the organic chemistry of carbohydrates · 2020. 3. 24. · simple carbohydrates are...
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Organic Chemistry6th Edition
Paula Yurkanis Bruice
Chapter 22
The Organic Chemistry of
Carbohydrates
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Carbohydrates• General molecular formula is Cn (H2 O)n .• Structurally, are polyhydroxy aldehydes or ketones.• The most abundant carbohydrate in nature is glucose.
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Simple carbohydrates are monosaccharides
Complex carbohydrates contain two or more sugar unitslinked together:
• Disaccharides• Oligosaccharides• Polysaccharides
Polyhydroxy aldehydes are aldosesPolyhydroxy ketones are ketoses
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D and L notations are used to describe the configurationsof carbohydrates:
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Configurations of Aldoses
Aldotetroses have two asymmetric centers and four stereoisomers:
Aldopentoses have three asymmetric centers and eightstereoisomers
Aldohexoses have four asymmetric centers and 16stereoisomers
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Diastereomers that differ in configuration at only oneasymmetric center are called epimers:
D-Mannose is the C-2 epimer of D-glucoseD-Galactose is the C-4 epimer of D-glucose
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• A ketose has one fewer asymmetric center than an aldose.
• Therefore, ketoses have fewer stereoisomers than aldoses.
• Naturally occurring ketoses have the ketone group in the 2-position.
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Mechanism for the base-catalyzed epimerization of a monosaccharide:
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Mechanism for the base-catalyzed enediol rearrangement of a monosaccharide:
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Redox Reactions of Monosaccharides
The carbonyl of aldoses and ketoses can be reduced bythe carbonyl-group reducing agents:
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Oxidation
The aldehyde groups can be oxidized by Br2
Ketones and alcohols cannot be oxidized by Br2
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Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent:
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A strong oxidizing agent such as HNO3 can oxidize both the aldehyde and the alcohol groups:
A primary alcohol is the one most easily oxidized
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Osazone Formation
Aldoses and ketoses react with three equivalents ofphenylhydrazine to form osazones:
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The C-2 epimers of aldoses form identical osazones…
…because the configuration of the C-2 carbon is lost during osazone formation
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The C-1 and C-2 carbons of ketoses also react with phenylhydrazine:
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The carbon chain of an aldose can be increased by onecarbon in a Kiliani–Fischer synthesis:
This reaction leads to a pair of C-2 epimers
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The Wohl degradation shortens an aldose chain by one carbon:
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D-Glucose exists in three different forms:
anomer
anomer
The specific rotation of pure - D-glucose or -D-glucose changes over time to reach an equilibrium (mutarotation)
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If an aldose can form a five- or six-membered ring, it willexist predominantly as a cyclic hemiacetal:
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• Six-membered rings are called pyranoses.• Five-membered rings are called furanoses.• The prefix - indicates the configuration about
the anomeric carbon.
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Ketoses also exist predominantly in cyclic forms:
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-D-Glucose is the most stable of all aldohexoses and predominates at equilibrium:
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Formation of Glycosides
The acetal (or ketal) of a sugar is called a glycoside:
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Mechanism for glycosideformation
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Formation of an N-Glycoside
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Hilbert–Johnson Reaction
Oxocarbocation formation:
Selective -anomer formation:
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-Nucleophilic attack:
Hydrolysis:
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Glycated hemoglobin test: • Without diabetes, 5.5 to 9% glycated hemoglobin• With diabetes, 10.9 to 15.5% glycated hemoglobin
Glucose can form glycosides with amine groups on hemoglobin
Glycoside Formation in Diabetes
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The Anomeric EffectThe preference of certain substituents bonded to theanomeric carbon for the axial position is called theanomeric effect:
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Reducing and Nonreducing Sugars
A sugar with an aldehyde, a ketone, a hemiacetal, or ahemiketal group is a reducing sugar
A sugar without one of these groups is a nonreducingsugar
Nonreducing sugars cannot reduce Ag+ or Br2
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DisaccharidesComposed of two monosaccharide subunits linked
together by an acetal linkage:
In maltose, the OH group bonded to the anomericcarbon is axial, hence the ‘’ in the linkage descriptor
Maltose is a reducing sugar
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In cellobiose, the two subunits are linked together by a-1,4-glycosidic linkage
Cellobiose is a reducing sugar
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In lactose, the two different subunits are joined by a-1,4-glycosidic linkage
Lactose is a reducing sugar and undergoes mutarotation
The -1,4
glycosidic linkage is cleaved by the enzyme lactase
Without lactase, lactose is turned into gases by flora in the lower bowel, producing flatulence and bloating
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Sucrose: The most common disaccharide
Sucrose is not a reducing sugar and does not exhibit mutarotation
Sucrose []D = + 66.5 o Glucose + fructose (1:1) []D = - 22.0 ohydrolysis
H2 O
Hence also called ‘invert sugar’. Bees have invertase
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The trisaccharide raffinose is found in beans and a variety of grains and vegetables
Galactose
Glucose
FructoseSucrose
Cleaved by -galactosidase (-GAL)
-GAL is not in the human digestive tract.
• Raffinose is not cleaved to sucrose and galactose.
• Lower gut flora converts raffinose to gases; flatulence results.
How Does Beano Work?
Beano contains fungal -GAL that cleaves raffinose and other polysaccharides
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Determination of disaccharide structure (sequence)
Consider lactose: disaccharide containing galactose and glucose
How are they linked?
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Polysaccharides
Amylose is a component of starch:
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Amylopectin is another polysaccharide component of starch that has a branched structure:
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Cellulose is the structural material of higher plants:
-1,4-Glycosidic linkages are easier to hydrolyze than -1,4-glycosidic linkages
because of the anomeric effect
OO
O
OO
OH
OH
O O
CH2OH
OH
HO
OH CH2OH
OHCH2OH
intramolecular hydrogen bond
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Chitin: The structural component of the shells of crustaceans
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Naturally occuring derivatives of carbohydrates
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D-Ribose is the sugar component of ribonucleic acid
D-2-Deoxyribose is the sugar component of deoxyribonucleic acid
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Vitamin C (L-ascorbic acid) is synthesized from D-glucose
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Proteins bonded to oligosaccharides are called glycoproteins
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Blood type is determined by the sugar bound to the protein on the surface of red blood cells
(Fucose = 6-deoxygalactose)
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