L3 - Chemical Properties of Monosaccharide

Chemical Properties of
Monosaccharides
Hold on tight…
… here comes
a quick review
of organic
chem.
Aldehyde and Ketone Reactions Con’t.
Formation of an Acetal The
addition of alcohols to aldehydes
produces an unstable hemiacetal
intermediate that in-turn, produces an
acetal.
The alcohol here is a
reactant not a catalyst
Reactions, cont.
General formulas
R
Anomeric
Carbon
R
Acetal
Hemiacetal
• The hemiacetal is formed
by the addition of an alcohol
(R-O-H) by breaking the C = O
carbonyl double bond and
attaching the ether group
(-O-R) to the carbonyl carbon
and the hydrogen to the
carbonyl oxygen.
• The unstable hemiacetal is
then converted into an acetal
by the substitution of the
newly formed hydroxyl (-O-H)
group by a new ether from a
second alcohol (R-O-H), also
producing water (H 2 O).
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Reactions Con’t.
Formation of a
Ketal The addition of
alcohols to ketones
produces an unstable
hemiketal intermediate
that in-turn, produces an
ketal.
The alcohol here is a
reactant not a catalyst
• The hemiketal is formed
by the addition of an alcohol
(R-O-H) by breaking the C = O
carbonyl double bond and
attaching the ether group
(-O-R) to the carbonyl carbon
and the hydrogen to the
carbonyl oxygen.
• The unstable hemiketal is
then converted into a ketal by
the substitution of the newly
formed hydroxyl (-O-H) group
by a new ether from a second
alcohol (R-O-H), also
producing water (H 2 O).
Even though hemiacetals and
hemiketals are relatively unstable,
cyclical hemiacetals and hemiketals are
commonly found because they are more
stable than open chains.
Cyclic Structures of Monosaccharides
All monosaccharides with at least five carbon
atoms exist predominantly as cyclic
hemiacetals and hemiketals.
A Haworth structure can be used to depict
the α and β anomers of a monosaccharide.
Anomers are stereoisomers that differ in the
3-D arrangement of groups at the anomeric
carbon of an acetal, ketal, hemiacetal, or
hemiketal group.
• Sixmembered
pyranose
ring system
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• When the newly formed hydroxyl group forms
below the ring structure, it is said to form the
alpha anomer and when it forms above the ring
structure, it is said to form the beta anomer.
• Six-membered pyranose ring system
• Anomers are sugars that differ in stereochemistry
only at the hemiacetal or hemiketal carbon.
Five-membered furanose ring system
Naming Cyclic Monosaccharides
• Designate the anomeric form of the cyclic
sugar structure by preceding the
monosaccharide name with α or β
Classic Naming of Cyclic Monosaccharides
• Identify as 5 carbon ring (furanose) or 6
carbon ring (pyranose)
β-D-glucopyranose
α-D-glucopyranose
• Drop the –se from the straight chained
monosaccharide name and replace with
furanose or pyranose
• Designate the anomeric form by preceding
the entire name with α or β
α-D-fructofuranose
β-D-fructopyranose
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Drawing Haworth Projections
• We have been representing cyclic
monosaccharides using Haworth Projections,
a standardized way of representing
carbohydrate stereoisomerism in 3D space.
• Haworth projections are drawn directly from
Fischer Projections simply by following the
steps below.
1. Draw the correct Fischer projection for the
monosaccharide.
• Note, the α or β is not determined by D
or L.
Drawing Haworth Projections
2. Draw a primary pyranose or furanose ring
structure depending on the
monosaccharide.
3. Number the ring structure beginning with
the carbon directly right of the oxygen atom
in the ring (begin with the anomeric
carbon).
• Remember, the last carbon is above the
ring.
Drawing Haworth Projections
4. Assign hydroxyl group and hydrogens
to all carbons other than the anomeric
carbon following the following rule:
Hydroxyl groups pointing to the left in the
fissure projection are position upward on
the Haworth Projection and those hydroxyl
groups pointing to the right are positioned
downward.
1.Try drawing the Haworth Projections for
the following monosaccharides:
β-D-mannopyranose
α-D-xylofuranose
β-L-galactopyranose
5. Assign the acetal carbon a hydroxyl
group corresponding to its anomeric
designation (β up, α down)
Remember Benedict’s Test?
In the presence of aldehydes, Benedict’s
reagent (containing CuSO 4 ) produces a red
copper precipitate.
Benedict’s
reagent in
comparison
to 2 glucose
solutions
Monosaccharide Reactions
Oxidation of a Reducing Sugar
The oxidation of a carbonyl group on a
monosaccharide.
•Since all monosaccharides are in equilibrium
with their cyclic form, they are all reducing
sugars.
•Benedict’s reagent is commonly used to test for
the presence of reducing sugars:
Reducing sugar + Cu 2+ → oxidized compound + Cu 2
O
blue
orange-red
precipitate
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Monosaccharide Reactions, cont.
Esterfication The –OH groups of
monosaccharides can behave as
alcohols and react with acids (especially
phosphoric acid) to form esters.
Monosaccharide Reactions, cont.
Glycoside Formation Cyclic
monosaccharide hemiacetals and
hemiketals react with alcohols to form
acetals and ketals, referred to as
glycosides.
This is an important reaction
in the formation of Lipids
studied in the next chapter
• The unstable hemiacetal is
then converted into an acetal
by the substitution of the
newly formed hydroxyl (-O-H)
group by a new ether from a
second alcohol (R-O-H), also
producing water (H 2 O).
The dehydration of the hemiacetal or
hemiketal forms what is called a
glycosidic linkage.
A glycosidic linkage is an ether link
between a carbohydrate (poly hydroxyl,
or glycol compound) and another carbon
molecule.
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