Osazone Formation Mechanism

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Osazone is the derivative of carbohydrates that are formed by the reaction of carbohydrates with phenylhydrazine. As aldehyde, aldoses react with phenylhydrazine to form phenylhydrazones. In 1875, Emil Fischer found that reducing benzene diazonium chloride by sulfur dioxide yields phenylhydrazine.

Fischer reported that the phenylhydrazine he had discovered could be used as a powerful tool in the study of carbohydrates. If an excess of phenylhydrazine is used, the reaction proceeds further to yield products known as osazones, Which contain two phenylhydrazine residues per molecule a third molecule of the reagent is turned into aniline and ammonia.

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Osazone formation mechanism

One of the difficulties of working with carbohydrates is their tendency to form syrups. Treatment with phenylhydrazine converts carbohydrates into solid osazones, which are readily isolated and purified, and can be identified by characteristic crystalline forms.

Osazone
osazone mechanism
osazone formation mechanism

Fischer found osazone formation to be useful not only in identifying carbohydrates but also this was much more important in determining their configurations. For example; the two diastereoisomeric aldohexoses (+)-glucose and (+) mannose yields the same osazone. Osazone formation destroys the configuration of C-2 of aldose but does not affect the configuration of the rest of the molecule. It, therefore, follows that (+)-glucose and (+)-mannose differs only in the configuration of C-3, C-4, and C-5. We can see that whatever the configuration of either of these compounds is established, the configuration of the other is immediately known through this osazone relationship.

Osazone
Osazone mechanism
Osazone formation mechanism

A pair of diastereomeric aldoses that differ only in the configuration of C-2 are called epimers.

Mechanism of osazone formation

The proposed mechanism of osazone formation is given below:

mechanism of osazone formation
osazone of glucose

osazone

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