Chemistry 2100

Chapter 20

Carbohydrates

• Molecular formula (CH2O)n

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

Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.

Aldose: A monosaccharide containing an aldehyde group.Ketose: A monosaccharide containing a ketone group.

Enantiomers

Monosacharides

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

– D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection.

– L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection.

L-GlyceraldehydeD-Glyceraldehyde

CHOCHO

H OH

CH2OH CH2OH

HHO

[]25 = +13.5°D

[]25 = -13.5°D

Epimers

Hemiacetals and Hemiketals

Cyclization

C

CH2OH

OHH

HO H

H OH

H

O HH

O

C

CH2OH

OHH

HO H

H OH

H O

H

H

O1

2

3

5

6

4

-D-glucose -D-glucoseD-glucose

1

2

3

5

6

4

1

2

3

5

6

4

C

CH2OH

OHH

HO H

H OH

H

OH H

O

Pyranoses and

Furanoses

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

HO

HOCH2 OH

HHO

CH2OH

OHH

H

C=O

CH2OH

HOH

CH2OH

OHH

HO HOH

HOHOCH2

HO HCH2OH

OH

D-Fructose

1

2

5

5

5

1

2

2

()

-D-Fructofuranose(-D-Fructose)

-D-Fructofuranose(-D-Fructose)

()

1

O

CH2OH

OHOH

HOCH 2

HO

O

HOHO

OHOH

CH2OH

-D(–)-fructofuranose -D(–)-fructopyranose

(Honey)

Chair Conformations• For pyranoses, the six-membered ring

is more accurately represented as a chair conformation.

OCH2OH

HOHO

OHOH()

CHOH

HO

CH2OHOHHO

OHO

OH()HO

HO

CH2OHO

(-D-Glucose)

(-D-Glucose)

-D-Glucopyranose

-D-Glucopyranose

D-Glucose

anomericcarbon

Chain-ring Equilibrium and Reducing Sugars

The Glycosidic Bond

Disaccharides

SweetnessMonosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol.

Sweetness relative to sucrose:Carbohydrate

fructose

glucose

galactose

sucrose (table sugar)

lactose (milk sugar)

honey

SweetnessRelative to Sucrose

1.741.000.970.74

0.320.16

Artificial Sweetener

SweetnessRelative to Sucrose

maltose 0.33

saccharin 450acesulfame-K 200aspartame 180sucralose 600

starch

• amylose

• amylopectin

• dextrins

starch

• amylose

• amylopectin

• dextrins

starch

• amylose

• amylopectin

• dextrins

(1,4)

starch

starch• amylose

• amylopectin

• dextrins

(1,4)

starch• amylose

• amylopectin

• dextrins

(1,6) (1,4)

starch• amylose

• amylopectin

• dextrins

glycogen

β(1,4)

cellulose

Other Polymers

Chitin

Agar

Heparin

CH2OH

CH3

O

RBC

O

OHOH

OH

O

AcNHHO

OOH

CH2OHH

O

H

O

O

H

NAGlu

Fuc

Gal

Type A

HO

CH2OH

OHO

H AcNH

NAGal

Blood Groups (14)

(12)

(13) AcNH

CH2OH

CH3

O

RBC

O

OHOH

OH

O

AcNHHO

OOH

CH2OHH

O

H

O

O

H

NAGlu

Fuc

Gal

HO

CH2OH

OHO

H

Blood Groups (14)

(12)

(13) AcNH

Type B

OH

Gal

CH2OH

CH3

O

RBC

O

OHOH

OH

O

AcNHHO

OOH

CH2OHH

O

H

O

O

H

NAGlu

Fuc

GalBlood Groups (14)

(12)

(13) AcNH

H

Type O