Carbohydrate Representations
Based on a Fall 2005 Chemistry 14D honors project
Some Useful Vocabulary:
Aldose: A polyhydroxy aldehyde, i.e., a carbohydrate containing an aldehyde functional
group.
Ketose: A polyhydroxy ketone, i.e., a carbohydrate containing a ketone functional
group.
Furanose: A five-member closed chain form of a monosaccharide.
Pyranose: A six-member cyclic form of a monosaccharide.
Fischer Projection: A way of representing an acyclic (open chain) carbohydrate
structure. Vertical lines point away from the viewer and horizontal lines point toward the
viewer.
Haworth Projection: A way of representing a cyclic (closed chain) carbohydrate.
Substituents can either point up or down on this ring.
Chair Conformation: The most stable conformation of cyclohexane that resembles a
chair.
Monosaccharide: A single sugar. A carbohydrate that cannot be broken down into a
simpler carbohydrate.
Anomeric carbon: The carbon in a cyclic sugar that is the carbonyl carbon in the openchain (acyclic) form.
Carbohydrates are the most abundant class of bioorganic compounds in the biological world.
They constitute most of Earths biomass, from tiny structural components of cells, to food we eat
for metabolic energy. To better understand the role carbohydrates play in the biological world, a
basic chemical understanding of how carbohydrates are formed and represented in their simplest
form is essential.
In organic chemistry, monosaccharides, the simplest carbohydrates are represented in three
ways: the Fischer projection, Haworth projection, and the chair conformation of D-glucose
(Figure 1). By the time you are finished reading this tutorial, you will have learned how to
represent monosaccharides in these three ways.
Figure 1
Monosaccharides, also known as saccharides, are carbohydrates that cannot be broken down into
simpler carbohydrates. They can be polyhydroxy aldehydes or polyhydroxy ketones because
they have either an aldehyde or a ketone group, along with OH substituted carbons in a chain.
Polyhydroxy aldehydes are called aldoses. Polyhydroxy ketones are called ketoses. The suffix
ose means sugar, while the prefixes ald and ket are used for aldehyde and ketone,
respectively.
Classification of monosaccharides is based on the number of carbons they contain (Table 1).
Table 1
Number of
carbons in
monosaccharide
3
Name
Aldose Form
Triose
aldotriose
4
Ketose Form
ketotriose
Tetrose
aldotetrose
ketotetrose
Tutorial: Carbohydrate Representations
Pentose
aldopentose
ketopentose
6
Hexose
aldohexose
ketohexose
Monosaccharides can exist in an open chain (acyclic) form, or in closed chain (cyclic) form. The
open chain form of monosaccharides is illustrated with Fischer projections. A Haworth
projection can be used to represent the cyclic form of monosaccharides. The five-member closed
chain form of a monosaccharide is known as a furanose, while the six-member cyclic form of a
monosaccharide is known as a pyranose. Often six-member monosaccharide rings can also be
represented in chair conformation.
Figure 2 shows pyranose and furanose rings. Substituents have been omitted so that the ring
structure can be emphasized.
Figure 2
Imagine we are asked to show the Fischer, Haworth, and chair conformation of _ and _
D-glucose. How do we go about doing this? There are many things to consider; however, there
are also some rules that will simplify the process. Lets walk through how this problem can be
approached.
A Fischer projection shows the skeleton of the acyclic monosaccharide (Figure 3). Glucose is an
aldohexose. This means that the top of the Fischer projection of glucose contains an aldehyde
group and that there are six carbons in the polyhydroxy chain. In Figure 3, the structure on the
right shows wedges and dashes to indicate how the sugar looks in three dimensions.
Tutorial: Carbohydrate Representations
Figure 3
D and L Notation:
We need to make sure that the glucose is a D-glucose. We do this by looking at the
monosaccharide with the ketone or aldehyde group on top; if the OH group on the bottom-most
asymmetric carbon is on the left side, the notation is L; if the OH group on the bottom-most
asymmetric carbon is on the right side, the notation is D (Figure 3).
Figure 4
H
H
HO
This oxygen becomes protonated
and singly bonded to the carbon.
OH
H
OH
OH
This oxygen loses its hydrogen and
attacks the carbon of the carbonyl (C=O)
CH2OH
Next, we rotate the C-5 carbons substituents once over to put OH on the vertical plane. This
shift shows us what direction the CH2OH group with point in the Haworth projection.
A substituent on the right side of the vertical line in a Fischer projection will be pointing down in
a Haworth projection. Conversely, a substituent on the left side in a Fischer projection will be
pointing up in a Haworth projection. (Figure 5)
Figure 5
Tutorial: Carbohydrate Representations
Therefore, the CH2OH group will be pointing up in the Haworth projection. Moreover, theOH
group on the C-2 will be pointing down and its H substituent will be pointing up. And, the
OH group on the C-3 will be pointing up while its H substituent points down, and so on.
In the next step, the carbonyl oxygen becomes protonated and singly bonded to C-1. The OH
group on C-5 becomes deprotonated and forms an O-C bond with C-1, resulting in a sixmembered ring. At this point, it is important to recognize the difference between alpha () and
beta () glucose (Figure 6).
The or configuration is determined by looking at the anomeric carbon which is the carbon
that is between the two oxygen atoms of the hemiacetal or acetal. In a cyclic sugar, the anomeric
carbon is the carbonyl carbon in the acyclic form. (C-1 in our case)
In an monosaccharide, the OH group attached to the anomeric carbon is on the right side of
the Fischer projection, points down in the Haworth projection, and is axial in the chair
conformation.
In a monosaccharide, the OH group attached to the anomeric carbon is on the left side of the
Fischer projection, points up in the Haworth projection, and is equatorial in the chair
conformation.
