CARBOHYDRATES 43 



carbohydrate diet is "fattening." Possibly the carbohydrate 

 molecules are both split into fragments and reduced or dehy- 

 drated, and then recombined to form compounds with longer 

 chains, fatty acids. The exact mechanism of the process is 

 still unknown. It is quite probable that carbohydrates may give 

 up their oxygen to cells or microorganisms under conditions 

 where vital activities are going on in the absence of atmospheric 

 oxygen. This process is called anaerobic respiration. 



Formation of Osazones. Monosaccharides and many of the 

 disaccharides combine with phenylhydrazine to form osazones. 

 These are yellow compounds which crystallize in needles. The 

 crystals often group together with points at a common center, 

 thus forming rosettes, or fans, or sheaves like grain sheaves. The 

 different osazones have slightly differing crystal forms, but they 

 are best recognized by their melting points; identifying an 

 osazone serves to identify the sugar from which it was formed. 

 Glucose and fructose form the same osazone, since the struc- 

 ture of these two sugars differs only around the carbon atoms 

 to which the phenylhydrazine molecules become attached. This 

 test thus will not distinguish between these two sugars. Sac- 

 charose, for a reason to be seen later, does not form an osa- 

 zone, nor do the polysaccharides. 



The reaction takes place in three stages as follows: 



CH 2 OH CHOH 



CH OH (CH OH) 3 



I I 



CH OH - CH OH 



I I 



CH OH CH =N NH C 6 H 5 +H 2 



CH OH 



I 

 CHO +H 2 N NH C 6 H 5 



The product is a hydrazone. A second molecule of phenylhy- 

 drazine then removes two H atoms from the group next the end 

 carbon atom, and is converted into aniline and ammonia. 



