Fructose reaction with phenylhydrazine

Simple sugars undergo reaction with phenylhydrazine, PhNHNH2, to yield crystalline derivatives called osazones. The reaction is a bit complex, however, as shown by the fact that glucose and fructose yield the same osazone. 

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. 

Although D-fructose is not an epimer of D-glucose or D-mannose (D-fructose is a ketohex-ose), all three yield the same phenylosazone. (a) Using Fischer projection formulas, write an equation for the reaction of fructose with phenylhydrazine. (b) What information about the stereochenfistry of D-fructose does this experiment yield ... 

Setting Up Heat a large beaker of water to boiling. In each of three test tubes, separately dissolve 0.2-g portions of D-glucose. D-fructose. and sucrose in 4 mL of water. In a fourth test tube, place 4 mL of fhe solution that was used for rotation experiments for the hydrolysis of sucrose (Sec. 23.3). Add 0.5 mL of saturated sodium bisulfite solution to each test tube to suppress oxidation of the phenylhydrazine during the reaction this avoids contaminating the phenylosazone with tarry by-products. 

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