54 THE CARBOHYDRATES 



a compound the property of optical activity, by which is meant 

 the power of the substance to rotate to the right or to the 

 left the plane of a beam of circularly polarized light passing 

 through it. This phenomenon may be made use of for the 

 estimation or identification of a sugar in solution (pp. 91, 92). 



GENERAL REACTIONS OF SUGARS. 



There are no reagents except that of Molisch (p. 56) which 

 are of general application for the characterization of sugars, 

 but there are two, namely phenylhydrazine and Fehling's solu- 

 tion, which react with by far the greater number of sugars 

 and are consequently very largely used. 



Phenylhydrazine, which was discovered by Fischer, reacts 

 only with sugars containing either an aldehyde or ketone group 

 to form, in the first place, phenylhydrazones, which in many 

 cases are characteristic crystalline solids, but are usually 

 soluble in water ; this reaction may be illustrated thus : — 



CHoOH(CHOH),CHO+H2NNHCsH8=CHoOH(CHOH)4CH : NNHCeHs+H^O 

 Dextrose or Glucose Glucose Phenylhydrazone 



If, however, an excess of phenylhydrazine be employed, a 

 second hydrazine complex is introduced into the compound, 

 and the resulting substance is termed an osazone. Both 

 glucose and levulose yield the same osazone, 



CH„0HfCH0H)3— C— CH : NNHCgHs 



II 

 N.NHCeHs 



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 



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



glucose, p. 60. 



