no ORGANIC AGRICULTURAL CHEMISTRY 



PENTOSES, C5H10O5 



The four-carbon monosaccharoses or tetroses are not im- 

 portant, but the five-carbon compounds, pentoses, axe commonly 

 occurring substances in many plants. When certain gums such 

 as gum arable, cherry gum or wood gum, are boiled with acids, 

 sugars known as arabinose and xylose are obtained which are 

 pentoses, and have the formula C5H10O5. 



These sugars are contained in the gums, not as sugars, but as 

 complex compounds which, because they yield pentose sugars, 

 are known as pentosans. The substances occur in the woody 

 or cellular parts of many plants which are used as animal foods, 

 and are directly utilizable by the animal as food nutrients. 

 The amount of pentosan compounds in an animal food is, 

 therefore, an important item. We shall consider pentosans 

 more at length as constituents of plants in Chapter XV, p. 272. 



The only carbohydrates which do not contain the elements 

 hydrogen and oxygen in the proportion H2:0 are the substi- 

 tuted sugars. The best known is rhamnose, which has the com- 

 position C6H12O5. It is a methyl substitution product of a 

 pentose, i.e. (CH3) — C5H9O5. 



EXPERIMENT STUDY XXI 

 Pentosans and Pentoses 



(i) {a) Place about 2.0 g. of wheat bran in a flask and boil with 

 100 CO. of dilute HCl (sp. gr. 1. 15), distilling through a condenser until 

 about 25 c.c. of distillate have collected. Now add to the distillate 

 some phloroglucinol solution. A green precipitate turning black 

 shows the presence oi furfural. The furfural has been produced from 

 the pentose carbohydrates obtained by hydrolysis of the pentosans 

 present in the bran, {h) The same experiment may be performed, 

 using gum arable instead of bran. 



HEXOSES, CsHiaOe 



By far the most important group of monosaccharose sugars 

 is that of the hexoses which have six carbon atoms and the for- 

 mula C6H12O6. The most common members of this group are : 



