Chapter 21

Chapter 21
Carbohydrates
Carbohydrates
They have the molecular formulas Cn(H2O)m
Compounds that can be hydrolyzed to polyhydroxy
aldehydes or polyhydroxy ketones are also classified as
carbohydrates
Simple Carbohydrates Are Monosacharides
Complex carbohydrates contain two or more sugar units
linked together
•disaccharides
•oligosaccharides
•polysaccharides
Polyhydroxy aldehydes are aldoses
Polyhydroxy ketones are ketoses
D and L notations are used to describe the configurations
of carbohydrates
1
2
3
4
5
6
D-(+)-Glyceraldehyde
L-(–)-Glyceraldehyde
Figure 6.18
Configurations of Aldoses
Aldotetroses have two stereogenic centers and
four stereoisomers
Diastereomers that differ in configuration at only one
stereogenic carbon are called epimers
A ketose has one less stereogenic center than
aldoses, therefore they have fewer stereoisomers
Redox Reactions of Monosaccharides
The carbonyl of aldoses and ketoses can be reduced by
the carbonyl-group reducing agents
Oxidation
The aldehyde groups can be oxidized by Br2
Ketones and alcohols cannot be oxidized by Br2
In a basic solution, ketoses are converted into aldoses
A strong oxidizing agent such as HNO3 can oxidize the
aldehyde and the alcohol groups
Osazone Formation
Aldoses and ketoses react with three equivalents of
phenylhydrazine
The C-2 epimers of aldoses form identical osazones
Reaction of Ketoses with Phenylhydrazine
The carbon chain of an aldose can be increased by one
carbon in a Kiliani–Fischer synthesis
The Ruff degradation shortens an aldose chain by one
carbon
Preparation of the Calcium D-Gluconate for the
Ruff Degradation
Cyclic Structure of Monosaccharides
Hemiacetal Formation
anomer
anomer
The specific rotation of pure α-D-glucose or β-D-glucose
changes over time to reach an equilibrium (mutarotation)
Figure 27.2
Note …
• If an aldose can form a five- or six-membered ring, it will
exist predominantly as a cyclic hemiacetal
• Six-membered rings are called pyranoses
• Five-membered rings are called furanoses
• A sugar with an aldehyde, a ketone, a hemiacetal, or
a hemiacetal group is a reducing sugar
The structures of cyclic sugars are best represented by
the Haworth projections
Haworth projections allow us to see the relative
orientation of the OH groups in the ring
Ketoses also exist predominantly in cyclic forms
β-D-Glucose Is More Stable
β-D-glucose is the predominant form at equilibrium
Acylation of Monosaccharides
The OH groups of monosaccharides show the chemistry
of typical alcohols
Alkylation of the OH Groups
Formation of Glycosides
The acetal (or ketal) of a sugar is called a glycoside
Mechanism of Glycoside Formation
Formation of an N-Glycoside
The Anomeric Effect
The formation of a glycoside favors the α-glucoside
product: the anomeric effect
Determination of Ring Size
Approach 1
The size of the ring can be determined from the structure
of the open-chain form
Determination of Ring Size
Approach 2
An acetal of the monosaccharide is oxidized with excess
HIO4
The α-hydroxyaldehyde formed from HIO4 oxidation is
further oxidized to formic acid and another aldehyde
Disaccharides
Composed of two monosaccharide subunits hooked
together by an acetal linkage
In α-maltose, the OH group bonded to the anomeric
carbon is axial
Maltose is a reducing sugar
In cellobiose, the two subunits are hooked together by a
β-1,4’-glycosidic linkage
Cellobiose is a reducing sugar
In lactose, the two different subunits are joined by a
β-1,4’-glycosidic linkage
Lactose is a reducing sugar
The most common disaccharide is sucrose
Sucrose is not a reducing sugar
Polysaccharides
Amylose is a component of starch
Structure of Chitin
Structure of Cellulose
Intramolecular hydrogen bonding
Amylopectin is another polysaccharide component of
starch that has a branched structure
Structure of amylose
The α-1,4-glycosidic linkages in amylose cause this polymer to form a left-handed helix.
An example of a naturally occurring product derived from
carbohydrates
Blood type is determined by the nature of the sugar
bound to the protein on the surface of red blood cells