Monosaccharides osazone formation

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

In the year 1884, Emil Fi8cheri prepared the first sugar osazones and suggested their use for identification purposes. He later used the fact that D-glucose and n-mannose give the same osazone in his determination of the structure of these two monosaccharides, and he accomplished, through osazone formation, the conversion of aldoses into ketoses. Since then, osazone formation has been used in a multitude of syntheses in the carbo-... 

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

Fischer was the first to synthesize and determine the structures of a number of the sugars used in the experiments in this section. He studied the hydrolysis of glucosides and disaccharides in much the same way you may if you perform the hydrolysis of sucrose (Sec. 23.3). He was also the first to synthesize phenylhydrazine, which you may use to form the same osazones he did in his pioneering work in characterizing sugars (Sec. 23.4). You can share his excitement as you observe the formation of the solid osazones from the combination of different monosaccharides with excess phenylhydrazine. 

We recall that hydrazine and 2,4-dinitrophenylhydrazine react with carbonyl compounds to give hydrazones. However, monosaccharides do not give simple phenylhydrazone derivatives. After the initial formation of a phenylhydrazone, further reaction occurs to give the osazone, which has two molecules of phenylhydrazine incorporated into it. 

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