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CarbohydratesClassification

MonosaccharidesChiral Carbon Atoms

Structures of Important MonosaccharidesCyclic Structures

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Carbohydrates

• Major source of energy from our diet

• Composed of the elements C, H and

O

• Produced by photosynthesis in

plants

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Types of Carbohydrates• Monosacchrides

• DisaccharidesContain 2 monosacchride units

• PolysacchridesContain many monosacchride units

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Monosacchrides

• Three Carbons = Triose

• Four Carbons = Tetrose

• Five Carbons = Pentose

• Six Carbons = Hexose

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Monosacchrides

• Aldoses are monosacchrides with an aldehyde group and many hydroxyl (-OH) groups.

• Ketoses are monosacchrides with a ketone group and many hydroxyl (-OH) groups.

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Learning Check C1Identify each as tetrose, pentose or hexose, and as aldose or ketose

A B

CC

CH2OHH OHC

OHH COHH

H OHC O

H

CC

CH2OHOHH CHHOO

CH2OH

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Solution C1

A Baldose, hexose ketose, pentose

CC

CH2OHH OHC

OHH COHH

H OHC O

H

CC

CH2OHOHH CHHOO

CH2OH

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Chiral Objects

• Chiral compounds have the same number of atoms arranged differently in space.

• A chiral carbon atom has four different groups attached

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Mirror Images• The three-dimensional structure of a chiral

compound has a mirror image. • Your hands are chiral. Try to superimpose

your thumbs, palms, back of hands, and little fingers. Is it possible? Why or why not?

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Tetrahedral carbon atombonded to four different groups

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ARE THERE CHIRAL C-ATOMS IN THESE MOLECULES?

CH3CHClCH2CH3

A

CH2ClCH3

B

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Learning Check C2

Determine if there is a chiral carbon in each compound.

A B

CCl

CH3

CH2CH3

H CCl

CH3

HH

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Solution C2

A Yes, 4 different B No, thegroups are attached 2 H atomsto the second C atom are identical

CCl

CH3

CH2CH3

H CCl

CH3

HH

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Glyceraldehyde

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D and L Notation

• D,L tells which of the two chiral isomers we are referring to.

• If the –OH group on the next to the bottom carbon atom points to the right , the isomer is a D-isomer; if it points left, the isomer is L.

• The D form is usually the isomer found in nature.

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D notation

CCCH2OH

OHHOHH

CO

H

Right = D

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Glucose

CC

CH2OHH OHC

OHH CHHO

H OHC O

H

D-Glucose

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Fructose

CC

CH2OHH OHC

OHH CHHOO

CH2OH

D-Fructose

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Galactose

D-galactose

CCCCCCH2OH

OH

OHHHO H

HHOOHH

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Cyclic Structures

• Monosaccharides with 5-6 carbon atoms form cyclic structures

• The hydroxyl group on C-5 reacts with the aldehyde group or ketone group

o O

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Haworth Structure for D-Isomers

The cyclic structure of a D-isomer has the final CH2OH group located above the ring.

oCH2OH

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Haworth Structure for D-Glucose

• Write –OH groups on the right (C2, C4) up

• Write –OH groups on the left (C3) down

• The new –OH on C1 has two possibilites: down for α anomer, up for β anomer

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Cyclization of glucose

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Haworth Structure for D-Glucose

oCH2OH

OHOH

OH

OH

oCH2OH

OHOH

OH

OH

α-D-Glucose β-D-Glucose

α

β

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d-fructose

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Mutarotation

• Mutarotation: A small amount of open chain is in equilibrium with the cyclic forms.

• The most stable form of glucose is β-D-glucose .

α-D-glucose D-glucose (open) β-D-glucose(36%) (trace) (64%)

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Learning Check C3

Write the cyclic form of α-D-galactose

CCCCCCH2OH

OH

OHHHO H

HHOOHH

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Solution C3

o

OH

OH

CH2OH

OH

OH

α-D-galactose