Textbook Question
Which of the D-aldopentoses will give optically active aldaric acids on oxidation with HNO3?
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 56c
Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(c) 6-O-(β-D-galactopyranosyl)-D-glucopyranose
Verified step by step guidance1
Step 1: Understand the structure of the given sugar, 6-O-(β-D-galactopyranosyl)-D-glucopyranose. This is a disaccharide where a β-D-galactopyranose unit is linked to a D-glucopyranose unit through an oxygen atom at the 6th position of the glucose ring.
Step 2: Recall the definition of a reducing sugar. A sugar is considered reducing if it has a free anomeric carbon (a carbon attached to a hydroxyl group and double-bonded to an oxygen atom) that can open up to form an aldehyde or ketone group in solution.
Step 3: Analyze the structure to determine if either of the two sugar units has a free anomeric carbon. In this case, the anomeric carbon of the β-D-galactopyranose is involved in the glycosidic bond, so it is not free. However, the anomeric carbon of the D-glucopyranose is free, making this sugar a reducing sugar.
Step 4: Recall the concept of mutarotation. Mutarotation occurs when a sugar with a free anomeric carbon interconverts between its α and β anomers in solution. Since the D-glucopyranose unit has a free anomeric carbon, it can undergo mutarotation.
Step 5: Conclude that 6-O-(β-D-galactopyranosyl)-D-glucopyranose is a reducing sugar because it has a free anomeric carbon on the D-glucopyranose unit, and it can undergo mutarotation for the same reason.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Reducing Sugars
Reducing sugars are carbohydrates that can donate electrons to other molecules, typically due to the presence of a free aldehyde or ketone group. Common examples include glucose and fructose. These sugars can reduce metal ions, which is a key characteristic used in various biochemical tests.
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Reducing Sugars
Mutarotation
Mutarotation is the change in optical rotation that occurs when an anomeric carbon in a sugar ring opens and closes, interconverting between its alpha and beta forms. This process is significant in sugars like glucose and galactose, where the anomeric carbon can exist in two different configurations, affecting their reactivity and properties.
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Glycosidic Bonds
Glycosidic bonds are covalent linkages formed between the anomeric carbon of a sugar and another molecule, which can be another sugar or a different type of compound. In the case of disaccharides or oligosaccharides, the presence of a glycosidic bond can prevent mutarotation and the reducing ability of the sugar, as it locks the anomeric carbon in a specific configuration.
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Related Practice
Textbook Question
Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(a) methyl β-D-glucopyranoside
(b) 2,3,4,6-tetra-O-methyl-D-mannopyranose
(c) 1,3,6-tri-O-methyl-D-fructofuranose
(d) methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside
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Textbook Question
Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(b) α-D-fructofuranosyl-β-D-mannopyranoside
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Textbook Question
An unknown reducing disaccharide is found to be unaffected by invertase enzymes. Treatment with an α-galactosidase cleaves the disaccharide to give one molecule of D-fructose and one molecule of D-galactose. When the disaccharide is treated with excess iodomethane and silver oxide and then hydrolyzed in dilute acid, the products are 2,3,4,6-tetra-O-methylgalactose and 1,3,4-tri-O-methylfructose. Propose a structure for this disaccharide, and give its complete systematic name.
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Textbook Question
Which of the D-aldotetroses will give optically active aldaric acids on oxidation with HNO3?
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Textbook Question
Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose
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