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Chapter 28: Question 28.18 (page 1125)

Draw a stepwise mechanism for the following reaction.

Short Answer

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Answer

The mechanism of the above reaction is followed by resonance and nucleophilic attack of H2O to form anomers.

Step by step solution

01

Glycoside

Glycoside is the molecule formed from monosaccharide, which undergoes condensation with a hydroxyl group and eliminates water.Glycosides have hemiacetal and hydroxyl groups, which on hydrolysis gives two anomers.

02

Formation of carbocation

Glycoside is hydrolyzed and forms an intermediate by the loss of H2O , which on resonance gives carbocation.

03

Formation of anomers

Carbocation was attacked by H2O on both sides (above and below). The carbocation forms an intermediate that undergoes nucleophilic attack to form αandβanomers, as shown below:

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Most popular questions from this chapter

The most stable conformation of the pyranose ring of most D-aldohexoses places the largest group, CH2OH, in the equatorial position. An exception to this is the aldohexose D-idose. Draw the two possible chair conformations of either the αorβ anomers of D-idose. Explain why the more stable conformation has the CH2OH group in the axial position.

(a) Convert each cyclic monosaccharide into a Fischer projection of its acyclic form. (b) Name each monosaccharide. (c) Label the anomer as α or β.

Draw the structure of each of the following compounds:

a. a polysaccharide formed by joining1→6-α D-glucosamine in glycosidic linkages

b. a disaccharide formed by joining D-mannose and D glucose in a -1→4-βglycosidic linkage using mannose’s anomeric carbon

c. an -N-glycoside formed from D-ribose and thymine

d. a ribonucleoside formed from D-ribose and thymine

a. Identify the glycosidic linkage in disaccharide C, classify the glycosidic bond as αorβ, and use numbers to designate its location.

b. Identify the lettered compounds in the following reaction.

A D-aldohexose A is formed from an aldopentose B by the Kiliani-Fischer synthesis. Reduction of A with forms an optically inactive alditol. Oxidation of B forms an optically active aldaric acid. What are the structures of A and B?

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