The table below summarizes the translation between Fischer projections, Haworth projections,
and chair conformation. The translations for the chair conformation are used to determine
and configurations and only concern the anomeric carbons substituent. The Fischer and
Haworth projection translations apply to all substituents on the carbon chain.
Figure 6
Table 2
Fischer Projection
Haworth Projection
Configuration
Chair Conformation
Right
Down
Axial
Left
Up
Equatorial
Tutorial: Carbohydrate Representations
In -D-glucose the anomeric carbons OH group is on the left. In the Haworth projection this
alcohol group points up. The other substituents point up in the Haworth projection if they are on
the left side in the Fischer projection, and point down if they are on the right side in the Fischer
projection. (Figure 7)
Figure 7
In -D-glucose the anomeric carbons OH group is on the right. In the Haworth projection of
-D-glucose illustrated below the OH group points down. Once again, the rest of the
substituents also follow the translation rule from Fischer to Haworth. (Figure 7)
Take a moment to compare the Fischer and Haworth projections and notice how one form is
translated to the other form.
When going from the Haworth projection to the chair conformation, the anomeric carbons
substituent that points down in the Haworth projection is going to be axial, and the substituent
that points up in the Haworth projection is going to be equatorial. An axial OH on the anomeric
carbon makes the sugar an sugar, while an equatorial OH on the anomeric carbon makes the
monosaccharide a sugar. Besides the substituents on the anomeric carbon, everything else is
drawn relative to the Haworth projection. In other words, all the other substituents are drawn
pointing up if they were pointing up in the Haworth projection, and pointing down if they were
pointing down in the Haworth projection. (Figure 8)
It helps to number the carbons in the monosaccharide in the Haworth projection and chair
conformation to prevent any careless mistakes.
Tutorial: Carbohydrate Representations
Figure 8
There is a shortfall in the method we have been using to determine the difference between an
and sugar in the chair conformation. (This is also the method used in Bruice.) For example, it
is true that in the chair conformational isomer that we drew that the axial OH group of the
anomeric carbon was while the equatorial OH group of the anomeric carbon was . But a
ring flip would make this fact untrue (Figure 9). An -D-glucose would still be an -D-glucose
after a ring flip, but the OH group attached to the anomeric carbon would no longer be axial!
(To learn more about ring flips, refer to your textbook and class notes)
Figure 9
Tutorial: Carbohydrate Representations
The method we have been using works, but only when the chair conformational isomer is drawn
so that the anomeric carbon is at the bottom right corner. However, a less restricted method that
is always true exists. As long as the primary alcohol, CH2OH, and the anomeric OH group are
on opposite sides of the ring, as in the trans isomer, it is . Moreover, if the primary alcohol and
the anomeric OH group are on the same side of the ring as in the cis isomer, it is . For
example, in the illustration below, notice that when CH2OH points up, the anomeric OH group
points down (opposite side of the pyranose). This is the trans isomer, and thus an -pyranose.
(Figure 10)
Figure 10
In -glucose, or any monosaccharide, the pyranose would be the cis isomer. Thus, when
CH2OH points above the plane of the ring, the anomeric OH group will point up as well.
Conversely, when the CH2OH points down, the anomeric OH group will also point down.
(Figure 11)
Figure 11
To get a better understanding of this concept, build models with your molecular model building
kit.
The cyclic monosaccharides in this example are pyranoses because they have a six-membered
ring. -D-glucose can also be called -D-glucopyranose. You may be asked specifically to
form another type of cyclic sugar, such as a furanose. In that case the OH that becomes
deprotonated and attacks the carbonyl carbon may be different than the one from the last
stereocenter. (Figure 12)
Tutorial: Carbohydrate Representations
Figure 12
As long as all the rules are applied consistently, the transition from Fischer projection to
Haworth projection to chair conformation will be a simple one. The chair conformation can only
be done with pyranoses. The following are some practice problems to help you strengthen this
skill.
Practice Problems Solutions start on page 10.
1. Make a table with the Fischer, Haworth, and chair conformation (when applicable, i.e. when
a six-membered ring exists) for each of the following sugars: (a) b-D-galactose, (b) a-Dribose, and (c) b-D-fructose.
1. Draw the Haworth projection of as a -pyranose anomer of the sugar given below:
1. Draw the chair conformation of -D-glucopyranose.
1. The following questions apply to the sugar A.
(a) Is this sugar a ketose or an aldose?
(b) Draw an arrow pointing to the anomeric carbon.
(c) Box the OH group attached to C-4.
(d) Is this the or anomer?
Tutorial: Carbohydrate Representations
Sugar A
Practice Problem Solutions
1.
Fischer projection
Haworth projection
Chair conformation
(a)
(b)
HO
Furanoses do not have chair
conformations
OH
HO
HOCH2
OH
OH
(c)
HO
CH2OH
Furanoses do not have chair
conformations
HO
Hydrogens can be omitted from Haworth projections to avoid cluttering, but do not draw a
stick instead of a hydrogen atom. As in all bond-line or stick structures, a stick that is only
attached to one other atom represents a methyl group.
HOCH2
OH
HOCH2
OH
HO
HO
CH2OH
HO
Correct Haworth projection
CH2OH
HO
Too many methyl groups!
CH2OH
1.
HO
O OH
CH2OH
OH
OH
1.
1. (a) Ketose.
Tutorial: Carbohydrate Representations
10
(b)-(d)
Works Cited
1. Lecture Notes and practice problems from Professor William Nguyen. (Summer 2004, 14C)
2. Bruice, Organic Chemistry, Fourth Edition.
3. 14C Thinkbook for Fall 2004, Professor Hardinger.
4. I want to thank Professor Hardinger and Mark Smuckler for helping me edit this tutorial.
Tutorial: Carbohydrate Representations
11
