16 16-1 copyright © 2000 by john wiley & sons, inc. all rights reserved. introduction to...

1616

16-1Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Introduction to Introduction to Organic Organic

ChemistryChemistry2 ed2 ed

William H. BrownWilliam H. Brown

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CarbohydratesCarbohydrates

Chapter 24

Chapter 16Chapter 16

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CarbohydratesCarbohydrates• CarbohydrateCarbohydrate: a polyhydroxyaldehyde or

polyhydroxyketone, or a substance that gives these compounds on hydrolysis

• MonosaccharideMonosaccharide: 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

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MonosaccharidesMonosaccharides• monosaccharides are classified by their number of

carbon atoms

hexose

heptose

octose

triose

tetrose

pentose

FormulaName

C3

H6

O3

C4

H8

O4

C5

H10

O5

C6

H12

O6

C7

H14

O7

C8

H16

O8

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MonosaccharidesMonosaccharides• There are only two trioses

• often aldo- and keto- are omitted and these compounds are referred to simply as trioses

• although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons

D ihydroxyacetone

(a ketotriose)

Glyceraldehyde

(an aldotriose)

CHO

CHOH

CH2

OH

CH2

OH

C=O

CH2

OH

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MonosaccharidesMonosaccharides• Glyceraldehyde contains a stereocenter and

exists as a pair of enantiomers

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Fischer ProjectionsFischer Projections• Fischer projectionFischer projection: a two dimensional

representation for showing the configuration of tetrahedral stereocenters• horizontal lines represent bonds projecting forward • vertical lines represent bonds projecting to the rear

(R)-Glyceraldehyde

CHO

CH OH

CH2

OH

(R)-Glyceraldehyde

convert to

a Fischer

projectionH OH

CHO

CH2

OH

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D,L MonosaccharidesD,L Monosaccharides• In 1891, Emil Fischer made the arbitrary

assignments of D- and L- to the enantiomers of glyceraldehyde

L-GlyceraldehydeD-Glyceraldehyde

[ α ]25

= +13.5°D

[ α ]25

= -13.5°D

C H O

C H2

O H

O HH

C H O

C H2

O H

HH O

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D,L MonosaccharidesD,L Monosaccharides• According to the conventions proposed by

Fischer• D-monosaccharideD-monosaccharide: a monosaccharide that, when

written as a Fischer projection, has the -OH on its penultimate carbon on the right

• L-monosaccharideL-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the left

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• Following are • the two most common D-aldotetroses and • the two most common D-aldopentoses

D,L MonosaccharidesD,L Monosaccharides

D-Erythrose D-Threose D-Ribose 2-Deoxy-D-

ribose

C H O

C H2

O H

O HH

O HH

C H O

C H2

O H

HH O

O HH

C H O

C H2

O H

O HH

O HH

O HH

C H O

C H2

O H

HH

O HH

O HH

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D,L MonosaccharidesD,L Monosaccharides• and the three most common D-aldohexoses

C H O

C H 2 O H

O HH

HH O

O HH

O HH

D-GlucosamineD-Glucose D-Galactose

C H O

C H 2 O H

O HH

HH O

HH O

O HH

C H O

C H 2 O H

N H2

H

HH O

HH O

O HH

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Cyclic StructureCyclic Structure• Monosaccharides have hydroxyl and carbonyl

groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals• anomeric carbonanomeric carbon: the new stereocenter resulting from

cyclic hemiacetal formation• anomersanomers: carbohydrates that differ in configuration at

their anomeric carbons

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Haworth ProjectionsHaworth Projections• Haworth projections

• five- and six-membered hemiacetals are represented as planar pentagons or hexagons, as the case may be, viewed through the edge

• most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right

• the designation - means that -OH on the anomeric carbon is cis to the terminal -CH2OH; α- means that it is trans

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D-Glucose

- -D Glucopyranose

( - - )D Glucose

C

H O H

H

H O

H

O H

H

C H2

O H

O H

O H ( α )

H O H

H

H O

HH

O H

H

C H2

O H

O

O

H

H

H O H

H

H O

H

O H ( β )

O H

H

C H2

O H

O

α -D-Glucopyranose

( α -D-Glucose)

+

anomeric

carbon

5

5 5

5

C H O

C H2

O H

O HH

HH O

O HH

O HH

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Haworth ProjectionsHaworth Projections• six-membered hemiacetal rings are shown by the infix

-pyranpyran- • five-membered hemiacetal rings are shown by the infix

-furanfuran-

OO

PyranFuran

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Conformational FormulasConformational Formulas• five-membered rings are close to being planar so that

Haworth projections are adequate to represent furanoses

O

O H ( α )

H

H

H O O H

H H

α -D-Ribofuranose

( α -D-Ribose)

O

H

O H ( β )

H

H O O H

H H

β -D-Ribofuranose

( β -D-Ribose)

H O C H2

H O C H2

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Conformational FormulasConformational Formulas• for pyranoses, the six-membered ring is more

accurately represented as a strain-free chair conformation

- D-Glucopyranose

(chair conformation)

