Reducing Sugars
Lab Report
Simeon Wong
25 March 2011
SBI4UP
Mr. John
SBI4UP
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Simeon Wong
24 March 2011
Reducing Sugars Lab Report
Introduction
The aldehyde functional group, found in many organic molecules is a potential reducing agent.
One of the types of organic molecules in which it can be found includes many carbohydrates.
An experiment was performed in which the basic monosaccharaides: glucose, fructose and
galactose, as well as the common disaccharides: maltose, sucrose and lactose were tested for
the presence of a reducing agent—specifically an available aldehyde functional group.
Procedure
Approximately 2 grams of each of the sugars used (listed above) were put into separate test
tubes and fully dissolved in approximately 15mL of distilled water. The test tubes were capped
with a rubber stopper and agitated until the sugars were fully dissolved. Twelve drops of
Benedict’s solution were mixed into the sugar solutions, rendering the solution a light shade of
blue. The test tubes were then placed in a warm water bath of about 50oC – 60oC for
approximately 15 minutes.
Results
The test tubes were then examined for a cloudy orange precipitate:
Sugar
Galactose
•
Glucose
•
Fructose
•
Maltose
•
Lactose
•
Sucrose
•
Result
Has orange
precipitate
Has orange
precipitate
Has orange
precipitate, but
lighter shade
Has orange
precipitate
Has orange
precipitate
Clear light blue,
no apparent
change
Figure 1- Example of a sugar
solution forming an orange
precipitate with Benedict’s
solution
Figure 2 – Example of sugar
solution which does not form an
orange precipitate with
Benedict’s solution (Sucrose)
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Reducing Sugars Lab Report
Discussion
The availability of an aldehyde group is used to classify the sugar as either a reducing or nonreducing sugar. A reducing sugar is potential reducing agent, and thus has an available
aldehyde functional group, unlike non-reducing sugars. The orange precipitate formed is
caused by the reduction of copper(II) ions within Benedict’s solution, also known as Benedict’s
reagent. The testing for reducing sugars, or more generally, the presence of non-aromatic
aldehydes and alpha-hydroxy-ketones using this reagent, is known as Benedict’s test.
Originally, the copper(II) ions (Cu2+) are blue, giving Benedict’s solution its distinct light blue
colour. However, after reduction to copper(I) ions (Cu+), they bond with oxygen, forming orange
copper(I) oxide, and precipitate out of the solution, because of its insolubility. This copper(I)
oxide is the precipitate formed in the presence of a reducing sugar. The generalized formula for
the reaction is:
𝑅𝐶𝑂𝐻 + 2𝐶𝑢2+ + 4𝑂𝐻 − → 𝑅𝐶𝑂𝑂𝐻 + 𝐶𝑢2+ 𝑂 + 2𝐻2 𝑂
All the basic monosaccharaides are
reducing sugars. In their ordinary ringed
form, they do not appear to have an
available aldehyde functional group to
oxidize. However, due to mutarotation
between the various forms of the sugars in
solution, they constantly change between
their various ringed forms and their
straight chain forms. The straight chain
forms of glucose and galactose provide
the critical aldehyde group required for a
Figure 3 – An illustration of the mutarotation between the 𝛼
redox reaction to occur (see Figure 4). The
and 𝛽 diastereomers of glucose. The aldehyde group made
exception is fructose, which becomes a
available in this process is circled in red.
ketohexose in its straight-chain form.
Figure 4 – The open chain form of glucose, with
an aldehyde functional group.
The ketone group in the straight chain form of
fructose cannot be readily oxidized any further,
meaning that it is not a reducing agent. However,
fructose, in solution, undergoes several tautomeric
shifts, essentially becoming an aldohexose. This
transforms the ketone group on carbon #2 into a
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Reducing Sugars Lab Report
aldehyde group on carbon #1, which then becomes available for a redox reaction. This is
indicated by the lighter colour observed in the final product.
In fructose, the tautomeric shift that occurs is
also known as a keto-enol tautomeric shift,
where the ketone group present in fructose
becomes an enol, a highly unstable molecule.
It then shifts back either into its straight-chain
form of fructose, or into the straight-chain
form of glucose (see Figure 5). This then
makes an aldehyde group available for the
redox reaction with the copper(II) ions in
Benedict’s reagent.
Furthermore, Maltose and Lactose, both
Figure 5 – The tautomeric shifts that fructose undergoes
in solution in order to free an aldehyde group for a redox
disaccharides were also found to form an
reaction.
orange precipitate with Benedict’s solution.
Both contain an extremely strong glycoside linkage, which is only broken using enzymes or
strong acids. However, since only one of the anomeric carbons, out of the two present, one
from each component monosaccharide, is involved in the glycoside linkage, the other
anomeric carbon can become a free aldehyde group (see Figure 5). This occurs, again, due to
mutarotation of sugars in solution. The monosaccharide with the free anomeric carbon
undergoes mutarotation, breaking the ether bond to return to its straight chain form, which can
thus undergo a redox
reaction with the
copper(II) in Benedict’s
solution.
However, Sucrose was
not found to be a
reducing sugar,
implying that it does not have any available aldehyde groups. This is due to its glycoside
linkage involving both anomeric carbons (see Figure 7), from both its glucose and fructose
components. Because of the strength of the glycoside linkage, this prevents either
monosaccharide from undergoing mutarotation, and thus exposing a free aldehyde group for a
redox reaction. Since the reduction of copper(II) does not occur, no precipitate is formed when
an aqueous sucrose solution is mixed with Benedict’s solution.
Figure 6 – The mutarotation of maltose from ringed to straight-chain form. The freed
aldehyde group is circled in red.
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The ability of Benedict’s reagent to detect the
presence of reducing sugars is what renders it a
common indicator in the laboratory, as a method
of detecting the presence of basic sugars such
as monosaccharaides and disaccharides. It is
commonly used, for example, in the medical field
as a preliminary test to diagnose kidney failure
and diabetes by testing for the presence of
glucose in urine. A positive result means that
Figure 7 – Structural formula for sucrose. The
further tests should be conducted to investigate glycoside linkage between the two anomeric carbons
is circled in red.
the issue since urine from healthy adults should
not contain sugars of any sort.
Benedict’s reagent is also used within the medical field to test the effectiveness of digestive
enzymes on starch, notably amylase. Amylase breaks down complex starches, which typically
do not have an available aldehyde group, and thus are not reducing sugars, into
monosaccharaides, which is detected by Benedict’s reagent. Thus, the effectiveness of a
sample of human saliva can be measured through the amount of time necessary before
Benedict’s reagent detects the presence of monosaccharaides.
Although Benedict’s reagent cannot technically detect the presence of sucrose because of its
non-reducing nature, in certain circumstances, a sample containing sucrose can be placed in
an acidic bath, thus cleaving the glycoside linkage, splitting the sucrose molecule into its
glucose and fructose components, which can then be detected using Benedict’s reagent.
Therefore, due to the nature of the reducing sugars: glucose, fructose, galactose, maltose and
lactose, their presence can be detected using Benedict’s reagent. Benedict’s reagent forms an
orange precipitate in the presence of a reducing sugar as its copper(II) ion is reduced to
copper(I) and bonds with an oxygen atom to form the insoluble copper(I) oxide. Thus,
Benedict’s solution is an essential indicator in many laboratory tests and experiments.