Chapter 20 Chapter 20 CarbohydratesCarbohydrates

CarbohydratesCarbohydrates

Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.

Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula

CCnnHH2n2nOOnn, where nn varies from 3 to 8.

• AldoseAldose:: A monosaccharide containing an aldehyde group.

• KetoseKetose:: A monosaccharide containing a ketone group.

MonosaccharidesMonosaccharidesThe suffix -ose-ose indicates that a molecule is a

carbohydrate.The prefixes tri-tri-, tetratetra, pentapenta, and so forth

indicate the number of carbon atoms in the chain.

Those containing an aldehyde group are classified as aldosesaldoses. .

Those containing a ketone group are classified as ketosesketoses..

There are only two trioses:

MonosaccharidesMonosaccharides There are only two trioses:

◦ Often aldo- and keto- are omitted and these compounds are referred to simply as trioses.

◦ Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.

HC

HC

H2C OH

O

Glyceraldehydean aldotrioses

OH C

H2C

H2C OH

OH

O

Dihydroxyacetonea ketotrioses

MonosaccharideMonosaccharide Monosaccharides with

◦ three carbons: trioses◦ Five carbons: pentose◦ Six carbons: hexose ◦ And so on …

M M M M M M

Polysaccharide

hydrolysisn M

monosaccharide

MonosacharidesMonosacharides

Figure 12.1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.

EnantiomersEnantiomers Enantiomers: a molecule has a nonsuperimposable

mirror image◦ Chiral molecule – has four different groups

MonosaccharidesMonosaccharidesFischer projection:Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward

from the stereocenter. • Vertical lines represent bonds projecting to the rear.• Only the stereocenter is in the plane.

MonosacharidesMonosacharidesIn 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

• D-monosaccharide:D-monosaccharide: the –OH is attached to the bottom-most assymetric center (the carbon that is second from the bottom) is on the right in a Fischer projection.

HC

HC

H2C OH

O

OH

achiral carbon

D-Glyceraldehyde

MonosacharidesMonosacharides

• L-monosaccharide:L-monosaccharide: the -OH is on the left in a Fischer projection.

CH

HC

H2C OH

O

HO

achiral carbon

L-Glyceraldehyde

D,L-MonosaccharidesD,L-Monosaccharides• The most common D-tetroses and D-pentoses are:

D,L-MonosaccharidesD,L-Monosaccharides The three most common D-hexoses are:

Amino SugarsAmino SugarsAmino sugars contain an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature: D-

glucosamine, D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.

Cyclic StructureCyclic Structure• Aldehydes and ketones react with alcohols to form

hemiacetalshemiacetals • Cyclic hemiacetals form readily when the hydroxyl and

carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.

EpimersEpimers Diastereomers that differ in configuration at only on

asymmetric center

HC O

OHH

OHH

OH

CH2OH

H

HC O

HHO

OHH

OH

CH2OH

H

D-ribose D-arabinose

1

2

3

4

5

1

2

3

4

5

C2-epimers*dif ferent configuration at C2

HC O

HHO

OHH

HHO

HC O

HHO

HHO

OHHO

CH2OH CH2OH

H OH OHH

C3-epimers*different conf iguration at C3

D-iodose D-talose

Table 20-1 p532

Table 20-2 p532

ExamplesExamples Draw Fisher projections for all 2-ketopentoses.

Which are D-2-ketopentoses, which are L-2-ketopentoses? Prefer to table 12.2 (your textbook) to write their names

Haworth ProjectionsHaworth Projections

• Figure 12.2 D-Glucose forms these two cyclic hemiacetals.

Haworth ProjectionsHaworth Projections

• A five- or six-membered cyclic hemiacetal is represented as a planar ring, lying roughly perpendicular to the plane of the paper.

• Groups bonded to the carbons of the ring then lie either above or below the plane of the ring.

• The new carbon stereocenter created in forming the cyclic structure is called the anomeric carbonanomeric carbon.

• Stereoisomers that differ in configuration only at the anomeric carbon are called anomersanomers.

• The anomeric carbon of an aldose is C-1; that of the most common ketose is C-2.

Haworth ProjectionsHaworth Projections In the terminology of carbohydrate chemistry,

◦ means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH.

◦ means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH.

◦ A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

Haworth ProjectionsHaworth Projections◦ Aldopentoses also form cyclic hemiacetals.◦ The most prevalent forms of D-ribose and other pentoses

in the biological world are furanoses.

◦ The prefix “deoxydeoxy” means “without oxygen.” at C2

Haworth ProjectionsHaworth ProjectionsD-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.

ExamplesExamples Give structure of the cyclic hemiacetal formed by

◦ 4-hydroxybutanal

◦ 5-hydroxypentanal

Chair ConformationsChair Conformations• For pyranoses, the six-membered ring is more accurately

represented as a strain-free chair conformationstrain-free chair conformation.

Chair ConformationsChair Conformations• In both Haworth projections and chair conformations, the

orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up.

O

CH2OH

OH

OH

OH

1

23

4

5

6

OH

HOH2C

HOHO

OH123

45

6

-D-glucose

OH

opposite = trans

Chair ConformationsChair Conformations

O

OH

CH2OH

OH

OH

OH

HOH2C

1

23

4

5

6

HOHO

OHOH

123

45

6

D-glucosesame = cis

ExamplesExamples Which OH groups are in the axial position in β-D-mannopyranose

β-D-idopyranose

MutarotationMutarotation Mutarotation: Mutarotation: The change in specific rotation that

accompanies the equilibration of - and -anomers in aqueous solution.◦ Example: When either -D-glucose or -D-glucose is

dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

Formation of GlycosidesFormation of Glycosides• Treatment of a monosaccharide, all of which exist almost

exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.

Formation of GlycosidesFormation of Glycosides

• A cyclic acetal derived from a monosaccharide is called a glycosideglycoside.

• The bond from the anomeric carbon to the -OR group is called a glycosidic bondglycosidic bond.

• Mutarotation is not possible for a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound.

Formation of GlycosidesFormation of Glycosides• Glycosides are stable in water and aqueous

base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide.

• Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -ee is replaced by -ide-ide.

ExamplesExamples Draw a Haworth projection and a chair conformation

for methyl -D-mannopyranoside. Label the anomeric carbon and glycosidic bond