Reducing sugars are those carbohydrates that can be oxidized by Benedict's reagent. The name reducing sugar comes from the reaction in which the aldehyde group of the carbohydrate is oxidized while Cu(II) ions in the reagent are reduced to Cu(I). A positive test with Benedict's reagent is readily discerned when the solution changes from a blue color to brick red as the reduced copper precipitates in the form of Cu2O.
In aqueous solution, most carbohydrates exist primarily in the closed hemiacetal form (A); however, the hemiacetal is always in equilibrium with the open aldehyde (B). Only a small amount of the open form is required for the sugar to give a positive test. As the open form reacts with Benedict's reagent and is oxidized to the carboxylic acid (C), more of the hemiacetal will open to replenish the supply of aldehyde. This process will continue until all of the sugar has reacted with the reagent.
Although the ketohexose fructose does not appear to be a reducing sugar, it readily isomerizes to glucose under the reaction conditions used for the test, so it also gives a positive test as a reducing sugar.
Unlike the hemiacetal group, the acetal group that joins two sugars together in a disaccharide is stable in water and aqueous base. Cleavage of an acetal only occurs by treatment with hot aqueous acid. Depending upon how the acetal in a disaccharide is formed, the disaccharide may or may not be a reducing sugar. If the hemiacetal groups from both monosaccharides are involved in the formation of the acetal linkage, then the disaccharide cannot be a reducing sugar. Sucrose is an example and is classified as a nonreducing sugar. If the hemiacetal group from only one of the monosaccharides is involved in the acetal linkage, then the other hemiacetal can still react with the reagent. Thus, a disaccharide such as maltose will give a positive test and is considered to be a reducing sugar.
Polysaccharides containing a few sugar units will give positive results, since the concentration of hemiacetal groups is still relatively high. Large polymers of glucose, such as starch, are not reducing sugars, since the concentration of hemiacetal groups is very low. However, starches can be readily identified when tested with iodine solution. In water, iodine is yellow-brown; in the presence of starch, iodine forms a deep blue complex with the starch. This complex is not observed with monosaccharides. The acetal linkages in starch are hydrolyzed in hot aqueous acid, giving glucose. Therefore, starches can be distinguished from other reducing sugars by the distinctive reaction with iodine. A positive Benedict's test can only occur after the starch has been hydrolyzed.
Before coming to lab, look up the structures of glucose, fructose, lactose, maltose, sucrose, and starch; draw them on your laboratory record sheet.
A. Identify models of sugars.
B. Test dilute aqueous solutions of glucose, lactose, fructose, maltose, and sucrose with Benedict's reagent to determine which are reducing sugars C. Determine the effect of hydrolysis on the behavior of sucrose with Benedict's D. Test starch with iodine and with Benedict's both before and after hydrolysis
A. Identifying models of sugars
Numbered models of some of the sugars from the prelab activity will be found in the laboratory. Identify the sugar from the model, draw its structure on your data sheet, and write the complete name of the sugar. These sugars may be in the open or ring form. If the sugar is in the ring form, be sure to include the ( or ( orientation and the pyranose or furanose designation.
To convert from a ring to a Fischer projection, do the following: draw the skeleton of the Fischer projection,...