518
CHAPTER
l6 Carbohydrates
P R A C T I CEEX E R C I SI E
6.I6
How many functional groups can you identiff in the following structure?
CH2OH
o
I
CHr
j_io
t/'
v
NOH /
HO\-J
tl
H
o-c
I
I
H
OH
g # $wffiARV
Simple sugars, called monosaccharides,are polyhydroxy aldehydes (aldoses)or ketones (ketoses).
All sugarscontain at least one asyrnmetric carbon.
An important aspect of sugar chemistry is stereochemistry -relating the positions of groups of a
compound in space.There is one pair of stereoisomers for compounds containing one asymmetric
carbon. Stereoisomershave the same molecular
formula, but they are nonsuperimposable mirror
images.Stereoisomersmay be called optical isomers becausethey rotate plane-polarized light.
Straight-chainaldosesand ketosesform hemiacetal and hemiketal rings, respectively.The heterocyclic ether rings are five-membered (furanoses)and
six-membered(pyranoses).Fischerproj ectionsrepresent straight-chain sugars;Haworth projections represent sugils in ring forms. Cyclization of a straightchain sugar to a pyranose or furanose ring createsa
new asyrnmetric carbon in the molecule. Molecules
whose stereochemistrydiffers only at the newly created asymmetric carbon are called anomers.
Simple sugars are often linked to other sugars
through glycosidic bonds to create disaccharides.
Polysaccharides are sugars containing many
monosaccharideunits.
Starchesare polymers of glucose from plants.
One form, which is a linear polyrner, is amylose.
Another, which is a branched polymer, is amylopectin. Glycogen is the animal form of starch; it
is more highly branched than amylopectin. Cellulose is another linear poll'rner of glucose.Cellulose
is more resistant to hydrolysis than starch. Many
other polymers of monosaccharides are found in
nature, where they have many diverse functionsfrom forming the hard shells of crabs,like chitin, to
acting as a blood anticoagulant, like heparin.
Reducing sugars give positive results in the
form of a red precipitate with Benedict'sreagent or
a silver mirror with Tollens' reagent. Nonreducing
sugars do not. Most monosaccharides and disaccharides except sucroseare reducing sugars.Most
polysaccharidesare nonreducing sugars.
KffiY?€RffiS
Acid mucopolysaccharide
(16.10)
Aldose(16.1)
Anomer (16.4)
AsJ,rynmetric
carbon (16.1)
Carbohydrate(16.1)
Cellulose(16.6)
Disaccharide(16.7)
Fischerprojection (16.1)
Furanosering system
(16.3)
Glycogen(16.6)
Glycoside(16.5)
Glycosidicbond (16.5)
Haworth projection (16.4)
Invert sugar (16.8)
Ketose(16.2)
Monosaccharide(16.2)
Nonreducingsugar (16.9)
Nonsuperimposable
mirror image (16.1)
Optical isomer (16.1)
Plane-polarized light
(i6.1)
Polysaccharide(16.6)
Pyranosering system
(16.3)
Reducingsugar (16.9)
Simple sugar (16.2)
Starch(16.6)
Stereoisomer(16.1)
Sugar(16.1)
Exercises 519
EXffiK€ESgS
lo
Carbohydrate
Structure
and Stereochemistry
(Section
l6.l)
I
HO -l-n
f 6.17 Identify the as).'rnmetriccarbon (if there is one) in
the following compounds.
(a) CH.GHOH
(b) CH3CHTCHOH
ll
cH2cH3
CH.
(c) CH:OHCHzOH (d) CH3CHCH2OH
OH
CI
16.lB Draw stereorepresentations,
and stereoisomersif
they exist, for each of the following.
OH
I
(a) CH.CHCHO
(b) CH3CH2CHO
16.19 Draw stereorepresentations
and Fischerprojection
formulas of l-glyceraldehyde and r-glyceraldehyde.
Explain the meaning of p and r.
16.20 Draw the structural formula for 3-hydroxybutanal,
and identify the asl.rnmetrical carbon.
Monosaccharidei
(Section16.2)
16.21 Name each m6nosaccharide, and characterizeit as
to chain length and functional group.
(a)
CHO
I
HO-J-
H
HIon
I
cH,oH
(c)
(b)
CHO
(c)
cH2oH
H lott
I
Ho-1-H
cH2oH
H
H
-f o"
To"
CH2OH
+"
-o
HO
H __FOH
H
Io,'
I
cH2oH
16.22 Draw Fischerprojection formulas for o-ribose and
o-2-deoxyribose.Number the carbons,and asterisk
those which are asyrnmetrical.
