osazone

An osazone is a crystalline derivative of a monosaccharide formed by reacting the sugar with phenylhydrazine under slightly acidic conditions. The reaction involves the condensation of phenylhydrazine with the aldehyde or ketone group of the sugar, followed by further reactions with the other hydroxyl groups. Osazones are characterized by their sharp melting points, which can be used to help identify different sugars.

The reaction was first developed by Emil Fischer in the late 19th century. He discovered that reducing sugars, which have a free aldehyde or ketone group, readily react with phenylhydrazine to form osazones. Non-reducing sugars, like sucrose, do not form osazones under these conditions. The structure of an osazone is formed when two molecules of phenylhydrazine react with a sugar molecule. One phenylhydrazine molecule reacts with the carbonyl group, and the second reacts with the adjacent carbon's hydroxyl group after it has been oxidized to a carbonyl.

The key feature of osazone formation is that it specifically reacts with the first two carbon atoms of a sugar. This means that different aldoses and ketoses differing only in the configuration of carbons beyond the second carbon will form the same osazone. For example, glucose, fructose, and mannose all yield the same osazone. This property makes osazones useful for determining the configuration of the first two carbons of a sugar but not for distinguishing between sugars with different configurations further down the chain. The melting point of the osazone can be used as a diagnostic tool.