Fundamentals of General, Organic, and Biological Chemistry

John McMurry, David S. Ballantine, Carl A. Hoeger

Chapter 20

Carbohydrates - all with Video Answers

Educators

Chapter Questions

Problem 1

Classify the following monosaccharides as an aldose or a ketose, and label each according its number of carbon atoms.

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Problem 2

Draw the structures of an aldopentose and a ketohexose.

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Problem 3

Notice in the following structures (a)-(d) that the bottom carbon and its substituents are written as $\mathrm{CH}_{2} \mathrm{OH}$ in every case. How does the $\mathrm{C}$ in this group differ in each case from the $\mathrm{C}$ atoms above it? Why must the locations of the $\mathrm{H}$ atoms and $-\mathrm{OH}$ groups attached to the carbons between this one and the carbonyl group be shown?

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Problem 4

From monosaccharides (a)-(d) in Problem $20.3,$ choose the one that is the enantiomer of the unlabeled monosaccharide shown.

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Problem 5

Aldoheptoses have five chiral carbon atoms. What is the maximum possible number of aldoheptose stereoisomers? Draw all of the aldoheptose stereoisomers.

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Problem 6

Draw the enantiomer of the following monosaccharides, and in each pair identify the D sugar and the $\mathrm{L}$ sugar.

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Problem 7

D-Talose, a constituent of certain antibiotics, has the open-chain structure shown next. Draw D-talose in its cyclic hemiacetal form.

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Problem 8

The cyclic structure of D-idose, an aldohexose, is shown in the margin. Convert this to the straight-chain Fischer projection structure.

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Problem 9

Draw the structure that completes the mutarotation reaction between the two cyclic forms of (a) galactose and
(b) fructose.

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Problem 10

Neomycin is an antibiotic used in topical applications to inhibit the growth of bacteria. It is an aminoglycoside, that is, some of the - OH groups on the sugars have been replaced by $-\mathrm{NH}_{2}$ or R groups. The four rings that constitute neomycin are joined by glycosidic bonds and two of the rings are amino sugars. In the structure shown, identify (a) the amino sugar rings by number, (b) the unmodified sugar ring structure, and
(c) the non-sugar ring structure. List how many carbon atoms are in each ring.

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Problem 11

In the monosaccharide hemiacetal shown in the margin, number all the carbon atoms, identify the anomeric carbon atom, and identify it as the $\alpha$ or $\beta$ anomer.

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Problem 12

Identify the chiral carbons in $\alpha$ -D-fructose, $\alpha$ -D-ribose, and $\beta$ -D-2-deoxyribose.

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Problem 13

L-Fucose is one of the naturally occurring $\mathrm{L}$ monosaccharides. It is present in the short chains of monosaccharides by which blood groups are classified (see the Chemistry in Action "Cell-Surface Carbohydrates and Blood Type" on p. 674 ). Compare the structure of $\mathrm{L}$ -fucose shown in the margin with the structures of $\alpha$ - and $\beta$ -D-galactose and answer the following questions.
(a) Is $\mathrm{L}$ -fucose an $\alpha$ or $\beta$ anomer?
(b) Compared with galactose, on which carbon is $\mathrm{L}$ -fucose missing an oxygen?
(c) How do the positions of the - OH groups above and below the plane of the ring on carbons $2,3,$ and 4 compare in D-galactose and $\mathrm{L}$ -fucose?
(d) "Fucose" is a common name. Is 6 -deoxy-L-galactose a correct name for fucose? Why or why not?

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Problem 14

Draw the structure of the $\alpha$ and $\beta$ anomers that result from the reaction of methanol and ribose. Are these compounds acetals or hemiacetals?

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Problem 15

Refer to the cellobiose structure in Worked Example 20.5 . How would you classify the link between the monosaccharides in cellobiose?

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Problem 16

Refer to the cellobiose structure in Worked Example $20.5 .$ Show the structures of the two monosaccharides that are formed on hydrolysis of cellobiose. What are their names?

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Problem 17

Identify the following disaccharides. Give a natural source for each of these disaccharides. (a) The disaccharide that contains two glucose units joined by an $\alpha$ -glycosidic linkage. (b) The disaccharide that contains fructose and glucose.
(c) The disaccharide that contains galactose and glucose.

