MONOSACCHARIDES 69 



and the resulting substance is termed an osazone. Both 

 glucose and levulose yield the same osazone, 



CH 2 OH(CHOH) 3 C CH : NNHC 6 H 5 

 II 

 N.NHC 6 H 6 



which is called glucosazone.* 



The osazones being, for the most part, insoluble in water, 

 serve as a valuable means of isolating a sugar from a dilute 

 solution ; their identity can then be readily established by 

 means of their crystalline form, melting point, solubility and 

 optical activity. 



A second very important reagent for sugars, depending for 

 its utility, like phenylhydrazine, on the presence of the alde- 

 hyde or ketone group, is Fehling's solution. This substance, 

 which is an alkaline solution of cupric oxide, acts upon a warm 

 solution of a sugar as an oxidizing agent and, in parting with 

 its oxygen, is converted into cuprous oxide ; this reduction of 

 cupric oxide to cuprous oxide is accompanied by a visible 

 change from a deep blue solution to a colourless one, with 

 simultaneous deposition of the cuprous oxide as a reddish- 

 brown precipitate. Any easily oxidized substance will thus 

 reduce Fehling's solution, becoming itself oxidized ; and, inas- 

 much as the aldehyde and ketone groups are readily oxid T 

 ized, all sugars containing these groups will bring about this 

 change. The reducing power of all sugars, however, is not 

 the same, but it has been determined in most cases against a 

 properly standardized Fehling's solution, and hence can be em- 

 ployed as a means of identifying or estimating the strength of 

 a sugar solution. In view of what has been said, it will of 

 course be seen that the experimental determination of the 

 reducing power of a sugar is valueless if an unknown amount 

 of any other easily oxidized substance is present in solution. 



MONOSACCHARIDES. 

 A. PENTOSES. 



The pentoses, which are sugars containing five carbon 

 atoms, have the general formula C 5 H 10 O 5 ; they are not as a 

 rule found free in plants, but may occur in a combined state. 



* For details of the preparation of this substance, see under reactions for 

 glucose, p. 75. 



