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The times required for the formation of the osazones can be a valuable aid in distinguishing among various sugars. The following figures are the times required for the osazone to precipitate from the hot solution: fructose, 2 min; glucose, 4-5 min; xylose, 7 min; arabinose, 10 min; galactose, 15-19 min; raffinose, 60 min; lactose, osazone soluble in hot water; maltose, osazone soluble in hot water; mannose, 0.5 min (hydrazone); sucrose, 30 min (owing to hydrolysis and formation of glucosazone).
Osazone formation involves hydrazone formation at C-1 of an aldose (or C-2 of a ketose) and oxidation of C-2 (or C-1) of an alcohol group to a ketone (or an aldehyde). The new carbonyl group is also converted to a hydrazone. It has been suggested that the reaction stops here (rather than further oxidation at C-3, etc.) because of hydrogen-bonding stabilization of the osazone.
For identification purposes the carbonyl and adjacent alcohol functions will form phenylhydrazine derivatives known as osazones, which give characteristic melting points and exhibit definite crystalline structure. It should be noted that glucose , fructose and mannose yield the same osazone since the difference in structure and configuration about carbon atoms 1 and 2 are abolished.
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