O

CH2

OH

HO

HO

OH

OH ( β )

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Conformational FormulasConformational Formulas• compare the orientations of groups on carbons 1-5 in

the Haworth and chair representations of -D-glucopyranose

• in each case they are up-down-up-down-up

- -D Glucopyranose

( )chair conformation

O

C H 2 O H

H O

H O

O H

O H ( β )

β -D-Glucopyranose

(Haworth projection)

H

H O H

HH O

HO H ( β )

O H

H

C H2

O H

O

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MutarotationMutarotation• MutarotationMutarotation: the change in specific rotation that

occurs when the α or forms of a carbohydrate are converted to an equilibrium mixture of the two

+80.2

+80.2

+52.8

+150.7

- -D galactose

α - -D galactose

[ α ] after

Mutarotation

( )degrees

[ α ]

Monosaccharide

% Present at

Equilibrium

28

72

64

36α - -D glucose

- -D glucose

+112.0

+18.7

+52.7

+52.7

(degrees )

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44

4

( α - D-Galactose)

( β - D-Galactose) α - D-Galactopyranose

β - D-Galactopyranose

D-Galactose

C

HO H

H O

H O

C H 2 O H

O

O H

O H ( α )

O H

H O

H O

O

C H 2 O H

O

C H 2 O H

H O

H O

O HO H ( β )

[α ]25

= +52.8°D

[ α ]25

= +150.7°D

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Physical PropertiesPhysical Properties• monosaccharides are colorless crystalline solids, very

soluble in water, but only slightly soluble in ethanol• following are relative sweetnesses (table sugar = 100 is

the reference sweetness)

Monosaccharide

& Disaccharides

Other Carbohydrate

Sweetening Agent

D-fructose 174

D-glucose 74

D-galactose 0.22

sucrose (table sugar) 100

lactose (milk sugar) 0.16

honey 97

molasses 74

corn syrup 74

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Formation of GlycosidesFormation of Glycosides• GlycosideGlycoside: a carbohydrate in which the -OH of the

anomeric carbon is replaced by -OR

Methyl -D-glucopyranoside (methyl -D-glucoside)

O

C H2

O H

H

O H

O C H3

( )

H

H O

H

O HH

H

O

C H 2 O H

H O

H O

O H

O C H3

( )

Haworth projection

Chair conformation

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GlycosidesGlycosides• Glycoside bondGlycoside bond: the bond from the anomeric

carbon of the glycoside to an -OR group• Glycosides are named by listing the name of the

alkyl or aryl group attached to oxygen followed by the name of the carbohydrate with the ending -ee replaced by -ideide• methyl -D-glucopyranoside• methyl α-D-ribofuranoside

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N-GlycosidesN-Glycosides• The anomeric carbon of a cyclic hemiacetal

undergoes reaction with the N-H group of an amine to form an N-glycoside• N-glycosides of the following purine and pyrimidine

bases are structural units of nucleic acids

HN

N

O

O

H

N

N

NH2

O

H

HN

N

O

O

H

CH3

Uracil Thymine Cytosine

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N-GlycosidesN-Glycosides

N

N N

N

NH2

HAdenine

anomeric

carbon

a - -N glycoside

bond

H

H

H

OH O C H

2

H O O H

N H2

O

N

N

H

H N

NN

N

O

H

H2

N

Guanine

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Reduction to AlditolsReduction to Alditols• The carbonyl group of a monosaccharide can be

reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2/M

Ni

+

D-Glucitol

(D-Sorbitol)

D-Glucose

H 2

C H O

C H2

O H

O HH

HH O

O HH

O HH

C H2

O H

C H2

O H

O HH

HH O

O HH

O HH

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Oxidation to Aldonic AcidsOxidation to Aldonic Acids• Oxidation of the -CHO group of an aldose to a -

CO2H group can be carried out using Tollens’, Benedict’s, or Fehling’s solutions

Precipitates as

a silver mirror

+

O

O

R C H

A g ( N H3

)2

+R C O

- N H

4

+

A g

Tollens' solution

N H3

, H2

O+

citrate or

tartrate buffer

Precipitates

as a red solid

++

O

C u2 +

R C O-

C u2

O

O

R C H

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Oxidation to Aldonic AcidsOxidation to Aldonic Acids• 2-Ketoses are also oxidized by these reagents

because, under the conditions of the oxidation, 2-ketoses equilibrate with isomeric aldoses

An aldoseAn enediolA 2-ketose

CH 2 OH

C=O

CH 2 OH

C-OH

CH 2 OH

CHOH

CHOH

CH2

OH

CHO

(CH OH)n

(CH OH)n

(CH OH)n

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Glucose AssayGlucose Assay• The analytical procedure most often performed in

the clinical chemistry laboratory is the determination of glucose in blood, urine, or other biological fluid

