Chapter 20
Carbohydrates
Carbohydrates
Carbohydrate: A polyhydroxyaldehyde or
polyhydroxyketone, or a substance that gives these
compounds on hydrolysis.
Monosaccharide: A carbohydrate that cannot be
hydrolyzed to a simpler carbohydrate.
• Monosaccharides have the general formula CnH2nOn,
where n varies from 3 to 8.
• Aldose: A monosaccharide containing an aldehyde
group.
• Ketose: A monosaccharide containing a ketone group.
Monosaccharides





The suffix -ose indicates that a molecule is a
carbohydrate.
The prefixes tri-, tetra, penta, and so forth indicate the
number of carbon atoms in the chain.
Those containing an aldehyde group are classified as
aldoses.
Those containing a ketone group are classified as
ketoses.
There are only two trioses:
Monosaccharides

There are only two trioses:
◦ Often aldo- and keto- are omitted and these compounds
are referred to simply as trioses.
◦ Although “triose” does not tell the nature of the carbonyl
group, it at least tells the number of carbons.
Monosaccharide

Monosaccharides with
◦ three carbons: trioses
◦ Five carbons: pentose
◦ Six carbons: hexose
◦ And so on …
Monosacharides
Figure 12.1 Glyceraldehyde, the simplest aldose, contains one
stereocenter and exists as a pair of enantiomers.
Enantiomers

Enantiomers: a molecule has a nonsuperimposable mirror
image
◦ Chiral molecule – has four different groups
Monosaccharides
Fischer projection: A two-dimensional representation for
showing the configuration of tetrahedral stereocenters.
• Horizontal lines represent bonds projecting forward from the
stereocenter.
• Vertical lines represent bonds projecting to the rear.
• Only the stereocenter is in the plane.
Monosacharides
In 1891, Emil Fischer made the arbitrary assignments of D- and Lto the enantiomers of glyceraldehyde.
•
D-monosaccharide: the –OH is attached to the bottom-most
assymetric center (the carbon that is second from the bottom) is on
the right in a Fischer projection.
Monosacharides
• L-monosaccharide: the -OH is on the left in a Fischer
projection.
D,L-Monosaccharides
• The most common D-tetroses and D-pentoses are:
D,L-Monosaccharides

The three most common D-hexoses are:
Amino Sugars
Amino sugars contain an -NH2 group in place of an -OH group.
• Only three amino sugars are common in nature: D-glucosamine,
D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine
is an acetylated derivative of D-glucosamine.
Cyclic Structure
•
Aldehydes and ketones react with alcohols to form hemiacetals
• Cyclic hemiacetals form readily when the hydroxyl and carbonyl
groups are part of the same molecule and their interaction can
form a five- or six-membered ring.
Epimers

Diastereomers that differ in configuration at only on
asymmetric center
Table 20-1 p532
Table 20-2 p532
Examples

Draw Fisher projections for all 2-ketopentoses. Which are D2-ketopentoses, which are L-2-ketopentoses? Prefer to table
12.2 (your textbook) to write their names
Haworth Projections
•
Figure 12.2 D-Glucose forms these two cyclic hemiacetals.
Haworth Projections
• A five- or six-membered cyclic hemiacetal is represented as a
planar ring, lying roughly perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring then lie either above or
below the plane of the ring.
• The new carbon stereocenter created in forming the cyclic
structure is called the anomeric carbon.
• Stereoisomers that differ in configuration only at the anomeric
carbon are called anomers.
• The anomeric carbon of an aldose is C-1; that of the most
common ketose is C-2.
Haworth Projections
In the terminology of carbohydrate chemistry,
◦ b means that the -OH on the anomeric carbon is on the same side
of the ring as the terminal -CH2OH.
◦ a means that the -OH on the anomeric carbon is on the side of the
ring opposite from the terminal -CH2OH.
◦ A six-membered hemiacetal ring is called a pyranose, and a fivemembered hemiacetal ring is called a furanose because these ring
sizes correspond to the heterocyclic compounds furan and pyran.
Haworth Projections
◦ Aldopentoses also form cyclic hemiacetals.
◦ The most prevalent forms of D-ribose and other pentoses in the
biological world are furanoses.
◦ The prefix “deoxy” means “without oxygen.” at C2
Haworth Projections
D-Fructose (a 2-ketohexose) also forms a five-membered cyclic
hemiacetal.
Examples

Give structure of the cyclic hemiacetal formed by
◦ 4-hydroxybutanal
◦ 5-hydroxypentanal
Chair Conformations
•
For pyranoses, the six-membered ring is more accurately
represented as a strain-free chair conformation.
Chair Conformations
•
In both Haworth projections and chair conformations, the
orientations of groups on carbons 1- 5 of b-D-glucopyranose are up,
down, up, down, and up.
Chair Conformations
Examples

Which OH groups are in the axial position in
β-D-mannopyranose

β-D-idopyranose

Mutarotation

Mutarotation: The change in specific rotation that accompanies the
equilibration of a- and b-anomers in aqueous solution.
◦ Example: When either a-D-glucose or b-D-glucose is dissolved in
water, the specific rotation of the solution gradually changes to an
equilibrium value of +52.7°, which corresponds to 64% beta and
36% alpha forms.
Formation of Glycosides
•
Treatment of a monosaccharide, all of which exist almost exclusively
in cyclic hemiacetal forms, with an alcohol gives an acetal.
Formation of Glycosides
• A cyclic acetal derived from a monosaccharide is called
a glycoside.
• The bond from the anomeric carbon to the -OR group
is called a glycosidic bond.
• Mutarotation is not possible for a glycoside because an
acetal, unlike a hemiacetal, is not in equilibrium with
the open-chain carbonyl-containing compound.
Formation of Glycosides
• Glycosides are stable in water and aqueous base, but
like other acetals, are hydrolyzed in aqueous acid to
an alcohol and a monosaccharide.
• Glycosides are named by listing the alkyl or aryl
group bonded to oxygen followed by the name of the
carbohydrate in which the ending -e is replaced by ide.
Examples

Draw a Haworth projection and a chair conformation for
methyl -D-mannopyranoside. Label the anomeric
carbon and glycosidic bond