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 45
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.
Verified step by step guidance1
Step 1: Understand the structure of lactulose. Lactulose is a disaccharide composed of two monosaccharides: d-galactopyranose and d-fructofuranose. These are connected by a β-1,4′-glycosidic linkage. The β-1,4′ linkage means the anomeric carbon of d-galactopyranose is connected to the fourth carbon of d-fructofuranose.
Step 2: Analyze the reaction conditions. The treatment with CH3I/Ag2O methylates all free hydroxyl (-OH) groups in the molecule. This means that any hydroxyl group not involved in the glycosidic bond will be converted to a methoxy (-OCH3) group. The subsequent treatment with HCl/H2O hydrolyzes the glycosidic bond, breaking the disaccharide into its monosaccharide components.
Step 3: Determine the number of nonmethylated hydroxyl groups. After methylation and hydrolysis, the d-galactopyranose subunit has one nonmethylated hydroxyl group, and the d-fructofuranose subunit has two. This indicates that the glycosidic bond involves one hydroxyl group from each monosaccharide, leaving the remaining hydroxyl groups free for methylation.
Step 4: Draw the structure of α-lactulose. Start by drawing the d-galactopyranose subunit in its pyranose (six-membered ring) form, ensuring the hydroxyl groups are positioned correctly. Then, draw the d-fructofuranose subunit in its furanose (five-membered ring) form. Connect the anomeric carbon of d-galactopyranose to the fourth carbon of d-fructofuranose via a β-1,4′-glycosidic bond.
Step 5: Verify the stereochemistry and functional groups. Ensure the stereochemistry of the hydroxyl groups on both monosaccharides matches their natural configurations. Confirm that the glycosidic bond is correctly represented as β-1,4′ and that the remaining hydroxyl groups are free for methylation during the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Glycosidic Linkage
A glycosidic linkage is a type of covalent bond that connects two monosaccharides to form a disaccharide. In the case of lactulose, the bond is specifically a β-1,4' glycosidic linkage, indicating the orientation of the bond between the anomeric carbon of one sugar and the hydroxyl group of another. Understanding this linkage is crucial for determining the structure and properties of the resulting disaccharide.
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Monosaccharide Structure
Monosaccharides, the building blocks of carbohydrates, can exist in different forms, such as pyranose (six-membered ring) and furanose (five-membered ring). In lactulose, d-galactopyranose and d-fructofuranose are the two monosaccharide units. Recognizing the structural differences and functional groups of these monosaccharides is essential for understanding their reactivity and the overall structure of lactulose.
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Methylation and Hydrolysis
Methylation is a chemical reaction that involves the addition of a methyl group (CH3) to a molecule, often affecting its reactivity and solubility. In the context of lactulose, treatment with CH3I/Ag2O followed by HCl/H2O leads to selective methylation and hydrolysis, revealing the number of nonmethylated hydroxyl groups on each monosaccharide. This process is important for analyzing the structure and functional groups present in the disaccharide.
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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
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.
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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 student isolated a monosaccharide and determined that it had a molecular weight of 150. Much to his surprise, he found that it was not optically active. What is the structure of the monosaccharide?
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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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