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Ch. 23 - Carbohydrates and Nucleic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 23 - Carbohydrates and Nucleic AcidsProblem 20c,d
Chapter 23, Problem 20c,d

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.
(c) α-D-allopyranose
(d) ethyl β-D-ribofuranoside

Verified step by step guidance
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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 group (-CHO) or a free ketone group (-C=O) in their structure. These groups can reduce mild oxidizing agents like Benedict's or Fehling's solution.
Step 2: Analyze the structure of a-D-allopyranose. This is a six-membered ring (pyranose form) of the sugar D-allose. Check if the anomeric carbon (C1) has a free hydroxyl group (-OH) or if it is involved in a glycosidic bond. If the hydroxyl group is free, it is a reducing sugar.
Step 3: Examine ethyl b-D-ribofuranoside. This is a furanose form (five-membered ring) of D-ribose where the anomeric carbon (C1) is bonded to an ethyl group via a glycosidic bond. Since the anomeric carbon is not free, this compound is not a reducing sugar.
Step 4: Comment on sucrose as table sugar. Sucrose is a disaccharide composed of glucose and fructose linked by a glycosidic bond between the anomeric carbons of both monosaccharides. Since neither anomeric carbon is free, sucrose is not a reducing sugar.
Step 5: Summarize the findings. a-D-allopyranose is a reducing sugar because it has a free anomeric hydroxyl group, while ethyl b-D-ribofuranoside and sucrose are not reducing sugars due to the absence of a free anomeric carbon.

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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 chemical 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

Sucrose

Sucrose, commonly known as table sugar, is a disaccharide composed of glucose and fructose linked by a glycosidic bond. Unlike reducing sugars, sucrose does not have a free aldehyde or ketone group in its structure, making it a non-reducing sugar. This characteristic is important in various biochemical reactions and food chemistry.

Pyranose and Furanose Forms

Pyranose and furanose refer to the cyclic forms of sugars, where pyranose is a six-membered ring and furanose is a five-membered ring. These structures arise from the reaction of the carbonyl group with a hydroxyl group in the sugar molecule. Understanding these forms is crucial for identifying the reactivity and properties of different sugars, including their classification as reducing or non-reducing.
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Related Practice
Textbook Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(a) Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimers results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.

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Textbook Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has two –OH protons, and removing one or the other gives two different enolate ions.

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Textbook Question

Draw the structures of the compound methyl α-D-galactopyranoside.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(e)

(f)

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Textbook Question

Two sugars, A and B, are known to be glucose and galactose, but it is not certain which one is which. On treatment with nitric acid, A gives an optically inactive aldaric acid, while B gives an optically active aldaric acid. Which sugar is glucose, and which is galactose?

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Textbook Question

Draw the structures of the compound α-D-allopyranose.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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