CARBOHYDRATES 41 



water. Hence, they do not serve for the separation or identifica- 

 tion of the individual sugars. But if the solution in which they 

 are formed contains an excess of phenyl hydrazine and is heated 

 to the temperature of boiling water for some time, the alcoholic 

 group next to the aldehyde group (the terminal alcohol group in 

 ketoses) is first oxidized to an aldehyde and then a second molecule 

 of phenyl hydrazine is added on, as illustrated above, forming a 

 di-addition-product, known as an " osazone." The osazones are 

 generally more or less soluble in hot water, but on cooling they 

 crystallize out in yellow crystalline masses of definite melting 

 point and characteristic forms. All sugars which have active 

 aldehyde groups in the molecule form osazones. These afford 

 excellent means of identification of unknown sugars, or of dis- 

 tinguishing between sugars of different origin and type. 



Glucose, mannose, and fructose all form identical osazones. 

 This is because the structure of these three sugars is identical 

 except for the arrangement within the two groups at the aldehyde 

 end of the molecule (see formulas on page 44). Since it is to 

 these two groups that the phenyl hydrazine residue attaches itself, 

 it follows that the resulting osazones must be identical in structure 

 and properties. All other reducing sugars yield osazones of differ- 

 ent physical properties. 



When an osazone is decomposed by boiling with strong acids, 

 the phenyl hydrazine groups break off, leaving a compound con- 

 taining both an aldehyde and a ketone group. Such compounds 

 are known as " osones." The osones from glucose, mannose, and 

 fructose are identical. By carefully controlled reduction, either one 

 of the C = groups of the osone may be changed to an alcoholic 

 group, producing thereby one of the original sugars again. Hence, 

 it is possible to start with one of these sugars, convert it into the 

 osone and then reduce this to another sugar, thereby accomplishing 

 the transformation of one sugar into another isomeric sugar. 



Formation of Glucosides. By treatment with a considerable 

 variety of different types of compounds, under proper conditions, 

 it is possible to replace one of the hydrogen atoms of the terminal 

 alcoholic group of the hexose sugars with the characteristic group 

 of the other substance, forming compounds known, respectively, 

 as glucosides, fructosides, galactosides, etc. The structural 

 relation of methyl glucoside to glucose, for example, may be illus- 

 trated as follows : 



