Solved: Simple sugars undergo reaction with phenylhydrazine, PhNHNH2, to yield

Chapter 25, Problem 25.67

(choose chapter or problem)

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. 3 equiv PhNHNH2 3 equiv PhNHNH2 + NH3 + PhNH2 + 2 H2O D-Fructose H OH CH2OH H OH HO H C O CH2OH D-Glucose H OH CH2OH H OH HO H H OH CHO H OH CH2OH H OH HO H C N NHPh N NHPh C H (a) Draw the structure of a third sugar that yields the same osazone as glucose and fructose. (b) Using glucose as the example, the first step in osazone formation is reaction of the sugar with phenylhydrazine to yield an imine called a phenylhydrazone. Draw the structure of the product. (c) The second and third steps in osazone formation are tautomerization of the phenylhydrazone to give an enol, followed by elimination of aniline to give a keto imine. Draw the structures of both the enol tautomer and the keto imine. (d) The final step is reaction of the keto imine with 2 equivalents of phenylhydrazine to yield the osazone plus ammonia. Propose a mechanism for this step.