96 THE CARBOHYDRATES 



DISTINCTION BETWEEN ALDOSES AND KETOSES. 



To distinguish an aldose from a ketose use is made of the 

 fact that on heating with concentrated hydrochloric or hydro- 

 bromic acid a ketose is more readily converted into chloro- or 

 bromo-methylfurfural than is an aldose, as may be seen from 

 the formulae : — 



CHOH-CHOH CHOH-CHOH 



CHpHCHOH— CH CHOH CHjOH— CH COH CHjOH 



O O 



Aldose Ketose 



CH CH 



li II 



CHXIC C . CHO 



O 



Chloro-methj'lfurfural 



The production of the furfural derivative from the ketose 

 involves much less rearrangement than from the aldose. On 

 this fact depends the two reactions of Seliwanoff and of 

 Fenton.* 



Seliwanoff Reaction. — Warmed on a water bath with an 

 equal volume of concentrated hydrochloric acid and a crystal 

 of resorcin, a ketose solution turns rapidly red while a hexose 

 develops a colour much more slowly. 



There are no convenient general reactions for distinguish- 

 ing hexoses as a class from any other group of sugars, but 

 each of the hexoses occurring in nature is readily identified by 

 characteristic reactions. 



GLUCOSE OR DEXTROSE. 



Occurrence. 



The substance which is commonly known as grape sugar 

 occurs, together with levulose or fruit sugar, in a number of 

 sweet fruits, in honey, and in the seeds, leaves, roots, and 

 blossoms of a great many of the higher plants. Glucose is 

 formed by the hydrolysis of cane sugar, of glucosides, and of 

 many polysaccharides, such as starch, cellulose, etc. 



* Fenton and Gostling : " J. Chem. Soc," 1901, 79, 361, 807. 



