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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

Predict the products obtained when D-galactose reacts with each reagent.

(a) Br2 and H2O

(b) NaOH,H2O

(c)CH3OH, H+

(d) Ag(NH3)+2OH+

(e) H2, Ni

(f) excess Ac2O and pyridine

(g) excess CH3I ,Ag2O

(h) NaBH4

(i) Br2 , H2O then H2O2 and Fe2(SO4)3

(j) (1) KCN/HCN; (2) H2 Pd/BaSO4; (3) H3O+

(k)excess HIO4

In 1891, Emil Fischer determined the structures of glucose and seven other D-aldohexoses using only simple chemical reactions and clever reasoning about stereochemistry and symmetry. He received the Nobel Prize for this work in 1902. Fischer has determined that D-glucose is an aldohexose, and he used Ruff degradation to degrade it to (+)-glyceraldehyde. Therefore, the eight D-aldohexose structures shown in Figure 23-3 are the possible structures for glucose.

Pretend that no names are shown in Figure 23-3 except for glyceraldehyde, and sue the following results to prove which of these structures represent glucose, mannose, arabinose, and erythrose.

(a)Upon Ruff degradation, glucose and mannose gives the same aldopentose: arabinose.Nitric acid oxidation of arabinose gives an optically active aldaric acid. What are the two possible structures of arabinose?

(b) Upon Ruff degradation, arabinose gives the aldotetrose erythrose. Nitric acid oxidation of erythrose gives an optically inactive aldaric acid, meso-tartaric acid. What is the structure of erythrose?

(c) Which of the two possible structures of arabinose is correct? What are the possible structures of glucose and mannose?

(d) Fischer’s genius was needed to distinguish between glucose and mannose. He developed a series of reactions to convert the aldehyde group of an aldose to an alcohol while converting the terminal alcohol to an aldehyde. In effect, he swapped the functional groups on the ends. When he interchanged the functional groups on D-mannose, he was astonished to find that the product was still D-mannose. Show how this information completes the proof of the mannose structure, and show how it implies the correct glucose structure.

(e) When Fischer interchanged the functional groups on D-glucose, the product was an unnatural L sugar. Show which unnatural sugar he must have formed, and show how it completes the proof of the glucose structure.

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

Treatment of either anomer of fructose with excess ethanol in the presence of a trace of HCI gives a mixture of the α and β anomers of ethyl-D-fructofuranoside. Draw the starting materials, reagents, and products for this reaction. Circle the aglycone in each product.

Draw and name the enantiomers of the sugars shown in Figure 23-2. Give the relative configuration (D or L) and the sign of the rotation in each case.
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