Textbook Question
Predict the products formed when the following sugars react with excess acetic anhydride and pyridine.
(a) α-D-glucopyranose
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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 27b
Draw the expected product of the reaction of the following sugars with excess methyl iodide and silver oxide.
(b) β-D-galactopyranose
Verified step by step guidance1
Identify the functional groups in β-D-galactopyranose that can react with methyl iodide (CH₃I) in the presence of silver oxide (Ag₂O). These include the hydroxyl (-OH) groups and the anomeric hydroxyl group.
Understand the reaction mechanism: Methyl iodide acts as a methylating agent, and silver oxide facilitates the deprotonation of hydroxyl groups, allowing them to be methylated. Each hydroxyl group will be converted into a methoxy (-OCH₃) group.
Determine the stereochemistry of β-D-galactopyranose. The β-anomer means the anomeric hydroxyl group is in the equatorial position. This stereochemistry will be retained in the product.
Replace all hydroxyl (-OH) groups, including the anomeric hydroxyl group, with methoxy (-OCH₃) groups. Ensure that the stereochemistry of the sugar ring is preserved during this substitution.
Draw the final structure of the fully methylated β-D-galactopyranose, where all hydroxyl groups have been replaced with methoxy groups, and confirm that the stereochemistry of the sugar ring is consistent with the original β-anomer.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Glycosylation Reaction
Glycosylation is a chemical reaction where a sugar molecule (glycosyl donor) reacts with an alcohol or another sugar (glycosyl acceptor) to form a glycosidic bond. In this context, the reaction of b-D-galactopyranose with methyl iodide involves the substitution of the hydroxyl group on the sugar with a methyl group, leading to the formation of a methyl glycoside.
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General Reaction
Methyl Iodide as a Methylating Agent
Methyl iodide (CH3I) is a potent methylating agent commonly used in organic synthesis. It can transfer a methyl group to nucleophiles, such as the hydroxyl groups on sugars. In the presence of a base like silver oxide, methyl iodide facilitates the formation of methyl ethers from alcohols, which is crucial for understanding the expected product of the reaction.
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Silver Oxide as a Base
Silver oxide (Ag2O) acts as a mild base in organic reactions, promoting the deprotonation of alcohols to form alkoxide ions. This enhances the nucleophilicity of the hydroxyl group in b-D-galactopyranose, making it more reactive towards methyl iodide. Understanding the role of silver oxide is essential for predicting the outcome of the reaction and the stability of the resulting methyl glycoside.
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Related Practice
Textbook Question
What do the results of parts (a) and (b) imply about the hemiacetal structure of fructose?
Textbook Question
Propose a mechanism for methylation of any one of the hydroxy groups of methyl α-D-glucopyranoside, using NaOH and dimethyl sulfate.
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Textbook Question
Draw the expected product of the reaction of the following sugars with excess methyl iodide and silver oxide.
(a) α-D-fructofuranose
Textbook Question
Show that Ruff degradation of D-mannose gives the same aldopentose (D-arabinose) as does D-glucose.
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Textbook Question
Predict the products formed when the following sugars react with excess acetic anhydride and pyridine.
(b) β-D-ribofuranose
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