Carbohydrate Survey:
The Carbohydrate Family: Polyhydroxylated Aldehydes and Ketones
The most common representation of carbohydrates is the use of the Fischer Projection. Multiple
chiral centers are represented as “stacked” Fischer projections. The carbonyl carbon must appear at the
top of the Fischer projection, ending with the –CH2OH unit on the bottom.
Some examples of common carbohydrates would include:
O
H
H
OH
HO
H
H
O
O
H
O
H
HO
H
HO
H
H
OH
OH
HO
H
HO
H
HO
H
H
OH
OH
H
OH
H
H
OH
HO
H
H
OH
HO
H
OH
HO
H
H
OH
H
CH2 OH
D-glucose
CH2 OH
L-glucose
H
CH2 OH
CH2 OH
D-mannose
D-galactose
D-fructose
O
H
H
H
OH
H
OH
H
OH
H
OH
H
OH
D-ribose
O
CH2 OH
O
CH2 OH
CH2 OH
CH2
CH2 OH
2-deoxy-D-ribose
Carbohydrates possess chiral centers. Recall that the total number of stereoisomers for
n
molecules possessing chiral centers is equal to 2 , where n is the number of chiral centers. Based on
the configuration of the bottom-most chiral center (when drawn in a Fisher projection), the enantiomer
is considered to be either a D-series carbohydrate or an L-series carbohydrate. Shown on the next page
are the enantiomers of glyceraldehyde (which has only one chiral center and therefore only two
possible stereoisomers). Note that the D-series has the hydroxyl group on the right side of the chiral
center and the L-series has the hydroxyl group on the left side of the chiral center.
O
H
H
OH
CH2 OH
D-glyceraldehyde
O
HO
H
H
CH2 OH
L-glyceraldehyde
Any carbohydrate is a member of the D-series if the chiral center farthest from the carbonyl carbon has
the hydroxyl on the right side, and a member of the L-series if the chiral center farthest from the
carbonyl carbon has the hydroxyl on the left side.
Thus the symbolism of D- and L- signifies enantiomers in the carbohydrate families (see D-glucose
and L-glucose in the figure on the first page). This should not be confused with the concept of d- and
l- used in optical activity to indicate sign of rotation. There is no relationship between D- and L- and
the sign of rotation. You should be able to identify a D-series carbohydrate or an L-series
carbohydrate. Given a D-series carbohydrate, you should be able to draw an L-series
carbohydrate. You should understand that the relationship between D- and L- is enantiomeric.
Note that there are many different relationships between carbohydrates. Enantiomers exist when
looking at D- versus L-series of carbohydrates. Diastereomers occur any time some of the chiral
centers of the carbohydrate are mirror images but the rest are the same. A specific example of
diastereomers, where only one center differs, is the case of epimers. Glucose and galactose are
examples of epimers. Constitutional (structural) isomers can also exist, as in the case of glucose and
fructose, also shown below.
H
O
O
H
O
H
HO
H
H
OH
H
H
HO
OH
HO
H
H
H
HO
H
H
OH
OH
H
OH
H
OH
HO
HO
H
H
OH
H
CH2OH
D-glucose
enantiomers
OH
HO
H
L-glucose
H
H
HO
CH2OH
O
CH2OH
CH2OH
D-galactose
D-mannose
epimers
diastereomers
O
H
O
H
OH
H
HO
CH2OH
HO
H
H
OH
H
OH
H
OH
H
OH
CH2OH
D-fructose
CH2OH
D-glucose
Constitutional Isomers
Carbohydrates come in many forms, depending on the number of sugars involved.
Monosaccharides are those carbohydrates referred to as simple sugars. They are the simplest form of
carbohydrate and cannot be hydrolyzed to smaller carbohydrate molecules. The examples shown so
far are all monosaccharides. Carbohydrates, generally speaking, are easily recognized by their suffix,
“-ose”. Monosaccharides are classified based on the kind of carbonyl involved and the number of
carbons in the carbohydrate. Monosaccharides that have an aldehyde group in their structure are called
aldoses and those with ketone groups are called ketoses. Glucose is an aldose. Fructose is a ketose.
The number of carbon atoms can range from three (as in glyceraldehyde) to six or seven (as in
glucose). The prefixes tri-, tetr-, pent-, hex- and hept- indicate the number of carbons. When you
combine these terms, you can convey some basic information about the carbohydrate. For example,
ribose is an aldopentose (five-carbon aldehyde). Fructose is a ketohexose (six-carbon ketone). What
these classifications do not tell you are the configurations of the chiral centers. You should be able to
classify a carbohydrate based on carbonyl group and number of carbon atoms.