16.23 Identify each of the following monosaccharides as
H -]-""
H
To"
cH2oH
(d)
CHO
H
Io*,
I
H -lo'
H -]-ott
cH2oH
Closed-Chain
Structuresand HaworthProjections
(Sections
t6.5,16.4)
16.24 Draw (a) a structure of the cyclic ether furan and,
(b) the general structure of a hemiketal.
16.25 Draw (a) the general structure of a hemiacetal and
(b) a structure of the cyclic ether p1'ran.
16.26 Draw the Haworth projections of a-l-glucopl'ranose and B-o-glucopyranose,the cyclic forms of
o-glucose.
16.27 Draw the Haworth projections of a-o-fructofuranose and B-o-fructofuranose, the cyclic forms of
r-fructose.
16.28 Draw structures to show the relationship that
existsbetween the two cyclic hemiacetal forms of
o-glucose and the open-chain aldehyde form.
(sectionst6.5, 16.5)
Polysaccharides
16.29 Write the general structure of (a) an acetal and (b)
a ketal.
f 6.30 Explain the difference between a BG+4) and an
a(1+$) glycosidicbond.
16.31 Give one use for (a) starch, (b) cellulose,and
(c) glycogen.
16.32 Name a source of (a) cellulose, (b) glycogen,and
(c) starch.
16.33 Describe the structural and stereochemicaldifferencesbetween starch (amylose)and cellulose.\iVhatis
the biological significance of these differences?
16.34 lVhat is glycogen?How does it differ from amylopectin?
DOTL.
(a)
CHo
I
HO__FH
I
CH2OH
(b)
CHO
I
HToH
HToH
CH,OH
(Sections
Disaccharides
l6.Z t6.S)
16.35 Give sources,uses,and the products ofhydrolysis
of (a) sucroseand (b) maltose.
16.36 Give sources,uses,and the products gfhydrolysis
of (a)lactoseand (b) cellobiose.
520
CHAPTER
t6 Carbohvdrates
16.37 Draw a Haworth projection formula of the disaccharide maltose. Label the hemiketal, hemiacetal,
ketal, or acetal carbons.
16.38 Draw a Haworth projection formula of the disaccharide sucrose.Label the hemiketal, hemiacetal, ketal,
or acetalcarbons.
Reducing
and Nonreducint
Sugals(sectiont6.9)
16.39 Explain whylactose is a reducing sugar but sucrose is a nonreducing sugar.
16.40 \tVhyare polysaccharidesgenerally not regardedas
reducing sugars?
16.41 Would Benedict'sreagent be useful for distinguishing between
(a) glucose and starch
(b) fructose and sucrose
(c) fructose and glucose?
16.42 A solution of invert sugar is treated with (a) Tollens' reagent and (b) Benedict'sreagent.\fhat would
you expect to happen?
16.43 \tVhichof the following compounds exist as optical
isomers?
Br
I
CI-C-H
I
CI
(c)
H
I
CH"CH,CHC-O
"-l
Ho-fu
Ho-l-H
HoTH
C:O
I
HToH
CH2OH
cH2oH
(d)HocP.o\ot
AdditionalExercises
(a)
16.44 Drawthe Fischerprojection formula for l-galactose.
16.45 Draw the Haworth structures for a-o-galactose
and B-o-galactose.Identiff the anomeric carbon.
16.46 Draw the structure for the disaccharide formed
when two o-galactosemoleculesarejoined B[+4).
Would you expect this to be a reducing sugar?Explain.
16.47 Tell the number of as1'rnmetricalcarbons in the
open-chainstructureof (a) r-ribose, (b) o-2-deoxyribose,(c) n-glucose,(d) o-fructose,(e) r-galactose,and
(f) o-mannose.
16.48 Howwould you explain the water solubility of
monosaccharides?
16.49 Identiff these monosaccharides as D or L.
(a)
(b)
CHO
cH20H
(d)
cl
CH,:CHCH,C
--l
OH
I
H
\,4
w
OH OH
OH OH
16.50 Match the description on the right with the appropriate polysaccharides.A number may be used more
than once.
