MONOSACCHARIDES 55 



cupric oxide to cuprous oxide is accompanied by a visible 

 change from a deep blue solution to a colourless one, with 

 simultaneous deposition of the cuprous oxide as a reddish- 

 brown precipitate. Any easily oxidized substance will thus 

 reduce Fehling's solution, becoming itself oxidized ; and, inas- 

 much as the aldehyde and ketone groups are readily oxid- 

 ized, all sugars containing these groups will bring about this 

 change. The reducing power of all sugars, however, is not 

 the same, but it has been determined in most cases against a 

 properly standardized Fehling's solution, and hence can be em- 

 ployed as a means of identifying or estimating the strength of 

 a sugar solution. In view of what has been said, it will of 

 course be seen that the experimental determination of the 

 reducing power of a sugar is valueless if an unknown amount 

 of any other easily oxidized substance is present in solution. 



MONOSACCHARIDES. 



A. PENTOSES. 



The pentoses, which are sugars containing five carbon 

 atoms, have the general formula C^H^oOg ; they are not as a 

 rule found free in plants, but may occur in a combined state. 

 For example, in cherry or wood gum they occur in the form of 

 pentosanes or pentosides, which substances may be regarded 

 as anhydrides of pentoses, since they give rise to these sugars 

 on hydrolysis in much the same way that starch on hydrolysis 

 yields glucose, 



GENERAL PROPERTIES OF PENTOSES. 



1. They are not fermentable by yeast. ' 



2. On distillation with hydrochloric or sulphuric acid, they 

 are converted in furfural, which may be detected by its turning 

 a solution of aniline acetate red. 



qHioO,— 3H„0 = C.H^O.CHO 



This may easily be seen by boiling some wood shavings with . 

 concentrated hydrochloric acid in a test tube and allowing the n^ / 

 escaping steam to impinge upon a piece of filter paper moistened / 

 with aniline acetate ; * a pink colour is produced. 



* Prepared by mixing together equal parts of aniline, water and glacial acetic 

 acid. 



