V] CARBOHYDRATES 47 



■clusively be used for tests d, e and /. If a considerable quantity of gum is available, 

 crystallization of arabinose should be attempted by concentrating the aqueous sugar 

 solution, extracting this with 90 ^/q alcohol and again concentrating in a desiccator 

 (see p. 55). If a very small quantity of gum only is available, the tests a, h and c 

 should be performed directly on a small piece of the gum in a test-tube. 



A purer preparation of arabinose, which may be used for all the 

 tests of Expt. 39, can be obtained by the hydrolysis of araban (see 

 Expt. 48). 



Xylose. This sugar occurs very widely distributed in woody tissue 

 a,s the pentosan, xylan (see p. 56). A solution of xylose which will give 

 the pentose reactions can be obtained from the hydrolysis of straw, or 

 the presence of xylan giving the pentose reactions can be directly de- 

 monstrated in straw, bran or sawdust (see Expt. 49). 



A purer solution of xylose can be obtained from the hydrolysis of 

 xylan (see Expt. 51). 



When xylose is oxidized with bromine, it yields xylonic acid which 

 has a characteristic cadmium salt. The formation of this salt is used as 

 a, method for identifying the sugar (see Expt. 51). 



The methyl pentoses are pentoses in which one of the hydrogen 

 a-toms of the CH2OH group is replaced by the methyl group, CHg. 



Rhamnose, C5H9O5CH3, occurs as the constituent of many glucosides 

 <see pp. 113, 159). 



Hexoses. 

 If we examine the structural formula for a hexose, such as glucose : 



H~C = 

 H— C*— OH 

 OH— C*— H 

 H— C*— OH 



H— C»— OH 



I 

 H— C_H 



I 

 OH 



we see that there are four carbon atoms marked * which are united to 



