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Osazones are a class of carbohydrate derivatives found in organic chemistry formed when sugars are reacted with excess of phenylhydrazine.
The famous German chemist Emil Fischer developed and used the reaction to identify sugars whose stereochemistry differed by only one chiral carbon.
Osazones formation test involves the reaction of a reducing sugar (free carbonyl group) with excess of phenylhydrazine when kept at boiling temperature. All reducing sugars form osazones. Therefore, sucrose, for example, does not form osazone crystals because it is a non reducing sugar as it has no free carbonyl group.
The reaction involves formation of a pair of phenylhydrazone functionalities, concomitant with the oxidation of the hydroxymethyl group in alpha carbon (carbon atom adjacent to the carbonyl center).
The reaction can be used to identify monosaccharides. It involves two reactions. Firstly glucose with phenylhydrazine gives glucosephenylhydrazone by elimination of a water molecule from the functional group. The next step involves reaction of one equivalent of glucosephenylhydrazone with two equivalents of phenylhydrazine (excess). First phenylhydrazine is involved in oxidizing the alpha carbon to a carbonyl group, and the second phenylhydrazine involves in removal of one water molecule with the new-formed carbonyl group of that oxidized carbon and forming the similar carbon nitrogen bond. The alpha carbon is attacked here because its more reactive than the others.
Osazones are highly coloured and crystalline compounds and can be easily detected. Each sugar has a characteristic crystal form of osazones.