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Ch. 20 - The Organic Chemistry of Carbohydrates
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 20 - The Organic Chemistry of CarbohydratesProblem 70
Chapter 21, Problem 70

Trehalose, C12H22O11, is a nonreducing sugar that is only 45% as sweet as sugar. When hydrolyzed by aqueous acid or the enzyme maltase, it forms only D-glucose. When it is treated with excess methyl iodide in the presence of Ag2O and then hydrolyzed with water under acidic conditions, only 2,3,4,6-tetra-O-methyl-d-glucose is formed. Draw the structure of trehalose.

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Understand the problem: Trehalose is a disaccharide that, upon hydrolysis, yields only d-glucose. This indicates that trehalose is composed of two glucose units linked together. The problem also specifies that trehalose is a nonreducing sugar, which means that the anomeric carbons of both glucose units are involved in the glycosidic bond, preventing them from acting as reducing agents.
Analyze the methylation experiment: When trehalose is treated with excess methyl iodide (CH3I) in the presence of Ag2O, all free hydroxyl (-OH) groups are methylated. The subsequent hydrolysis step reveals that the product is 2,3,4,6-tetra-O-methyl-d-glucose. This indicates that the hydroxyl groups on the anomeric carbons of the glucose units are not free and are instead involved in the glycosidic bond.
Determine the glycosidic linkage: Since trehalose is a nonreducing sugar, the glycosidic bond must be between the anomeric carbons of the two glucose units. This type of linkage is called an α,α-1,1-glycosidic bond, where both glucose units are in the α-configuration at their anomeric carbons.
Draw the structure of trehalose: Start by drawing two glucose units. Ensure that both glucose units are in the α-configuration at their anomeric carbons (the -OH group on C1 is axial and points down in the Haworth projection). Connect the anomeric carbons (C1 of each glucose) via an oxygen atom to form the α,α-1,1-glycosidic bond.
Verify the structure: Confirm that the structure satisfies all the given conditions. The glycosidic bond between the anomeric carbons makes trehalose a nonreducing sugar. Upon hydrolysis, the bond breaks, yielding two d-glucose molecules. The methylation experiment confirms that all free hydroxyl groups are methylated, leaving the anomeric carbons involved in the glycosidic bond.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Structure of Trehalose

Trehalose is a disaccharide composed of two glucose units linked by an α,α-1,1-glycosidic bond. This unique linkage makes trehalose a nonreducing sugar, as it does not have a free aldehyde or ketone group. Understanding its structure is crucial for predicting its reactivity and the products formed during hydrolysis.
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Hydrolysis of Disaccharides

Hydrolysis is a chemical reaction that involves the breaking of bonds in a compound by the addition of water. In the case of trehalose, hydrolysis can occur under acidic conditions or enzymatically, resulting in the formation of d-glucose. This process is essential for understanding how trehalose can be converted into its monosaccharide components.
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Methylation and Its Products

Methylation is a chemical reaction that involves the addition of methyl groups to a molecule, often using reagents like methyl iodide. In the case of trehalose, treatment with excess methyl iodide in the presence of Ag2O leads to the formation of 2,3,4,6-tetra-O-methyl-d-glucose. This transformation is important for understanding the derivatization of sugars and their subsequent hydrolysis.
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Related Practice
Textbook Question

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in axial positions, the two groups can react to form an acetal. This is called the anhydro form of the sugar (it has 'lost water'). The anhydro form of d-idose is shown here. Explain why about 80% of d-idose exists in the anhydro form in an aqueous solution at 100 °C, but only about 0.1% of d-glucose exists in the anhydro form under the same conditions.

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Textbook Question

An unknown disaccharide gives a positive Tollens' test. A glycosidase hydrolyzes it to D-galactose and D-mannose. When the disaccharide is treated with methyl iodide and Ag2O and then hydrolyzed with dilute HCl, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-O-methylmannose. Propose a structure for the disaccharide.

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Textbook Question

Predict whether D-altrose exists preferentially as a pyranose or a furanose. (Hint: In the most stable arrangement for a five-membered ring, all the adjacent substituents are trans.)

Textbook Question

Propose a mechanism for the rearrangement that converts an ⍺-hydroxyimine to an ⍺-aminoketone in the presence of a trace amount of acid.

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Textbook Question

Draw the mechanism for the elimination step in the Wohl degredation.

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