68 THE CARBOHYDRATES 



glucose is obtained whose specific rotation is a D = + 110 ; if 

 crystallized from water at a temperature above 98, another 

 variety, known as /3 glucose (a D = + 19), is obtained ; if 

 either a glucose or /3 glucose is dissolved in water, a gradual 

 change in rotation is observed until a steady value of a D =52-5 

 is attained, which is regarded as the specific rotation of an 

 equilibrium mixture of a and /3 glucose. The attainment of 

 the stable condition is accelerated by acids, and is practically 

 instantaneous in presence of traces of alkali. 



The work of Fischer and others has also brought to light 

 the existence of yet another variety of glucose* having an 

 a-ethylene oxide ring structure 



CH 2 OH.CHOH.CHOH.CHOH.CH . CHOH 



V 

 o 



instead of the <y-butylene oxide, or 7 lactone, ring of ordinary 

 glucose. So far only derivatives of a-ethylene oxide glucose 

 are known, the free substance not having been isolated, but 

 there is reason to believe that its reactivity far exceeds that of 

 the ordinary a or (3 glucose. 



GENERAL REACTIONS OF SUGARS. 



There are no reagents except that of Molisch (p. 71) 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 : 



CH 2 OH(CHOH) 4 CHO+H 2 NNHC 6 H 5 =CHoOH(CHOH) 4 CH : NNHC 6 H 5 +H 2 O 

 Dextrose or Glucose Glucose Phenylhydrazone 



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

 second hydrazine complex is introduced into the compound, 



* Fischer: " Ber. deut. chem. Gesells.," 1914,47, 1980; Irvine, Fyfe, and 

 Hogg: "J. Chem. Soc,," 1915, 107, 524. 