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Problem 18

What is the structural difference between glucose and (a) $\beta$ -D-glucuronate,
(b) $\beta$ -D-glucosamine,
(c) $\mathrm{N}$ -acetyl- $\beta$ -D-glucosamine?

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Problem 19

In $N$ -linked glycoproteins, the sugar is usually attached to the protein by a bond to the $\mathrm{N}$ atom in a side-chain amide. Which amino acids can form such a bond?

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Problem 20

During the digestion of starch from potatoes, the enzyme $\alpha$ -amylase catalyzes the hydrolysis of starch into maltose. Subsequently, the enzyme maltase catalyzes the hydrolysis of maltose into two glucose units. Write an equation (in words) for the enzymatic conversion of starch to glucose. Classify each of the carbohydrates in the equation as a disaccharide, monosaccharide, or polysaccharide.

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Problem 21

Identify the following as diastereomers, enantiomers, and/
or anomers.
(a) $\alpha$ -D-fructose and $\beta$ -D-fructose
(b) D-galactose and L-galactose
(c) $\mathrm{L}$ -allose and $\mathrm{D}$ -glucose (both aldohexoses)

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Problem 22

Consider the trisaccharide $\mathrm{A}, \mathrm{B}, \mathrm{C}$ shown in Problem $20.23 .$
(a) Identify the hemiacetal and acetal linkages.
(b) Identify the anomeric carbon atoms, and indicate whether each is $\alpha$ or $\beta$.
(c) State the numbers of the carbon atoms that form glycosidic linkages between monosaccharide $\mathrm{A}$ and monosaccharide B.
(d) State the numbers of the carbon atoms that form glycosidic linkages between monosaccharide $\mathrm{B}$ and monosaccharide C.

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Problem 23

Hydrolysis of both glycosidic bonds in the following trisaccharide $\mathrm{A}, \mathrm{B}, \mathrm{C}$ yields three monosaccharides.
(a) Are any two of these monosaccharides the same?
(b) Are any two of these monosaccharides enantiomers?
(c) Draw the Fischer projections for the three monosaccharides.
(d) Assign a name to each monosaccharide.

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Problem 24

The trisaccharide shown with Problem 20.23 has a specific sequence of monosaccharides. To determine this sequence, we could react the trisaccharide with an oxidizing agent. Since one of the monosaccharides in the trisaccharide is a reducing sugar, it would be oxidized from an aldehyde to a carboxylate. Which of the monosaccharides $(\mathrm{A}, \mathrm{B},$ or $\mathrm{C})$ is oxidized? Write the structure of the oxidized monosaccharide that results after hydrolysis of the trisaccharide. How does this reaction assist in identifying the sequence of the trisaccharide?

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Problem 25

Are one or more of the disaccharides maltose, lactose, cellobiose, and sucrose part of the trisaccharide in Problem $20.23 ?$ If so, identify which disaccharide and its location. (Hint: Look for an $\alpha-1,4$ link, $\beta-1,4$ link, or 1,2 link, and then determine if the correct monosaccharides are present.)

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Problem 26

Cellulose, amylose, amylopectin, and glycogen are the polysaccharides of glucose that we examined in this chapter. The major criteria that distinguish these four polysaccharides include $\alpha$ -glycosidic links or $\beta$ -glycosidic links, 1,4 links or both 1,4 and 1,6 links, and the degree of branching. Create a table evaluating each polysaccharide using these five criteria.

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Problem 27

In solution, glucose exists predominantly in the cyclic hemiacetal form, which does not contain an aldehyde group. How is it possible for mild oxidizing agents to oxidize glucose?

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Problem 28

What is a carbohydrate?

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Problem 29

What is the family-name ending for a sugar?

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Problem 30

What is the structural difference between an aldose and a ketose?

Ricajoy Montero

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Problem 31

Classify the four carbohydrates (a)-(d) by indicating the nature of the carbonyl group and the number of carbon atoms present. For example, glucose is an aldohexose.

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Problem 32

How many chiral carbon atoms are present in each of the molecules shown in Problem $20.31 ?$

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Problem 33

How many chiral carbon atoms are there in each of the two parts of the repeating unit in heparin (p. 684 )? What is the total number of chiral carbon atoms in the repeating unit?

