Organic chemistry with biological applications

John E. McMurry

Chapter 21

Biomolecules: Carbohydrates - all with Video Answers

Educators

Chapter Questions

Problem 1

Classify each of the following monosaccharides:

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Problem 2

Convert the following Fischer projections into tetrahedral representations, and assign $R$ or $S$ stereochemistry to each:

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Problem 3

Which of the following Fischer projections of glyceraldehyde represent the same enantiomer?

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Mikayla Stephens

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Problem 4

Redraw the following molecule as a Fischer projection, and assign $R$ or $S$ configuration to the chirality center (green $=\mathrm{Cl}$ ):

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Problem 5

Assign $R$ or $S$ configuration to each chirality center in the following monosaccharides, and tell whether each is a $\mathrm{D}$ sugar or an $\mathrm{L}$ sugar:

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Problem 6

(+)-Arabinose, an aldopentose that is widely distributed in plants, is systematically named $(2 R, 3 S, 4 S)-2,3,4,5$ -tetrahydroxypentanal. Draw a Fischer projection of ( $+$ )-arabinose, and identify it as a D sugar or an L sugar.

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Problem 7

Draw the adjacent molecular model as a Fischer projection. Does it represent D-glyceraldehyde or L-glyceraldehyde?

Dr. Satish Ingale

Dr. Satish Ingale

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Problem 8

Only the $\mathrm{D}$ sugars are shown in Figure $21.3 .$ Draw Fischer projections for the following $L$ sugars:
(a) $\mathrm{L}$ -Xylose
(b) L-Galactose
(c) L-Allose

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Problem 9

How many aldoheptoses are there? How many are D sugars, and how many are L sugars?

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Problem 10

The following model is that of an aldopentose. Draw a Fischer projection of the sugar, name it, and identify it as a D sugar or an L sugar.

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Problem 11

Ribose exists largely in a furanose form, produced by addition of the C4 -OH group to the C 1 aldehyde. Draw D-ribose in its furanose form.

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Problem 12

Figure 21.5 shows only the $\beta$ -pyranose and $\beta$ -furanose anomers of $\mathrm{D}$ -fructose. Draw the $\alpha$ -pyranose and $\alpha$ -furanose anomers.

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Problem 13

Draw $\beta$ -D-galactopyranose and $\beta$ -D-mannopyranose in their more stable chair conformations. Label each ring substituent as either axial or equatorial. Which would you expect to be more stable, galactose or mannose?

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Problem 14

Draw $\beta$ -L-galactopyranose in its more stable chair conformation, and label the substituents as either axial or equatorial.

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Ricajoy Montero

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Problem 15

Identify the following monosaccharide, write its full name, and draw its openchain form in Fischer projection:

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Problem 16

Draw the products you would obtain by reaction of $\beta$ -D-ribofuranose with:
(a) $\mathrm{CH}_{3} \mathrm{I}, \mathrm{Ag}_{2} \mathrm{O}$
(b) $\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}$, pyridine

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Problem 17

Reduction of $\mathrm{D}$ -glucose leads to an optically active alditol (D-glucitol), whereas reduction of D-galactose leads to an optically inactive alditol. Explain.

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Problem 18

Reduction of L-gulose with $\mathrm{NaBH}_{4}$ leads to the same alditol (D-glucitol) as reduction of $\mathrm{D}$ -glucose. Explain.

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Problem 19

D-Glucose yields an optically active aldaric acid on treatment with $\mathrm{HNO}_{3}$, but D-allose yields an optically inactive aldaric acid. Explain.

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Problem 20

Which of the other six $\mathrm{D}$ aldohexoses yield optically active aldaric acids on oxidation, and which yield optically inactive (meso) aldaric acids? (See Problem $21.19 .$

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Problem 21

What product(s) would you expect from Kiliani-Fischer reaction of D-ribose?

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Problem 22

What aldopentose would give a mixture of L-gulose and L-idose on KilianiFischer chain extension?

Natalie Johns

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Problem 23

Two of the four $\mathrm{D}$ aldopentoses yield $\mathrm{D}$ -threose on Wohl degradation. What are their structures?

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Problem 24

Show how $N$ -acetylneuraminic acid can arise by an aldol reaction of $N$ -acetylmannosamine with pyruvate, $\mathrm{CH}_{3} \mathrm{COCO}_{2}^{-}$.

