40 CHEMISTRY OF PLANT LIFE 



conditions of volume of solutions used, length of time of boiling, 

 etc., are observed, the quantity of cuprous oxide precipitated bears 

 a definite ratio to the amount of sugar which is present, so that 

 if the precipitate be filtered off and weighed under proper condi- 

 tions, the weight of sugar present in the original solution can be 

 calculated. The proper conditions for carrying on such a deter- 

 mination and tables showing the amounts of the various " reducing 

 sugars " which correspond to the weight of cuprous oxide found, 

 are given in all standard text-books dealing with the analysis 

 of organic compounds. 



Fermentability. The common hexoses are all easily fermented 

 by yeast, forming alcohol and carbon dioxide, according to the 

 equation 



The importance and biochemical significance of this reaction will 

 be considered in detail in connection with the discussions of the 

 relation of molecular configuration to biochemical properties 

 (see page 56) and the nature of enzyme action (see page 194). 



Formation of Hydrazones and Osazones. Another property of 

 the hexoses which is due to the presence of an aldehyde group in 

 the molecule, is that of forming addition products with phenyl 

 hydrazine, known as " hydrazones "and " osazones." For exam- 

 ple, glucose reacts with phenyl hydrazine in acetic acid solution, in 

 two stages. The first, which takes place even in a cold solution 

 may be represented by the equation 



Glucose Phenyl-hydrazine ^.Glucose-hydrazone 



The structural relationships involved may be represented as 

 follows: 



CHO H 2 N-NH CH=N NH 



(CHOH) 4 + I | = (CHOH) 4 



CH 2 OH \/ CH 2 OH 



The hydrazones of the common sugars, with the exception of the 

 one from mannose, are colorless compounds, easily soluble in 