(a) starch (amylose)
(1) foundinplants
(b) starch (amylopectin)
(2) a(I+4)linkages
(c) glycogen
(3) F(l-G) linkages
(d) cellulose
(4) a(l-6) linkages
(5) found in animals
t6) F(l- ) Inkages
$eLF-Ys$T
{sEs*sw}
True/False
I l. A compoundwith optical isomers must have at least
one as]rynmetricalcarbon atom.
2. Anomers are stereoisomersthat result from ring
'"
closure.
3.
The
cyclic form of an aldose is an intramolecular
,
hemiacetal.
_,4. In polysaccharides,glycosidicbonds are formed
between the alcohol ofone sugar unit and the carbonyl group of a different sugar unit.
5. The disaccharidelactoseis composedof the monoi
saccharideso-glucoseand o-galactose.
A carbohydrate with a free aldehyde group will give a
,6.
'
positive Tollens' test and be classified as a reducing
sugar.
r 7. Moleculesof o-threoseand r-threoseare mirror
images.
B. A pyranose is a six-membered sugar ring system;a
P five-membered sugar ring system is a furanose.
9. A molecule of r(-) -glyceraldehydeis identical to a
molecule of o( + ) - glyceraldehyde.
10. Starch,cellulose,and glycogen €uecomposed of the
I
monosaccharideglucose.
Self-Test(Review)
521
MultipleChoice
1f . Vfhich of the following statementsis falsefor B-ofructofuranose?
(b) It is a hemiacetal.
(a) It is a reducing sugar.
(c) It has a five-membered ring.
(d) It is a structural isomer of o-glucose.
12. lVhen pure B-o-galactopyranoseis dissolved in an
acidic solution
(a) some of it is changed into B-r-galactopyranose.
(b) all of it changesto the open-chain form.
(c) some a-o-galactopyranose
is formed.
(d) no change takes place.
f3. All the following are aldohexosesexcept
(b) fructose.
(c) mannose.
(a) galactose.
(d) glucose.
14. a-o-Glucopyranose
(a) has the -OH group on carbon I pointing up.
(b) has a five-membered ring.
(c) is an intramolecular hemiacetal.'
(d) is an anomer of a-r-glucopyranose.
15. The disaccharidelactose
(a) is found in milk.
(b) is nonreducing.
(c) is made up of the simple sugarso-galactose and
D-fructose.
(d) all of the above are true.
16. All the following are nonreducing sugarsexcept
(c) methyl glucoside.
(a) starch.
(b) sucrose.
(d) none ofthe above.
17. The reactionof ethanol with a-o-ribofuranosewould
give
(a) a glycoside.
(b) ethyl a-p-ribofuranoside.
(c) anacetal.
(d) alloftheabove.'
lB. Complete hydrolysis of starch would give
(b) amylopectin.
(c) maltose.
, (a) o-glucose.
(d) o-fructose.
19. The Fischer projection formula for a stereoisomerof
xylose is
CHO
HO
H
OH
HO
cH2oH
From this formula we know that this stereoisomeris
(c) (-)-xylose.
(a) o-xylose.
(b) (+)-xylose.
(d) r-xylose.
20. \tVhichof the following statements is nottrue about
an aldotetrose?
(a) It has four asymmetrical carbons.
(b) It givesa positive Benedict'stest.
(c) Ithas an aldehyde group.
(d) It has three hydroxyl $oups.
21. \.Vhenstarch is partially hydrolyzed, the disaccharide
obtained is
(b) amylose.
(c) lactose.
(a) maltose.
(d) xylose.
22. For a carbohydrate molecule to have a mirror image,
it must
(a) be very complex.
(b) be an organic compound.
(c) not have a stereoisomer.
(d) have an asymmetrical carbon.
23. One thing we can say for certain about the pentose
r-ribose is that
(a) it rotates plane-polarized light to the left.
(b) it is a mirror image of o-ribose.
(c) it rotates plane-polarized light to the right.
(d) it has no asyrnmetrical carbons.
' 24. The most branched natural polysaccharide is
(c) glycogen.
(a) cellulose.
(b) amylose.
(d) amylopectin.
. 25. Human beings cannot use cellulose as a nutritional
sourcebecause
(a) it is found only in inedible materials.
(b) it contains B(1+4) glycosidic linkages.
(c) its structure is too highlybranched.
(d) all of the above are true.
o.
- 26. Sucroseis unique among the common disaccharides
because
(a) it is not a reducing sugar.
(b) it has a 1-2 linkage.
(c) it has no hemiacetal group.
(d) all of the above are true.