REACTIONS OF THE CARBOHYDRATES 



59 



of furfurol from the sugar by the concentrated acid. The furfurol 

 then reacts with the ct-naphthol yielding a colored product. 



If sugar be heated to a considerable degree the mass partially 

 carbonizes and turns deep brown. Numerous products are formed to 

 which the collective name of caramel is given. Caramel has distinc- 

 tively colloidal properties and very high coloring-power, upon which 

 depends its use in the artificial coloring of beverages. 



The sugars themselves are very soluble and on that account are 

 frequently difficult to characterize and to purify. They form, how- 

 ever, insoluble or sparingly soluble compounds with Phenylhydrazine, 

 which are of great service in characterizing the various sugars, enabling 

 us to identify them in many cases with a considerable degree of cer- 

 tainty. If an aldose, or sugar, that is, of the aldehyde type, be acted 

 upon by phenylhydrazine in the presence of excess of acetic acid, the 

 following reaction occurs: 



CHO 



CH(OH) 

 CH(OH) 



I 



CH(OH) 

 CH(OH) 

 2(OH) 



C 6 H 6 NH.NH 2 



CH:N.NHC 6 H 6 



I 

 CH(OH) 



= CH(OH) 

 CH(OH) 

 CH(OH) 



CH 2 (OH) 



Phenylhydrazone. 



H 2 O 



The Phenylhydrazones are, for the most part, readily soluble; so 

 that this stage of the reaction is easily overlooked, since the -subse- 

 quent secondary reactions which are about to be described produce 

 sparingly soluble substances. Mannose is, however, an exception to 

 this rule, the phenylhydrazone being very sparingly soluble and readily 

 detected and isolated. Other hydrazines such as methylphenyl- 

 hydrazine, benzylphenylhydrazine, and diphenylhydrazines also react 

 with sugars to form hydrazones which are in some cases sparingly 

 soluble and can readily be separated and purified by repeated recrystal- 

 lization, and identified by their melting-points. 



If excess of phenyl hydrazine be employed, the remainder of the 

 reagent which is not used up in converting the sugar into a hydrazone 

 acts as an oxidizing agent, converting a CH(OH) group into a CO 

 group, thus: 



CH:N.NHC 6 H 6 



I 



CH(OH) 



CH(OH) 

 CH(OH) 

 CH(OH) 



CH 2 OH 



Hydrazone + 



CH:N.NHC 6 H 6 



I 

 CO 



CH(OH) 



I 

 + C 6 H 5 NH.NH 2 = CH(OH) 



CH(OH) 



CH 2 OH 



phenylhydrazine = oxidation product 



C 6 H 6 NH 2 + NHs 



aniline + ammonia. 