• this need arises because of the high incidence of diabetes in the population

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Glucose AssayGlucose Assay• The glucose oxidase method is completely

specific for D-glucose

+

+

glucose

oxidase

D-Gluconic acid

Hydrogen

peroxide

- -D Glucopyranose

O H

O H

H O

H O

C H 2 O H

O

H 2 O 2

O 2 + H 2 O

C O2H

C H2O H

O HH

HH O

O HH

O HH

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Glucose AssayGlucose Assay• O2 is reduced to hydrogen peroxide H2O2

• the concentration of H2O2 is proportional to the concentration of glucose in the sample

• in one procedure, hydrogen peroxide is used to oxidize o-toluidine to a colored product, whose concentration is determined spectrophotometrically

peroxidase+colored product +o-toluidine H

2O

2H

2O

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Ascorbic Acid (Vitamin C)Ascorbic Acid (Vitamin C)• L-Ascorbic acid (vitamin C) is synthesized both

biochemically and industrially from D-glucoseC H O

C H2

O H

O HH

HH O

O HH

O HH

D-Glucose

C H2

O H

O HH

H

H O

O

O H

both biochemial

and industrial

syntheses

L-Ascorbic acid

(Vitamin C)

O

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Ascorbic Acid (Vitamin C)Ascorbic Acid (Vitamin C)• L-ascorbic acid is very easily oxidized to L-

dehydroascorbic acid. • both compounds are physiologically active and are

found in most body fluids

C H2

O H

O HH

H

H O

O

O

O H

C H2

O H

O HH

H

O

O

O

O

L-Ascorbic acid

(Vitamin C)

L-Dehydroascorbic acid

oxidation

reduction

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MaltoseMaltose• From malt, the juice of sprouted barley and other

cereal grains

O

O H

H O

H O

C H2

O H

OO

O H

H O

C H2

O H

O H

α -1,4-glycoside

bond

β -maltose because

this -OH is beta

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-Maltose-Maltose

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LactoseLactose• The principle sugar present in milk

• about 5% - 8% in human milk, 4% - 5% in cow’s milk

O

OH

HO

OH

CH2

OH

OO

OH

HO

CH2

OH

OH

-1,4-glycoside

bond

β -lactose because

this -OH is beta

D-galactose

unitD-glucose

unit

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-Lactose-Lactose

1616


SucroseSucrose• Table sugar, from the juice of sugar cane and

sugar beet

O

OH

HO

HO

CH2

OH

O

α -1,2-glycoside

bond

β− D-glucopyranose

unit

O

CH 2 OH

HCH

2OH

OH H

HHO

1

1

2

β− D-fructopyranose

unit

β -2,1-glycoside

bond

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Blood Group SubstancesBlood Group Substances• Membranes of animal plasma cells have large

numbers of relatively small carbohydrates bound to them• these membrane-bound carbohydrates are part of the

mechanism by which cell types recognize each other; they act as antigenic determinantsantigenic determinants

• among the first discovered of these antigenic determinants are the blood group substancesblood group substances

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ABO Blood ClassificationABO Blood Classification• In the ABO system, individuals are classified

according to four blood types: A, B, AB, and O• at the cellular level, the biochemical basis for this

classification is a group of relatively small membrane-bound carbohydrates

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ABO Blood ClassificationABO Blood Classification

NAGal Gal NAGluCell membrane

of erythrocyte

(α -1,4-) ( -1,3-) ( -1-)

Fuc

(α -1,2-)

= - - -NAGal N acetyl D galactosamine

= -Gal D galactose

= - - -NAGlu N acetyl D glucosamine

= -Fuc L fucose

missing in

type O blood

- D galactose in

type B blood

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L-FucoseL-Fucose• L-fucose is synthesized biochemically from D-

glucose (see Problem 24.32)

C H O

O H

C H3

HH O

O HH

H

HH O

An L-monosaccharide

because this -OH is on

the left in the Fischer

projection

rather than -CH2

OH

Carbon 6 is -CH3

L-Fucose

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StarchStarch• Starch is used for energy storage in plants

• it can be separated into two fractions; amylose and amylopectin. Each on complete hydrolysis gives only D-glucose

• amyloseamylose is composed of continuous, unbranched chains of up to 4000 D-glucose units joined by α-1,4-glycoside bonds

• amylopectinamylopectin is a highly branched polymer of D-glucose. Chains consist of 24-30 units of D-glucose joined by α-1,4-glycoside bonds and branches created by α-1,6-glycoside bonds

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GlycogenGlycogen• The reserve carbohydrate for animals

• a nonlinear polymer of D-glucose units joined by α-1,4- and α-1,6-glycoside bonds bonds

• the total amount of glycogen in the body of a well-nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle

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CelluloseCellulose• Cellulose is a linear polymer of D-glucose units

joined by -1,4-glycoside bonds• it has an average molecular weight of 400,000,

corresponding to approximately 2800 D-glucose units per molecule

• Both rayon and acetate rayon are made from chemically modified cellulose

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CarbohydratesCarbohydrates

End Chapter 16End Chapter 16

16 16-1 copyright © 2000 by john wiley & sons, inc. all rights reserved. introduction to...

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n o n n monosaccharides

brown slide

rear slide

left slide

common daldohexoses

common daldopentoses

monosaccharides glyceraldehyde