Carbohydrates Their Characterization and Identification

Determining the complete structure of an unknown monosaccharide was a formidable challenge to early organic chemists (see the Historical Highlight at the end of this chapter). Many carbohydrates, particularly when impure, have a tendency to form syrups rather than crystallize from solution, and this sometimes makes it difficult to obtain pure compounds for characterization and identification. The number and nature of functional groups and the number of stereocenters present in carbohydrates further exacerbates the problem of assigning their structures. 

The structural elucidation of polysaccharides represents an even greater challenge. The individual monosaccharide subunits that constitute the unknown polysaccharide must first be identified. Then the ring size and position in the polysaccharide sequence must be elucidated for each monosaccharide. Finally, the nature of the gly-cosidic linkages that form the polysaccharide backbone must be defined. Toward this end, it is necessary to establish which hydroxyl group on one monosaccharide is involved in the formation of the acetal or ketal that forms the glycosidic bond to the adjoining monosaccharide the stereochemistry at this anomeric center must also be determined. 

A number of useful qualitative chemical tests have been devised to obtain information for unknown carbohydrates. Some of these tests are used to classify such molecules according to their structural type. In the experiments that follow, you will use such tests to identify certain structural features that are found in mono-and polysaccharides. Data derived from these experiments provide information that may be used to prove the structure of an unknown carbohydrate. 

Tollens s test is designed to distinguish between aldehydes and ketones. A complete discussion of this test together with an experimental procedure is included with the classification tests for aldehydes in Section 25.7C. 

This test is based on the fact that cupric ion will oxidize aliphatic aldehydes, including a-hydroxyaldehydes, but not aromatic aldehydes. The reagent used in this test is a solution of cupric sulfate, sodium citrate, and sodium carbonate. The citrate ion forms a complex with the Cu(II) ion so that Cu(OH)2 does not precipitate from the basic solution. A positive test for the presence of the aliphatic aldehyde group in an aldose is evidenced by the formation of a red precipitate of cuprous oxide, CU2O (Eq. 23.5). A yellow precipitate is occasionally observed as a positive test. This yellow solid has not been characterized, but its formation seems to depend on the amount of oxidizing agent present. 

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