Reducing Sugars

Another characteristic of monosaccharides is that they can act as mild reducing agents. This is because the aldehydo group that is present can be oxidized to form a carboxylic acid group, or in the presence of a base, a carboxylate ion group.

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Fructose can also act as a reducing sugar, even though it has a ketone group instead of an aldehyde group. Under basic conditions, the fructose molecules can, essentially, have the location of the carbonyl bond switched to convert them into a glucose molecule. This occurs in a number of steps involving removing hydrogens from the #1-C and its oxygen and moving them to the #2-C and its oxygen.

In one sense, monosaccharides that are in the ring form are not reducing sugars because they don't have the aldehydo group that can be oxidized. However, because they're in equilibrium with the open form, any monosaccharide molecule that's in a ring form will, within a fraction of a second, be in the open form and, thus, be able to react with the oxidizing agent and reduce it.

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Lab Work

Your experiment for this week's lesson is to test a variety of carbohydrates to see whether or not they are reducing sugars. In a sense, that test can be used to determine the presence of a monosaccharide because a monosaccharide is a reducing sugar and, therefore, presumably, if you have a reducing sugar, you have a monosaccharide. But for reasons that you will see soon, that presumption is not entirely correct. If you wish, you could do the experiment at this time, although I would recommend that you wait until after we've talked about disaccharides and polysaccharides.

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E-mail instructor: Sue Eggling

Clackamas Community College
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