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Chapter 20: Q68P (page 985)

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. Proposea structure for the disaccharide.

Short Answer

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Disaccharides are sugars created when two monosaccharides are connected by glycosidic linkage. Condensation reactions are also involved in the generation of disaccharides. Some examples are sucrose and lactose.

Step by step solution

01

Disaacharide

Disaccharides are sugars created when two monosaccharides are connected by glycosidic linkage. Condensation reactions are also involved in the generation of disaccharides. Some examples are sucrose and lactose.

02

Proposing a structure for the disaccharide 

The silver oxide raises the leaving tendency of the iodide ion from the methyl iodide, thereby allowing the nucleophilic substitution to happen with the poor nucleophilic alcohol groups.

As a mannose is missing in the methyl substituent on the oxygen at C-6, the disaccharide must be formed using the C-6 OH group of the mannose and the anomeric carbon of the galactose.

Structure of the disaacharide

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Most popular questions from this chapter

Calculate the percentages of α-d-glucose and β-d-glucose present at equilibrium from the specific rotations of α-d-glucose, β-d-glucose, and the equilibrium mixture. Compare your values with those given in Section 20.10.

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.)

What is the main structural difference between

a. amylose and cellulose?

b. amylose and amylopectin?

c. amylopectin and glycogen?

d. cellulose and chitin?

a. What other monosaccharide is reduced only to the alditol obtained from the reduction of 1. D-talose? 2. D-glucose? 3. D-galactose?

b. What monosaccharide is reduced to two alditols, one of which is the alditol obtained from the reduction of 1. D-talose? 2. D-allose?

The disaccharide lactulose consists of a d-galactose pyranose subunit and a d-fructo furanose subunit joined by a b-1,4′-glycosidic linkage. After treatment of lactulose with

1. excess CH3I ,Ag2O ; 2 HCl,H2O, the d-galactose pyranose subunit was found to have one nonmethylated OH group, whereas the d-fructo furanose subunit had two. Draw the structure of α-lactulose.
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