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Chapter 13: Carbohydrates

Chem 20

El Camino College

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Carbohydrates

Carbohydrates have the formulas of Cn(H2O)n

Glucose is formed by photosynthesis:

6 CO2 + 6 H2O → C6 (H2O)6 + 6O2

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D and L The first carbon (C #1) is the carbon on the top In the L version, the OH on the last chiral carbon is on

the left In the D version, the OH on the last chiral carbon is on

the right

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Examples of D and L Isomers of Monosaccharides

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Cyclic Structure of Glucose The carbonyl group of D-glucose can react with the

–OH on carbon 5 to form a ring of six atoms.

It has one more chiral center (carbon 1) than the open chain.

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Drawing the Cyclic (Haworth) Structure

1. Starting with the aldehyde carbon, number the carbons

2. Draw the structure laying on the right sideMake sure you don’t change the locations of the

OH groups

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H

OHH

OH

C

H H

OH OH

C C CH

O

CHOCH2

Drawing the Cyclic Structure for Glucose

STEP 1 Number the carbon chain and turn on its side to the right

HHO

H

CH2OH

OHC

H

H

OH

OH

C

C

C

OH

C1

2

3

4

5

6

6 5 4 3 2 1

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Cyclic Structures

3. Draw the following cyclic structureNote that there is an oxygen in the ringAlways draw the structure in the exact same way

O

1

23

4

5

6

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Cyclic Structures 4. In your straight chain drawing, note if the OH is

down or up on carbons 2, 3, and 4Put those OH groups down or up on the ringCH2OH is up, OH on C1 can go up or down Fill in the rest with hydrogens

HOH2C

H

OH

OHH

OH

OH

H

OH

H

1

2345

6

downup

down

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Cyclic Structures

HOH2C

H

OH

OHH

OH

OH

H

OH

H

1

2345

6

downup

down

O

OH

OH

OH

1

23

4

5

6

O

OH

H

H

OH

OH

H

H

OH

CH2OH

H

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Cyclic Structure of Glucose In the Haworth structure, we suppose that the ring

is flat. the group -CH2OH is above the ring. The –OH group on carbon 1 is written down in the

alpha form and up in the beta form.

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Cyclic Structure of Glucose Solid glucose is α-D-glucose. In aqueous solution, there is equilibrium of three forms: 36%

α-D-glucose, 64% β-D-glucose and trace of open chain.

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Cyclic Structure, D-Galactose

CH2OH

HO

H OH

HHO

HHO

OHH

HOH2C

H

OH

OHH

OH

H

OH

OH

H

1

2345

6

downupup

O

OH

H

H

OH

OH

H

OH

H

CH2OH

H 1

23

4

5

6

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Cyclic Structure, D-Galactose

O

OH

H

H

OH

OH

H

OH

H

CH2OH

H 1

23

4

5

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The cyclic structure of galactose is similar to glucose, except that the –OH on carbon 4 is up.

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Cyclic Structure D-Galactose

The cyclic D-galactose can be in alpha and beta forms

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Cyclic Structure

CH2OH

HO

H OH

OHH

OHH

OHH

HOH2C

H

OH

OHOH

H

OH

H

OH

H

down down down

1

2345

6

OH

OH

H

OH

H

OHOH

H

CH2OH

H

23

4

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Cyclic Structure Fructose

The hydroxyl group on carbon 5 reacts with the ketone group on carbon 2 to form a five-atom ring. The cyclic D-fructose can be in alpha and beta forms

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Chemical Properties

Reduction of monosaccharides: The carbonyl group (-C=O) can be reduced to –OH group. For example D-glucose is reduced to sorbitol (D-glucitol)

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Oxidation of Monosaccharides

Aldoses are called “reducing sugars” The aldehyde group (-CHO) in aldose can be

oxidized by silver ammonia ion Ag(NH3)2+ of Tollens’

reagent to form silver In the Benedict test, the aldehyde group is oxidized

to a carboxylic acid group by Cu(II) citrate complex and a brick-red ppt (Cu2O) forms

Fructose is also a reducing sugar since it is converted to glucose in basic solution.

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+ Cu2O(s)

D-gluconic acidD-glucose

+ Cu2+

H OH

H OH

HHO

H OH

O

OH

CH2OH

C

C

C

C

C

H OH

H OH

HHO

H OH

O

H

CH2OH

C

C

C

C

C

Benedict Test of D-Glucose

[O]

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Disaccharides Disaccharides can be broken down into two

monosaccharidesMaltose + H+ + H2O glucose + glucose

Lactose + H+ + H2O glucose + galactose

Sucrose + H+ + H2O glucose + fructose

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Alpha and Beta Linkages There are different types of linkages in

disaccharidesIn -linkages, both OH groups involved in

bonding are pointing downIn -linkages, one OH involved in bonding is up

and the other is down.

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Formation of alpha lactose from glucose + galactose (formation of ether bridge)

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Formation of Beta Lactose

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Formation of Alpha Maltose

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Sucrose from glucose + fructose

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Polysaccharides Long chains of monosaccharides are called

polysaccharides Amylose and amylopectin are plant starches Glycogen is animal starch—it provides storage

for glucose in muscles

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Polysaccharides Cellulose (in wood and cotton) is a structural

material for plants

People don’t have the enzymes to digest the -linkages in cellulose, but cows do!

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