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Problem 25

Show the product you would obtain from the reaction of cellobiose with the following reagents:
(a) $\mathrm{NaBH}_{4}$
(b) $\mathrm{Br}_{2}, \mathrm{H}_{2} \mathrm{O}$
(c) $\mathrm{CH}_{3} \mathrm{COCl}$, pyridine

Mikayla Stephens

Mikayla Stephens

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Problem 26

Identify the following aldoses, and tell whether each is a $\mathrm{D}$ or $\mathrm{L}$ sugar:

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Problem 27

Draw Fischer projections of the following molecules, placing the carbonyl group at the top in the usual way, and identify each as a $\mathrm{D}$ or L sugar:

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Problem 28

The following structure is that of an $L$ aldohexose in its pyranose form. Identify it, and tell whether it is an $\alpha$ or $\beta$ anomer.

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Problem 29

The following model is that of an aldohexose:
(a) Draw Fischer projections of the sugar, its enantiomer, and a diastereomer.
(b) Is this a D sugar or an L sugar? Explain.
(c) Draw the $\beta$ anomer of the sugar in its furanose form.

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Problem 30

Classify each of the following sugars. (For example, glucose is an aldohexose.)

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Problem 31

Write open-chain structures for the following
(a) A ketotetrose
(b) A ketopentose
(c) A deoxyaldohexose
(d) A five-carbon amino sugar

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Problem 32

What is the stereochemical relationship of $\mathrm{D}$ -ribose to $\mathrm{L}$ -xylose? What generalizations can you make about the following properties of the two sugars?
(a) Melting point
(b) Solubility in water
(c) Specific rotation
(d) Density

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Problem 33

Does ascorbic acid (vitamin C) have a $\mathrm{D}$ or L configuration?

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Ricajoy Montero

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Problem 34

Draw the three-dimensional furanose form of ascorbic acid (Problem 21.33 ), and assign $R$ or $S$ stereochemistry to each chirality center.

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Problem 35

Assign $R$ or $S$ configuration to each chirality center in the following molecules:

Grigoriy Sereda

Grigoriy Sereda

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Problem 36

Draw Fischer projections of the following molecules:
(a) The $S$ enantiomer of 2 -bromobutane
(b) The $R$ enantiomer of alanine, $\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{CO}_{2} \mathrm{H}$
(c) The $R$ enantiomer of 2 -hydroxypropanoic acid
(d) The $S$ enantiomer of 3 -methylhexane

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Problem 37

Draw Fischer projections for the two $\mathrm{D}$ aldoheptoses whose stereochemistry at $\mathrm{C} 3, \mathrm{C} 4, \mathrm{C} 5,$ and $\mathrm{C} 6$ is the same as that of $\mathrm{D}$ -glucose at $\mathrm{C} 2, \mathrm{C} 3, \mathrm{C} 4,$ and $\mathrm{C} 5$.

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Problem 38

The following cyclic structure is that of allose. Is this a furanose or pyranose form? Is it an $\alpha$ or $\beta$ anomer? Is it a $\mathrm{D}$ or $\mathrm{L}$ sugar?

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Problem 39

What is the complete name of the following sugar?

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Problem 40

Write the following sugars in their open-chain forms:

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Problem 41

Draw D-ribulose in its five-membered cyclic $\beta$ -hemiacetal form.

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Problem 42

Look up the structure of D-talose in Figure $21.3,$ and draw the $\beta$ anomer in its pyranose form. Identify the ring substituents as axial or equatorial.

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Problem 43

Draw structures for the products you would expect to obtain from reaction of $\beta$ -D-talopyranose with each of the following reagents:
(a) $\mathrm{NaBH}_{4}$ in $\mathrm{H}_{2} \mathrm{O}$
(b) Warm dilute $\mathrm{HNO}_{3}$
(c) $\mathrm{Br}_{2}, \mathrm{H}_{2} \mathrm{O}$
(d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{HCl}$
(e) $\mathrm{CH}_{3} \mathrm{I}, \mathrm{Ag}_{2} \mathrm{O}$
(f) $\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}$, pyridine

Mikayla Stephens

Mikayla Stephens

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Problem 44

All aldoses exhibit mutarotation. For example, $\alpha$ -D-galactopyranose has $[\alpha]_{\mathrm{D}}=+150.7,$ and $\beta$ -D-galactopyranose has $[\alpha]_{\mathrm{D}}=+52.8 .$ If either anomer is dissolved in water and allowed to reach equilibrium, the specific rotation of the solution is $+80.2 .$ What are the percentages of each anomer at equilibrium? Draw the pyranose forms of both anomers.

