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Chapter 23: Q38P (page 1236)

Does lactose mutarotate? Is it a reducing sugar? Explain. Draw the two anomeric forms of lactose.

Short Answer

Expert verified

Lactose can mutarotate since it has free anomeric end which is shown by the figure below.

Free anomeric end which is able to mutarotate.

The two anomeric forms of lactose (伪 and 尾) are shown below.

伪 anomer

尾 anomer

Step by step solution

01

A concept:

Lactose can be obtained commercially from cow鈥檚 milk after the removal of emulsified fat and casein. Hydrolysis of lactose with dilute acids forms an equimolar mixture of D-glucose and D-galactose which means that the lactose molecule is made up of one unit each of D-glucose and D-galactose.

02

Lactose mutarotate or not:

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

Lactose can mutarotate since it has free anomeric end which is shown by the figure below.

Free anomeric end which is able to mutarotate.

The two anomeric forms of lactose (伪 and 尾) are shown below.

伪 anomer

尾 anomer

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Most popular questions from this chapter

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.

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.

Question. Cellulose is converted to cellulose acetateby treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibres. Show the structure of cellulose acetate.

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

Ruff degradation of D-arabinose gives D-erythrose. The Kiliani-Fischer synthesis converts D-erythrose to a mixture of D-arabinose and D-ribose. Draw out these reactions and give the structure of D-ribose.
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