Osazones are the phenyl hydrazones of sugars. Unlike the normal hydrazones of aldehydes or ketones, however, osa zones cannot be described as genuine derivatives of the original sugar, because in the course of the reaction a redox reaction changes the structure of the sugar at the
Emil Fischer discovered phenylhydrazine in 1874 while working on the diazotization of aniline; it was later used to characterize sugars.
Aldoses form dual hydrazones with three equivalents of phenylhydrazine at the
-Atom, so that a chiral center disappears and the identification of the
-Epimere becomes possible. The formation of the osazone is shown as follows:
In this process, a monosaccharide(1, abbreviated by R ), for example glucose, is reacted with phenylhydrazine with elimination of water to form a phenylhydrazone(2). If further phenylhydrazine is reacted in the reaction, this dehydrates the alcohol group of the sugar adjacent to the carbonyl group. This produces aniline and ammonia. An osonhydrazone is formed as an intermediate(3). Finally, a third molecule of phenylhydrazine reacts with the carbonyl group to form an ozone(4a). This is stabilized by an intramolecular hydrogen bond(4b).
It is not known why an ozone cannot be reacted with another molecule of phenylhydrazine. In the case of glucose, the hydroxyl group on the C3 atom would come into question.
Osazones are yellow, easily crystallizing substances. After exposure to strong acids or by boiling in benzaldehyde, an ozone can be formed from an ozone. Osones are α-ketoaldehydes.
Osazonation is mainly applied for the separation and characterization or identification of sugars above the melting point. The
-Epimers glucose and mannose (both aldoses) and fructose (a ketose) yield the same ozone upon conversion.
- Wittko Francke and Wolfgang Walter: Lehrbuch der Organischen Chemie. S. Hirzel Verlag Stuttgart; 24th revised edition 2004, ISBN 3-7776-1221-9; p. 460.
L. Mester, H. El Khadem, D. Horton: Structure of saccharide osazones. In: Journal of the Chemical Society C: Organic. No. 18, 1970, pp. 2567-2569, doi:10.1039/J39700002567.