44 CARBOHYDEATES [ch. 



strong acid. The reaction is due to the formation of a furfural compound 

 (see p. 46), and the colour depends on the particular sugar and phenol 

 used. Thus, with strong hydrochloric acid and orcinol, the colour is 

 violet-blue for pentoses and orange-red for hexoses; with the same acid 

 and phloroglucinol, the colour is red in both cases ; with a-naphthol and 

 strong sulphuric acid, the colour is purple in all cases. A variation of 

 this reaction provides a distinction between a ketone and an aldehyde 

 sugar. Thus, if hydrochloric acid diluted with its own volume of water 

 is used, a red colour is produced with resorcinol and a ketone sugar, 

 e.g. laevulose (Seliwanoff 's reaction). With an aldehyde sugar, e.g. glucose, 

 the colour is produced only by using concentrated acid. 



Pentoses, Methyl Pentoses. 



The pentoses contain five carbon atoms, and have the general formula 

 C5H10O5. They are said to be present in the free state to some extent in 

 leaves (Davis and Sawyer, 12). In plants they occur chiefly, however, as 

 condensation products formed with elimination of water. These products 

 are termed the pentosans, and are widely distributed ; on hydrolysis they 

 yield pentoses again. The various gums found in plants consist largely of 

 pentosans, and the pectins also contain pentose groups; both consequently 

 yield pentoses on hydrolysis (see pp. 63 and QQ). A pentose is also a 

 component of plant nucleic acid (see p. 141). It has recently been shown 

 (Spoehr, 33) that the metabolism of some succulent plants (Cactaceae) 

 is especially favourable to the production of pentoses. By condensation, 

 pentosan-mucilage is formed and this has the water-retaining properties 

 characteristic of succulents. 



If we examine the structural formula of a pentose, as for example 

 arabinose : 



H— C = O 



I 

 HO— C*--H 



H— C*— OH 



I 



H— C*— OH 



I 

 H— C— H 



I 

 OH 



we see that each of the three carbon atoms marked * is united to four 

 different atoms or groups of atoms. Each of these carbon atoms is there- 

 fore asymmetric, and, with regard to it, there are two possible isomers 



