Textbook Question
Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?
(c) 6-O-(β-D-galactopyranosyl)-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 56
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
Verified step by step guidance1
Step 1: Understand the concept of reducing sugars. Reducing sugars are carbohydrates that can act as reducing agents due to the presence of a free aldehyde or ketone group. This typically requires the sugar to have a free anomeric carbon (not involved in a glycosidic bond).
Step 2: Analyze mutarotation. Mutarotation occurs when a sugar has a free anomeric carbon that can interconvert between alpha and beta forms in solution. This requires the sugar to be in equilibrium between its cyclic and open-chain forms.
Step 3: Examine compound (a), methyl β-D-glucopyranoside. The anomeric carbon is involved in a glycosidic bond (methyl group attached), so it is not a reducing sugar and cannot undergo mutarotation.
Step 4: Examine compound (b), 2,3,4,6-tetra-O-methyl-D-mannopyranose. The anomeric carbon is free (not involved in a glycosidic bond), so it is a reducing sugar and can undergo mutarotation.
Step 5: Examine compounds (c) and (d). For (c), 1,3,6-tri-O-methyl-D-fructofuranose, the anomeric carbon is free, so it is a reducing sugar and can undergo mutarotation. For (d), methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside, the anomeric carbon is involved in a glycosidic bond, so it is not a reducing sugar and cannot undergo mutarotation.
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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. This property allows them to reduce certain reagents, such as Benedict's or Fehling's solutions. Common examples include glucose and fructose, which can exist in open-chain forms that contain these reactive groups.
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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 for sugars that can exist in both cyclic and open-chain forms, allowing them to exhibit different optical activities. Only sugars with a free anomeric carbon can undergo mutarotation.
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03:06Mechanism
Methyl Glycosides
Methyl glycosides are derivatives of sugars where the hydroxyl group on the anomeric carbon is replaced by a methoxy group (-OCH3). This modification typically prevents mutarotation because the anomeric carbon is no longer free to interconvert between its alpha and beta forms. As a result, methyl glycosides are generally non-reducing sugars since they lack a reactive carbonyl group.
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Related Practice
Textbook Question
Draw the structures (using chair conformations of pyranoses) of the following disaccharides.
(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose
(b) α-D-fructofuranosyl-β-D-mannopyranoside
(c) 6-O-(β-D-galactopyranosyl)-D-glucopyranose
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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
Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:
G = C and A = T
(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?
(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?
(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?
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Textbook Question
Draw the following sugar derivatives.
(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
1
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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
2
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