Chemistry Worksheet
Aldohexoses: Isomers of Glucose
Name: ___________________________
Block: ___________
Introduction:
Simple sugars are a class of chemical compounds that have a significant role in the chemistry of
biology. Glucose, a six-carbon simple sugar with the molecular formula of C6H12O6, is perhaps
the most pervasive of all sugars, as it is a product of photosynthesis, and is also found in
chemical processes in numerous animals. As a six-carbon sugar with an aldehyde on the number
1 carbon (C1), glucose belongs to a class of sugars known as aldohexoses. Aldohexoses, like all
sugars, are carbohydrates, as they contain a carbon backbone with hydrogen and oxygen in the
same ratio as water.
Aldohexoses exists in two basic forms – the acyclic (chain) form, and the cyclic (ring) form;
glucose is used as an example in table 1, below. The acyclic form has 4 chiral carbon atoms,
thus it has 24 (=16) isomeric forms, of which D-glucose and L-glucose are two. Others isomers
include the D and L forms of allose, altrose, galactose, gulose, idose, mannose, and talose. Only
7 of the 16 are found in biological systems.
The cyclic form of glucose is the most prevalent form around pH 7, although interconversion
happens readily, thus both forms are in equilibrium in solutions of moderate pH. Since blood pH
is only slightly more basic than 7, the cyclic form is most commonly associated with
physiological processes. Here is a way to visualize the formation of the cyclic ring from the
acyclic form. Examine the acyclic structure of glucose seen in the first column of table 1, below.
Rotation about the C1–C2 and C3–C4 single bonds yields the alternate view in column 2. This is
like folding the number 1 carbon (the aldehyde carbon) around backwards until it abuts the
hydroxyl of C5. Visualize a covalent bond forming between C1 and the oxygen of the C5
hydroxyl to form a six-member ring comprised of five carbons and one oxygen. The hydrogen
of the C5 hydroxyl migrates to the oxygen on C1 thereby turning the aldehyde into an hydroxyl.
The resulting hydroxyl on the carbon adjacent to the ether oxygen forms a functional group
known as a hemiacetal – a common product resulting from the reaction between and alcohol and
an aldehyde. In the case of aldohexoses, the alcohol (hydroxyl) and the aldehyde are part of the
same carbon chain, thus the ring is formed.
The simplest ring compound formed by five carbons and one oxygen is called pyran, (although
pyran itself is not a sugar.) Since the ring of the cyclic form of glucose resembles pyran, it is
often called glucopyranose, to designate the that ring has formed. Examine the two structures on
the right in table 1 to see the two possible glucopyranose structures that can result from the
formation of the hemiacetal ring. These structures, α-D-glucopyranose and β-D-glucopyranose,
are known as anomers, making C1 the anomeric carbon. By the formation of the ring, the
aldehyde carbon, C1, has become a chiral center, thus increasing the number of isomers of the
cyclic form of glucose to 32, including the two shown on the right, below.
OH
OH
OH
OH
O
O
H
HO
OH
OH
HO
OH
(acyclic form)
H
OH H
D-glucose
(alternate view)
OH
HO
OH
H
OH
D-glucose
OH
OH
OH
α-D-glucopyranose
H
H
OH H
O OH
HO
H
H
OH
β-D-glucopyranose
Table 1
Copyright 2007 by Alan D. Crosby, Newton South High School, Newton, MA 02459
Page 1
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Chemistry Worksheet
Aldohexoses: Isomers of Glucose
Name: ___________________________
Block: ___________
The designation D (dextrorotatory) or L (levorotatory) refers to the direction of rotation of plane
polarized light by the two isomers of glyceraldehyde. Glyceraldehyde, a three-carbon
carbohydrate has only one chiral center, thus it is used as the configurational standard for all
carbohydrates. The configuration standard is necessary to avoid arbitrary designation since the
direction of rotation of plane polarized light cannot be predicted by examination of a structure.
Thus if the stereochemistry on C2 of a particular isomeric sugar (a carbohydrate) has the same
configuration as C2 on D-glyceraldehyde, then that isomer of the sugar is a D isomer, irrespective
of the direction it will rotate plane polarized light. Use of this standard makes it easier to
compare structures.
The designation of an isomer as α or β pertains to the orientation of the hydroxyl on C1 relative
to the hydroxymethyl (−CH2OH) on C5, when a sugar such as glucose cyclizes into the pyranose
ring form. If the C5 hydroxymethyl is trans to the C1 hyroxyl, the sugar is given the α
designation. If, however, the C5 hydroxymethyl is cis to the C1 hyroxyl, the sugar is given the β
designation.
This overview of the structure of hexoses is by no means complete, but it is sufficient to lead you
through an exercise in drawing isomers of glucose.
Assignment:
Read the following assignment carefully. You may want to practice drawing the various isomers
on scratch paper before attempting to draw them on the data tables. It is best to start by using a
sharp pencil that can easily be erased in the event of an error. It is important that your drawings
be accurate and legible. Draw each structure carefully. Only hand-drawn structures that clearly
show the stereochemistry will be accepted.
1. One representative carbon chain drawing of the acyclic form of glucose is shown in the
data table on the following page. Complete the table with the remaining 15 acylic
isomers of glucose. (You do not need to name each structure.)
2. The data table on page 4 shows the nearly closed, but still acyclic form of 4 different
aldohexoses, including D-glucose. (The views shown are known as topical stereo
projections.) Using the examples shown for glucose, draw similar structures for the
remaining aldohexoses clearly showing the structure of the different D and L isomers in
both the α and β configurations. Correct isomers drawn in the incorrect position of the
data table will be deemed incorrect.
Copyright 2007 by Alan D. Crosby, Newton South High School, Newton, MA 02459
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Chemistry Worksheet
Aldohexoses: Isomers of Glucose
OH
Name: ___________________________
Block: ___________
OH
O
HO
OH
D-Glucose
OH
(acyclic)
Copyright 2007 by Alan D. Crosby, Newton South High School, Newton, MA 02459
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Chemistry Worksheet
Aldohexoses: Isomers of Glucose
OH
HO
HO
O
OH
OH
D-glucose
O
HO
OH
OH
HO
α-D-glucopyranose
O
HO
OH
(acyclic)
OH
HO
Name: ___________________________
Block: ___________
OH
OH
OH
α-L-glucopyranose
HO
O
HO
OH
OH
OH
β-D-glucopyranose
HO
O
HO
OH
OH
OH
β-L-glucopyranose
O
HO
HO
OH
OH
D-galactose
HO
(acyclic)
OH
HO
O
OH
OH
D-mannose
HO
(acyclic)
OH
HO
O
OH
OH
D-altrose
(acyclic)
Copyright 2007 by Alan D. Crosby, Newton South High School, Newton, MA 02459
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