CH2 OH
O
H
H
OH
HO
H
OH
H
OH
H
OH
H
OH
O
CH2 OH
H
CH2 OH
D-ribose
an aldopentose
D-fructose
a ketohexose
Other ways of representing monosaccharides, besides the Fischer projection, would include
chair structures (generally considered difficult by most introductory students) and the Haworth
structure. The Haworth structure is considered a more convenient representation of cyclic structures of
carbohydrates. Both the chair structure and the Haworth structure of glucose are shown below, as well
as a Haworth structure for fructose.
OH
CH2OH
OH
HO
HO
O
H
H
OH
H
H
OH
OH
OH
OH
OH
Chair form - glucose
O H
Haworth structure - glucose
OH
O
H
HO
H
CH2OH
OH
H
Haworth structure - fructose
Notice how the Haworth does not adequately demonstrate stereochemistry like the chair structure, yet
the flattened cyclohexane ring of a Haworth structure still depicts “up” substituents as “up” and
“down” substituents as “down”. The concept of axial versus equatorial is not utilized.
Hemiacetal Formation of Monosaccharides:
As solids, or when dissolved in water, monosaccharides contain virtually no free carbonyl
group (aldehyde or ketone). Instead, they exist as cyclic hemiacetals, owing to the internal addition of
one of the hydroxyl groups across the carbonyl group to produce stable five or six membered rings.
Two isomeric hemiacetals are formed because of the two possible approaches to the faces of the
carbonyl group, which is planar. These isomers are referred to as the α- and β -anomers. In aqueous
solutions, the isomeric hemiacetals are in equilibrium with one another and with a small amount of the
carbonyl-containing compound. The carbon bearing the new hydroxyl group of the hemiacetal is
called the anomeric carbon atom. The equilibrium that exists between the anomers due to the opening
and closing of the hemiacetal causes what is called mutarotation of the anomeric center. You should
understand what the concept of mutarotation is.
OH
H H
OH
O H
H H OH O
H
OH
H
OH
OH
H
OH
OH
H H
OH
H
H
!-anomer
H
H
OH
OH
OH
O OH
H
OH
OH
"-anomer
D-glucose
Chair structures can be used to depict hemiacetals as well. By viewing the chair structures, one can tell
why the β-anomer is the more stable of the two. In glucose, all the substituents attached to the ring are
equatorial substituents!
OH
OH
O
HO
HO
HO
HO
OH
O
OH
OH
OH
!-anomer
"-anomer
When carbohydrates cyclize to form five membered rings, they are called furanose rings while six
membered rings are called pyranose rings. Upon cyclization, glucose becomes glucopyranose.
Including the nomenclature for the anomeric center, as well as the enantiomer, D- versus L- series, one
obtains a more complete name. You should recognize the terminology of furanose and pyranose
as applying to five and six membered hemiacetals.
OH
HO
HO
O
OH
OH
!-D-glucopyranose
You should be able to identify the anomeric center in a chair structure or a Haworth structure
and distinguish between α and β anomers. Given the open form you should recognize that the
carbonyl carbon is that which becomes the anomeric carbon in the closed form (the hemiacetal
carbon). You should be able to predict which of two forms is more stable, based on your
evaluation of axial versus equatorial substituents.
When the hemiacetal reacts with an equivalent of alcohol, an acetal forms and mutarotation can
no longer occur. The opening and closing of the ring is a reaction of hemiacetals. Acetals are stable in
aqueous solutions, both neutral and basic. Upon reaction with an alcohol to form an acetal, the
nomenclature of the carbohydrate is changed from using the suffix “-ose” to “-oside”. The ether
group (-OR) is also named, before that of the rest of the carbohydrate, similar to the nomenclature for
esters.
OH
OH
O
HO
HO
CH3OH
OH
OH
O
HO
HO
H+
OH
OCH3
methyl !-D-glucopyranoside
!-D-glucopyranose
Monosaccharides can bond together to form dimers called disaccharides. They may be the
same carbohydrate or different carbohydrates. The bonding occurs between the hemiacetal center of
one carbohydrate and a non-anomeric hydroxyl of another carbohydrate. This forms an acetal group
between the two carbohydrates. An example of a disaccharide is lactose, which is a dimer formed
from a glucose unit and a galactose unit.
OH
OH
OH
O
HO
OH
O
HO
O
OH
OH
The term “oligosaccharide” generally refers to carbohydrates that are composed of two to
eight units of monosaccharides. “Polysaccharides” are formed by more than eight monosaccharides.
A common example of a polysaccharide is starch (found in flour and cornstarch). Cellulose is also a
polysaccharide. Both of these are formed exclusively from D-glucose. A structural difference is that
cellulose contains b-linkages and starch contains α-linkages. Other differences would include
molecular weight, branching and water-solubility. Partial structures are shown on the next page.
OH
O
HO
O
OH
OH
OH
O
HO
O
O
O
O
HO
OH
OH
OH
Cellulose
Starch
You should be able to recognize α- and β - linkages and acetal linkages.
O
HO
O
OH
O