Textbook Question
Predict the products obtained when d-galactose reacts with each reagent.
(b) NaOH, H2O
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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 52c
Predict the products obtained when d-galactose reacts with each reagent.
(c) CH3OH, H+
Verified step by step guidance1
Step 1: Recognize that D-galactose is an aldose sugar, specifically an aldohexose, meaning it contains an aldehyde group (-CHO) and six carbon atoms. In the presence of an acid catalyst (H⁺) and methanol (CH₃OH), the reaction will involve the formation of a glycoside.
Step 2: Understand that the reaction occurs at the anomeric carbon (C-1) of D-galactose. The aldehyde group reacts with methanol under acidic conditions to form an acetal, specifically a methyl glycoside.
Step 3: Note that the reaction produces two stereoisomers (anomers) because the newly formed glycosidic bond can adopt either an α- or β-configuration. These configurations depend on the orientation of the methoxy group (-OCH₃) relative to the ring structure.
Step 4: Write the structures of the products. The α-anomer will have the methoxy group (-OCH₃) on the opposite side of the ring relative to the CH₂OH group at C-5, while the β-anomer will have the methoxy group on the same side as the CH₂OH group.
Step 5: Conclude that the products of the reaction are α-D-galactopyranoside and β-D-galactopyranoside, where the pyranose form of D-galactose is retained, and the methoxy group replaces the hydroxyl group at the anomeric carbon.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Monosaccharide Structure
D-galactose is a monosaccharide, specifically an aldohexose, which means it contains an aldehyde group and six carbon atoms. Understanding its structure is crucial for predicting how it will react with various reagents. The configuration of the hydroxyl groups around the carbon atoms influences its reactivity and the types of products formed during chemical reactions.
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Monosaccharides - Common Structures
Acid-Catalyzed Reactions
The presence of H+ indicates an acid-catalyzed reaction, which can facilitate the conversion of alcohols and sugars. In this context, the acid can protonate the hydroxyl groups of d-galactose, making them more reactive towards nucleophiles like methanol (CH3OH). This process often leads to the formation of glycosidic bonds or other derivatives, depending on the reaction conditions.
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Methanolysis
Methanolysis refers to the reaction of a compound with methanol, often resulting in the formation of methyl ethers. In the case of d-galactose reacting with methanol in the presence of acid, the hydroxyl groups can be converted into methoxy groups, yielding methyl galactosides. This transformation is significant in carbohydrate chemistry for modifying sugars and studying their properties.
Related Practice
Textbook Question
Predict the products obtained when d-galactose reacts with each reagent.
(f) excess Ac2O and pyridine
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Textbook Question
Predict the products obtained when D-galactose reacts with each reagent.
(e) H2, Ni
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Textbook Question
Predict the products obtained when D-galactose reacts with each reagent.
(d) Ag(NH3)2+ –OH
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Textbook Question
(a) Give the products expected when (+)-glyceraldehyde reacts with HCN.
(b) What is the relationship between the products? How might they be separated?
(c) Are the products optically active? Explain.
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Textbook Question
The relative configurations of the stereoisomers of tartaric acid were established by the following syntheses:
(1) D-(+)-glyceraldehyde diastereomers A and B (separated)
(2) Hydrolysis of A and B using aqueous Ba(OH)2 gave C and D, respectively.
(3) HNO3 oxidation of C and D gave (-)-tartaric acid and meso-tartaric acid, respectively.
(a) You know the absolute configuration of D-(+)-glyceraldehyde. Use Fischer projections to show the absolute configurations of products A, B, C, and D.
(b) Show the absolute configurations of the three stereoisomers of tartaric acid: (+)-tartaric acid, (-)-tartaric acid, and meso-tartaric acid.
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