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Problem 45

How many D-2-ketohexoses are possible? Draw them.

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Problem 46

One of the $\mathrm{D}-2$ -ketohexoses is called sorbose. On treatment with $\mathrm{NaBH}_{4}$, sorbose yields a mixture of gulitol and iditol. What is the structure of sorbose?

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Problem 47

Another D-2-ketohexose, psicose, yields a mixture of allitol and altritol when reduced with $\mathrm{NaBH}_{4}$. What is the structure of psicose?

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Problem 48

L-Gulose can be prepared from D-glucose by a route that begins with oxidation to D-glucaric acid, which cyclizes to form two six-memberedring lactones. Separating the lactones and treating them with sodium amalgam, $\mathrm{Na}(\mathrm{Hg}),$ reduces the $-\mathrm{CO}_{2} \mathrm{H}$ group to a primary alcohol and the lactone to an aldehyde, giving D-glucose and L-gulose. What are the structures of the two lactones, and which one is reduced to L-gulose?

Mikayla Stephens

Mikayla Stephens

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Problem 49

What other $\mathrm{D}$ aldohexose gives the same alditol as $\mathrm{D}$ -talose?

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Problem 50

Which of the eight $\mathrm{D}$ aldohexoses give the same aldaric acids as their L enantiomers?

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Problem 51

Which of the other three $\mathrm{D}$ aldopentoses gives the same aldaric acid as D-lyxose?

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Problem 52

Draw the structure of L-galactose, and then answer the following questions:
(a) Which other aldohexose gives the same aldaric acid as L-galactose on oxidation with warm $\mathrm{HNO}_{3} ?$
(b) Is this other aldohexose a $\mathrm{D}$ sugar or an $\mathrm{L}$ sugar?
(c) Draw this other aldohexose in its most stable pyranose conformation.

Mikayla Stephens

Mikayla Stephens

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Problem 53

Gentiobiose, a rare disaccharide found in saffron and gentian, is a reducing sugar and forms only D-glucose on hydrolysis with aqueous acid. Reaction of gentiobiose with iodomethane and $\mathrm{Ag}_{2} \mathrm{O}$ yields an octamethyl derivative, which can be hydrolyzed with aqueous acid to give 1 equivalent of 2,3,4,6 -tetra-O-methyl-D-glucopyranose and 1 equivalent of 2,3,4 -tri- $O$ -methyl-D-glucopyranose. If gentiobiose contains a $\beta$ -glycoside link, what is its structure?

Mikayla Stephens

Mikayla Stephens

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Problem 54

Amygdalin, or laetrile, is a cyanogenic glycoside isolated in 1830 from almond and apricot seeds. Acidic hydrolysis of amygdalin liberates HCN, along with benzaldehyde and 2 equivalents of D-glucose. If amygdalin is a $\beta$ -glycoside of benzaldehyde cyanohydrin with gentiobiose (Problem 21.53), what is its structure? [A cyanohydrin has the structure $\mathrm{R}_{2} \mathrm{C}$ (OH)CN and is formed by reversible nucleophilic addition of HCN to an aldehyde or ketone.]

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Problem 55

Trehalose is a nonreducing disaccharide that is hydrolyzed by aqueous acid to yield 2 equivalents of D-glucose. Methylation followed by hydrolysis yields 2 equivalents of 2,3,4,6 -tetra-O-methylglucose. How many structures are possible for trehalose?

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Problem 56

Trehalose (Problem 21.55 ) is cleaved by enzymes that hydrolyze $\alpha$ -glycosides but not by enzymes that hydrolyze $\beta$ -glycosides. What is the structure and systematic name of trehalose?

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Problem 57

Trehalose-6-phosphate (T6P) has been found to trigger the onset of blooming in flowering plants. Draw the structure of T6P.

Marisa A

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Problem 58

Isotrehalose and neotrehalose are chemically similar to trehalose (Problems 21.55 and 21.56 ) except that neotrehalose is hydrolyzed only by $\beta$ -glycosidases, whereas isotrehalose is hydrolyzed by both $\alpha$ - and $\beta$ -glycosidases. What are the structures of isotrehalose and neotrehalose?

