CHARACTERIZATION 87 



All monosaccharides reduce Fehling's solution, but some 

 disaccharides, such as sucrose and trehalose, are so constituted 

 that the reducing aldehydic or ketonic group is masked, and 

 is only set tree after hydrolysis. 



If the sugar is non-reducing, boil a fresh portion for a short 

 time with a little dilute hydrochloric acid ; neutralize and 

 test once more with Fehling's solution as above. The solution 

 should now reduce owing to the hydrolysis of the di- or tri- 

 saccharide to monosaccharides. 



It must be borne in mind that other substances besides 

 sugars reduce Fehling's solution, and consequently due pre- 

 caution must be taken to exclude the presence of these before 

 applying the test. 



2. Ascertain whether the substance is a pentose (for 

 tests see p. 90) or a hexose. 



3. If a pentose is not found, distinguish between aldo- 

 hexose and ketohexose (for tests see p. 96). 



4. If the substance is a reducing sugar, whether pentose 

 or hexose, its further identification usually depends upon the 

 production of a crystalline derivative by means of phenyl- 

 hydrazine or a similar compound. 



Phenylhydrazine reacts 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 : — 



CH20H(CHOH)4CHO + H2NNHC6H5 = CH20H(CHOH)4CH : NNHCsHs + 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, fructose, and mannose yield the same osazone : — 

 CH20H(CHOH)3— C— CH : NNHCgHg 



N . NHQHs 



which is called glucosazone. 



* This is due to the fact that these three sugars differ only in the con- 

 figuration of their two terminal carbon atoms, a difference which is elimin- 

 ated when they are converted into their osazones (cf. p. 95). 