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Problem 34

Draw the open-chain structure of a ketoheptose.

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Problem 35

Draw the open-chain structure of a 4-carbon deoxy sugar.

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Problem 36

Name four important monosaccharides and tell where each occurs in nature.

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Problem 37

Name a common use for each monosaccharide listed in Problem 20.36

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Problem 38

How are enantiomers related to each other?

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Problem 39

What is the structural relationship between $\mathrm{L}$ -glucose and D-glucose?

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Problem 40

Only three stereoisomers are possible for 2,3 -dibromo-2, 3-dichlorobutane. Draw them, indicating which pair are enantiomers (optical isomers). Why does the other isomer not have an enantiomer?

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Problem 41

In Section $15.6,$ you saw that aldehydes react with reducing agents to yield primary alcohols $\left(\mathrm{RCH}=\mathrm{O} \longrightarrow \mathrm{RCH}_{2} \mathrm{OH}\right)$ The structures of two
D-aldotetroses are shown. One of them can be reduced to yield a chiral product, but the other yields an achiral product. Explain.

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Problem 42

Sucrose and D-glucose rotate plane-polarized light to the right; D-fructose rotates light to the left. When sucrose is hydrolyzed, the glucose-fructose mixture rotates light to the left.
(a) What does this indicate about the relative degrees of rotation of light of glucose and fructose?
(b) Why do you think the mixture is called "invert sugar"?

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Problem 43

What generalization can you make about the direction and degree of rotation of light by enantiomers?

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Problem 44

What does the term reducing sugar mean?

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Problem 45

What structural property makes a sugar a reducing sugar?

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Problem 46

What is mutarotation? Do all chiral molecules do this?

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Problem 47

What are anomers, and how do the anomers of a given sugar differ from each other?

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Problem 48

What is the structural difference between the $\alpha$ hemiacetal form of a carbohydrate and the $\beta$ form?

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Problem 49

D-Gulose, an aldohexose isomer of glucose, has the cyclic structure shown here. Which is shown, the $\alpha$ form or the $\beta$ form?

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Problem 50

In its open-chain form, D-mannose, an aldohexose found in orange peels, has the structure shown here. Coil mannose around and draw it in the cyclic hemiacetal $\alpha$ and $\beta$ forms.

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Problem 51

In its open-chain form, D-altrose has the structure shown here. Coil altrose around and draw it in the cyclic hemiacetal $\alpha$ and $\beta$ forms.

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Problem 52

Treatment of D-glucose with a reducing agent yields sorbitol, a substance used as a sugar substitute by people with diabetes. Draw the structure of sorbitol.

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Problem 53

Reduction of $\mathrm{D}$ -fructose with a reducing agent yields a mixture of D-sorbitol along with a second, isomeric product. What is the structure of the second product?

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Problem 54

Treatment of an aldose with an oxidizing agent such as Tollens' reagent (Section 15.5 ) yields a carboxylic acid. Gluconic acid, the product of glucose oxidation, is used as its magnesium salt for the treatment of magnesium deficiency. Draw the structure of gluconic acid.

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Problem 55

Oxidation of the aldehyde group of ribose yields a carboxylic acid. Draw the structure of ribonic acid.

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Problem 56

What is the structural difference between a hemiacetal and an acetal?

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Problem 57

What are glycosides, and how can they be formed?

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Problem 58

Look at the open-chain form of D-mannose (Problem 20.50) and draw the two glycosidic products that you expect to obtain by reacting D-mannose with methanol.

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Problem 59

Draw a disaccharide of two cyclic mannose molecules attached by an $\alpha-1,4$ glycosidic linkage. Explain why the glycosidic products in Problem 20.58 are not reducing sugars, but the product in this problem is a reducing sugar.

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Problem 60

Give the names of three important disaccharides. Tell where each occurs in nature. From which two monosaccharides is each made?

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Problem 61

Lactose and maltose are reducing disaccharides, but sucrose is a nonreducing disaccharide. Explain.

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Problem 62

Amylose (a form of starch) and cellulose are both polymers of glucose. What is the main structural difference between them? What roles do these two polymers have in nature?

