Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 20e,f

Chapter 23, Problem 20e,f

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

Verified step by step guidance

Step 1: Analyze the chemical structures provided in the images. The first image shows a disaccharide with glycosidic bonds, and the second image is labeled as sucrose, which is also a disaccharide.

Step 2: Determine whether the sugars are reducing or non-reducing. A reducing sugar must have a free anomeric carbon (a carbon attached to the oxygen in the hemiacetal or hemiketal form) that can open up to form an aldehyde or ketone group. In sucrose, both anomeric carbons are involved in the glycosidic bond, preventing the sugar from acting as a reducing sugar.

Step 3: Comment on the common name 'sucrose' for table sugar. Sucrose is widely known as table sugar and is a non-reducing sugar due to the absence of a free anomeric carbon. It is composed of glucose and fructose linked by an α-1,2-glycosidic bond.

Step 4: Compare sucrose with other disaccharides like maltose or lactose, which are reducing sugars because they have a free anomeric carbon capable of undergoing oxidation.

Step 5: Conclude that sucrose is a non-reducing sugar and explain its significance in food and biochemistry, emphasizing its stability and inability to participate in reducing reactions like Benedict's test.

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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, which can reduce certain chemical reagents. In contrast, sucrose, which is a disaccharide composed of glucose and fructose, does not have a free aldehyde or ketone group in its structure, making it a non-reducing sugar.

Sucrose

Sucrose, commonly known as table sugar, is a disaccharide formed from the linkage of glucose and fructose through a glycosidic bond. This bond involves the anomeric carbon of glucose and the hydroxyl group of fructose, resulting in a molecule that lacks a free aldehyde or ketone group. Consequently, sucrose does not exhibit reducing properties, distinguishing it from other sugars that can participate in redox reactions.

Glycosidic Bond

A glycosidic bond is a type of covalent bond that connects a carbohydrate (sugar) molecule to another group, which can be another carbohydrate or a different type of molecule. In the case of sucrose, the glycosidic bond forms between the anomeric carbon of glucose and the hydroxyl group of fructose. This bond is crucial for the formation of disaccharides and polysaccharides, influencing their chemical properties and biological functions.

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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

Draw the structures of the compound ethyl β-D-ribofuranoside.

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

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