CARBOHYDRATES 107 



distinctive reactions characteristic of aldehydes and ketones, 

 viz. they form addition products with hydrogen cyanide, HCN, 

 they yield oximes with hydroxyl amine, H2N— OH, and 

 they form hydrazones when treated with phenyl hydrazine, 

 H2N— NH — CeHs. These reactions were referred to in con- 

 nection with aldehydes (p. 44) and they have been of the 

 greatest importance in the study of the carbohydrates. 



It will not be necessary to discuss these reactions more in 

 detail nor to dwell further upon ideas in regard to the constitu- 

 tion of the carbohydrates, for these facts are not essential to an 

 understanding of their agricultural importance. The more 

 important study for us is the occurrence and distribution of 

 the carbohydrates in nature, their relation to each other and 

 to a few other compounds such as alcohol and lactic acid, and 

 their economic uses. Before dismissing the question of consti- 

 tution, however, it should be stated that recent work has shown 

 that the aldehyde and ketone structure is probably not the form 

 in which the carbohydrates actually exist, but that they take 

 this form when acted upon by the reagents considered. 



Classification. — The carbohydrates are subdivided into 

 several smaller groups, depending both upon the number of 

 carbon atoms in the molecule and upon their general com- 

 plexity and relation to each other. In the first place there are 

 two main groups which are known as simple carbohydrates and 

 compound carbohydrates. The simple carbohydrates, or, as they 

 are more generally termed, the simple sugars, are known more 

 definitely as mono-saccharoses. The name signifies the fact 

 that the compounds of this subgroup are the unit sugars. This 

 is shown by the fact that they cannot be split or broken down 

 by hydrolysis into any simpler units and also because they are 

 the unit parts of more complex carbohydrates. The general 

 formula is C,XH02)„. Typical examples of monosaccharoses 

 are the two common sugars already mentioned, glucose and 

 fructose. These both have six carbon atoms in the molecule 

 and their formula is, therefore, C6(H20)6 or C6H12O6. While 

 all the monosaccharoses are alike in this unit character, the 