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Problem 63

How are amylose and amylopectin similar to each other, and how are they different from each other?

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Problem 64

Which of the following is not a use for cellulose?
(a) lumber for building
(b) fodder for cattle
(c) raw material for computer chips
(d) fabric for t-shirts

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Problem 65

Which of the following foods can someone who has lactose intolerance eat?
(a) ice cream
(b) french fries
(c) a chocolate milkshake

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Problem 66

Gentiobiose, a rare disaccharide found in saffron, has the following structure. What simple sugars do you obtain on hydrolysis of gentiobiose?

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Problem 67

Does gentiobiose (Problem 20.66 ) have an acetal grouping? A hemiacetal grouping? Do you expect gentiobiose to be a reducing or nonreducing sugar? How would you classify the linkage $(\alpha$ or $\beta$ and carbon numbers) between the two monosaccharides?

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Problem 68

Trehalose, a disaccharide found in the blood of insects, has the following structure. What simple sugars would you obtain on hydrolysis of trehalose?

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Problem 69

Does trehalose (Problem 20.68 ) have an acetal grouping? A hemiacetal grouping? Do you expect trehalose to be a reducing or nonreducing sugar? Classify the linkage between the two monosaccharides.

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Problem 70

Amylopectin (a form of starch) and glycogen are both $\alpha$ -linked polymers of glucose. What is the structural difference between them?

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Problem 71

What is the physiological purpose of starch in a seed or other plant tissue? What is the physiological purpose of glycogen in a mammal?

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Problem 72

What modified sugars makeup heparin, hyaluronate, and chondroitin-6-sulfate?

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Problem 73

What is the function of heparin, hyaluronate, and chondroitin-6-sulfate?

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Problem 74

Are the $\alpha$ and $\beta$ forms of monosaccharides enantiomers of each other? Why or why not?

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Problem 75

Are the $\alpha$ and $\beta$ forms of the disaccharide lactose enantiomers of each other? Why or why not?

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Problem 76

D-Fructose can form a six-membered cyclic hemiacetal as well as the more prevalent five-membered cyclic form. Draw the $\alpha$ isomer of $\mathrm{D}$ -fructose in the six-membered ring.

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Problem 77

Raffinose, found in sugar beets, is the most prevalent trisaccharide. It is formed by an $\alpha-1,6$ linkage of D-galactose to the glucose portion of sucrose. Draw the structure of raffinose.

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Problem 78

Write the open-chain structure of the only ketotriose. Name this compound and explain why it has no optical isomers.

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Problem 79

Write the open-chain structure of the only ketotetrose. Name this compound. Does it have an optical isomer?

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Problem 80

What is lactose intolerance, and what are its symptoms?

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Problem 81

What is the group of disorders that result when the body lacks an enzyme necessary to digest galactose? What are the symptoms?

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Problem 82

When a person cannot digest galactose, its reduced form, called dulcitol, often accumulates in the blood and tissues. Write the structure of the open-chain form of dulcitol. Does dulcitol have an enantiomer? Why or why not?

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Problem 83

Describe the differences between mono-, di-, and polysaccharides.

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Problem 84

Name a naturally occurring carbohydrate and its source for each type of carbohydrate listed in Problem 20.83 .

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Problem 85

Compare and contrast lactose intolerance with galactosemia. (Hint: Make a table.)

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Problem 86

Many people who are lactose intolerant can eat yogurt, which is prepared from milk curdled by bacteria, without any digestive problems. Give a reason why this is possible. (Hint: Read the label on each of several yogurt containers. Do the ingredients make a difference?)

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Problem 87

Carbohydrates provide $16.7 \mathrm{~kJ}$ per gram. If a person eats $200 \mathrm{~g}$ per day of digestible carbohydrates, what percentage of an $8350 \mathrm{~kJ}$ daily diet would be digestible carbohydrate?

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Problem 88

A $33 \mathrm{cL}$ can of cherry-flavored cola contains 42 grams of sugar. If sugar provides $16.7 \mathrm{~kJ}$ per gram, how many kilojoules are in one can of cola?

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Problem 89

Explain why cotton fibers, which are nearly pure cellulose, are insoluble in water, while glycogen, another polymer of glucose, will dissolve in water.

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