-aldopentose is oxidized by nitric acid to an optically active aldaric acid. A Wohl degradation of the aldopentose leads to a monosaccharide that is oxidized by nitric acid to an optically inactive aldaric acid. Identify the D-aldopentose.
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Textbook Question
Draw the mechanism for the formation of β-lactose from ⍺-D-galactose and β-D-glucose in dilute HCl.
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