86 THE CARBOHYDRATES 



in the production of pentoses from hexoses. Thus, assuming 



a glucose molecule to have its aldehyde group protected from 



attack, it would, on oxidation, give glucuronic acid which 



by loss of carbon dioxide would yield xylose : — 



CH20H(CHOH)4 . CHO (protected) -> COOH (CHOH), . CHO 



Glucuronic acid 

 COOH . (CHOH)4 CH0-C02-> CUfiU {CUOH)^ . CHO 

 Glucuronic acid Xylose 



On the other hand, if a given glucose molecule were not so 



protected and were susceptible to oxidation at both ends, 



it could give rise to an isomeric glucuronic acid whose aldehyde 



and carboxyl groups were at the opposite ends by comparison 



with the previous one : — 



CH2OH (CHOH) CHO (unprotected) -> CHO (CHOH) . COOH 

 CHO . (CHOH)4 COOH— CO, -> CHO (CHOH)3 . CH,OH 



Arabinose 



The fact that in nature xylose, rather than arabinose, is 

 commonly associated with glucose, suggests that xylose is 

 produced from glucose by the oxidation of the primary alcohol 

 group,* the aldehyde group being protected from attack 

 owing to the form of combination in the complex molecule 

 concerned ; the same explanation may account for the 

 frequent association of cellulose with xylans. Spoehr f has 

 isolated the lactone of glucuronic acid from cactus gum, 

 and suggests that glucuronic acid is broken up under the 

 influence of sunlight into carbon dioxide and xylose. 



THE CHARACTERIZATION OF SUGARS. 



In order to characterize a sugar, the following procedure 

 may be followed : — 



I. Ascertain whether the substance is a reducing or non- 

 reducing sugar by adding a little of the neutral aqueous 

 solution to a little Fehling's solution previously diluted with 

 three times its volume of water and boiled to see that it is 

 not changed by boiling alone. Boil the mixture for about one 

 minute. If at the end of this time no red or brown precipitate 

 of cuprous oxide is formed the sugar is non-reducing. 



* De Chalmot : " Am. Chem. J.," 1893, 16, 610. 

 t Spoehr : " The Carbohydrate Economy of Cacti," Carnegie Inst. 

 Pub., Washington, 381, 42. 75. 



