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Chapter 23: Q20. (page 1225)

Which of the following sugars are reducing sugars? Comment on the common name sucrose for table sugar.

(a) methyl伪鈥�D鈥�galactopyranoside (b) 尾鈥�L鈥�idopyranose (an aldohexose)

(c) 伪鈥�D鈥�allopyranose (d) 尾鈥�L鈥�ribofuranoside

(e) (f)

Short Answer

Expert verified

(a) non-reducing sugar

(b) reducing sugar

(c) reducing sugar

(d) non-reducing sugar

(e) reducing sugar

(f) non-reducing sugar

Step by step solution

01

Reducing and non-reducing sugars

Carbohydrates are further divided into two categories: reducing sugars and non-reducing sugars. Reducing sugars are carbohydrates that can reduce Fehling's solution and Tollens reagent, but non-reducing sugars cannot reduce Fehling's solution or Tollens reagent. Except for sucrose, all monosaccharides and disaccharides are categorised as reducing sugars. Furthermore, hemiacetals are reducing sugars, whereas acetals are non-reducing sugars.

02

General rule for identifying reducing and non-reducing sugars

A sugar whose name ends with the suffix 鈥�-ose鈥� is a reducing sugar while the sugar whose name ends with suffix 鈥�-oside鈥� is a non-reducing sugar.

03

Identification of the given sugars as reducing or non-reducing.

(a) The name for the sugar methyl伪鈥�D鈥�galactopyranosideends with the suffix鈥�-oside鈥�. Hence, it is a non-reducing sugar (acetal).

(b) The name for the sugar 尾鈥�L鈥�idopyranoseends with the suffix鈥�-ose鈥�. Hence, it is a reducing sugar (hemiacetal).

(c) The name for the sugar ends with the suffix鈥�-ose鈥�. Hence, it is a reducing sugar (hemiacetal).

(d) The name for the sugar h ends with the suffix鈥�-oside鈥�. Hence, it is a non-reducing sugar (acetal).

(e) The structure of the sugar is shown below

Sugar

(f) Sucrose is a non-reducing sugar since their glycosidic bond is between their respective hemiacetal carbon atom. It is a disaccharide and often called table sugar or cane sugar.

Sucrose

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Most popular questions from this chapter

Some protecting groups can block two OH groups of a carbohydrate at the same time. One such group is shown here, protecting the 4-OH and 6-OH groups of 尾 -D-glucose.

(a) What type of functional group is involved in this blocking group?

(b) What did glucose react with to form this protected compound?

(c) When this blocking group is added to glucose, a new chiral center is formed. Where is it? Draw the stereoisomer that has the other configuration at this chiral center. What is the relationship between these two stereoisomers of the protected compound?

(d) Which of the two stereoisomers in part (c) do you expect to be the major product? Why?

(e) A similar protecting group, called an acetonide, can block reaction at the 2鈥� and 3鈥� oxygens of a ribonucleoside. This protected derivative is formed by the reaction of the nucleoside with acetone under acid catalysis. From this information, draw the protected product formed by the reaction.

All of the rings of the four heterocyclic bases are aromatic. This is more apparent when the polar resonance forms of the amide groups are drawn, as is done for thymine at left. Redraw the hydrogen-bonded guanine-cytosine and adenine-thymine pairs shown in figure 23-24, using the polar resonance forms of the amides. Show how these forms help to explain why the hydrogen bonds involved in these pairings are particularly strong. Remember that a hydrogen bond arises between an electron-deficient hydrogen atom and electron-rich pair of nonbonding electrons.

Question:

a) Draw D-allose, the C3 epimer of glucose.

b) Draw D-talose, the C2 epimer of D-galactose.

c) Draw D-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.

d) Draw the C4 鈥渆pimer鈥� of D-xylose. Notice that this 鈥渆pimer鈥� is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.

Cytosine, uracil and guanine have tautomeric forms with aromatic hydroxyl groups. Draw these tautomeric forms.

After a series of Kiliani鈥揊ischer 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

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