Mikayla Stephens

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Problem 59

D-Glucose reacts with acetone in the presence of acid to yield the nonreducing 1,2: 5,6 -diisopropylidene-D-glucofuranose. Propose a mechanism.

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Problem 60

D-Mannose reacts with acetone to give a diisopropylidene derivative (Problem 21.59) that is still a reducing sugar. Propose a likely structure for this derivative.

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Problem 61

Glucose and mannose can be interconverted (in low yield) by treatment with dilute aqueous $\mathrm{NaOH}$. Propose a mechanism.

Mikayla Stephens

Mikayla Stephens

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Problem 62

Raffinose, a trisaccharide found in sugar beets, is formed by a $1 \rightarrow 6 \alpha$ linkage of D-galactose to the glucose unit of sucrose. Draw the structure of raffinose.

Ramesh Singh

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Problem 63

Is raffinose (see Problem 14.62) a reducing sugar? Explain.

Emily Himsel

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Problem 64

Propose a mechanism to account for the fact that D-gluconic acid and D-mannonic acid are interconverted when either is heated in pyridine solvent.

Mikayla Stephens

Mikayla Stephens

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Problem 65

The cyclitols are a group of carbocyclic sugar derivatives having the general formulation cyclohexane-1,2,3,4,5,6-hexol. How many stereoisomeric cyclitols are possible? Draw them in their chair forms.

Mikayla Stephens

Mikayla Stephens

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Problem 66

What product(s) would you expect from Kiliani-Fischer reaction of D-ribose?

Ricajoy Montero

Ricajoy Montero

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Problem 67

What aldopentose would give a mixture of L-gulose and L-idose on Kiliani-Fischer chain extension?

Ricajoy Montero

Ricajoy Montero

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Problem 68

Which two of the four $\mathrm{D}$ aldopentoses yield D-threose on Wohl degradation?

Nicole Smina

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Problem 69

Compound $A$ is a $D$ aldopentose that can be oxidized to an optically inactive aldaric acid B. On Kiliani-Fischer chain extension, A is converted into $\mathbf{C}$ and $\mathbf{D} ; \mathbf{C}$ can be oxidized to an optically active aldaric acid E, but $\mathbf{D}$ is oxidized to an optically inactive aldaric acid $\mathbf{F}$. What are the structures of A-F?

Mikayla Stephens

Mikayla Stephens

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Problem 70

Simple sugars undergo reaction with phenylhydrazine, $\mathrm{PhNHNH}_{2}$, to yield crystalline derivatives called osazones. The reaction is a bit complex, however, as shown by the fact that glucose and fructose yield the same osazone.
(a) Draw the structure of a third sugar that yields the same osazone as glucose and fructose.
(b) Using glucose as the example, the first step in osazone formation is reaction of the sugar with phenylhydrazine to yield an imine called a phenylhydrazone. Draw the structure of the product.
(c) The second and third steps in osazone formation are tautomerization of the phenylhydrazone to give an enol, followed by elimination of aniline to give a keto imine. Draw the structures of both the enol tautomer and the keto imine.
(d) The final step is reaction of the keto imine with 2 equivalents of phenylhydrazine to yield the osazone plus ammonia. Propose a mechanism for this step.

Mikayla Stephens

Mikayla Stephens

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Problem 71

When heated to $100^{\circ} \mathrm{C}$, D-idose undergoes a reversible loss of water and exists primarily as 1,6 -anhydro-D-idopyranose.
(a) Draw D-idose in its pyranose form, showing the more stable chair conformation of the ring.
(b) Which is more stable, $\alpha$ -D-idopyranose or $\beta$ -D-idopyranose? Explain.
(c) Draw 1,6 -anhydro-D-idopyranose in its most stable conformation.
(d) When heated to $100{ }^{\circ} \mathrm{C}$ under the same conditions as those used for $\mathrm{D}$ -idose, $\mathrm{D}$ -glucose does not lose water and does not exist in a 1,6-anhydro form. Explain.

Mikayla Stephens

Mikayla Stephens

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Problem 72

Acetyl coenzyme A (acetyl CoA) is the key intermediate in food metabolism. What sugar is present in acetyl CoA?

Ricajoy Montero

Ricajoy Montero

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Problem 73

One of the steps in the biological pathway for carbohydrate metabolism is the conversion of fructose 1,6 -bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3 -phosphate. Propose a mechanism for the transformation.

Mikayla Stephens

Mikayla Stephens

Numerade Educator