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Chapter 23: Q39P (page 1237)

Is gentiobiose a reducing sugar? Does it mutarotate? Explain your reasoning.

Short Answer

Expert verified

Gentiobiose can mutarotate because it has free anomeric end which is shown by the figure below.

Step by step solution

01

A concept:

Gentiobiose is a disaccharide in which two glucose units are linked together by a β-1,6 glycosidic linkage. The anomeric carbon of one sugar is linked to the oxygen of the terminal carbon (C6) of another sugar in 1,6 linkage.

02

Explain is gentiobiose a reducing sugar or not:

Gentiobiose is a reducing sugar since it has only one of its two anomeric carbon atoms involved in the glycosidic bond. This means that gentiobiose can be converted to an open chain form with an aldehyde (-CHO) group.

Gentiobiose can mutarotate because it has free anomeric end which is shown by the figure below.

Free anomeric end which is able to mutarotate.

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Most popular questions from this chapter

After a series of Kiliani–Fischer syntheses on (+)-glyceraldehyde, an unknown sugar is isolated from the reaction mixture. The following experimental information is obtained:

(1) Molecular formula C6H12O6

(2) Undergoes mutarotation.

(3) Reacts with bromine water to give an aldonic acid.

(4) Reacts with HNO3 to give an optically active aldaric acid.

(5) Ruff degradation followed by HNO3 oxidation gives an optically inactive aldaric acid. (6) Two Ruff degradations followed by HNO3 oxidation give meso-tartaric acid.

(7) When the original sugar is treated with CH3I and Ag2O, a pentamethyl derivative is formed. Hydrolysis gives a tetramethyl derivative with a free hydroxy group on C5.

(a) Draw a Fischer projection for the open-chain form of this unknown sugar. Use Figure 23-3 to name the sugar.

(b) Draw the most stable conform

Trehalose is a nonreducing disaccharide (C12H22O11) isolated from the poisonous mushroom Amanita muscaria. Treatment with an α -glucosidase converts trehalose to two molecules of glucose, but no reaction occurs when trehalose is treated with a β -glucosidase. When trehalose is methylated by dimethyl sulfate in mild base and then hydrolyzed, the only product is 2,3,4,6-tetra-O-methylglucose. Propose a complete structure and systematic name for trehalose.

Fructose is found in many fruits. From memory, draw fructose in

  1. the Fischer projection of the open chain.
  2. The most stable chair conformation of the most stable pyranose anomer.
  3. The Haworth projection of the most stable pyranose anomer

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranoseis +150.70 , and that of the β anomer is +52.80 . When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.20. Determine the percentages of the two anomers present at equilibrium.

(a) Figure 23-2 shows that the degradation of D-glucose gives D-arabinose, an aldopentose. Arabinose is most stable in its furanose form. Draw D-arabinofuranose.

(b) Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.
See all solutions

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