CARBOHYDRATES 35 



As already stated, fructose, sometimes called levulose or fruit sugar, 

 occurs widely disseminated throughout the plant kingdom in company 

 with glucose. Its reducing power is somewhat weaker than that of 

 dextrose. Fructose does not ordinarily occur in the urine in diabetes 

 mellitus, but has been found in exceptional cases. With phenylhydra- 

 zine it forms the same osazone as glucose. With methylphenylhy- 

 drazine, levulose forms a characteristic methylphenylfructosazone. 



(For a further discussion of fructose see the section on Hexoses, 

 page 20.) 



EXPERIMENTS ON FRUCTOSE 



1-6. Repeat Solubility, Fehling's, Phenylhydrazine, Barfoed's, Nylander's, 

 and Fermentation tests as given under Glucose, pages 21-31. 



7. Resorcinol-Hydrochloric Acid Reaction (Seliwanoff). To 5 c.c. of Seli- 

 wanofPs reagent 1 in a test-tube add a few drops of a fructose solution and heat the 

 mixture to boiling. A positive reaction is indicated by the production of a red 

 color and the separation of a brown-red precipitate. The latter may be dissolved 

 in alcohol to which it will impart a striking red color. 



If the boiling be prolonged a similar reaction may be obtained with 

 solutions of glucose or maltose. This has been explained 2 in the case 

 of glucose as due to the transformation of the glucose into fructose 

 by the catalytic action of the hydrochloric acid. The precautions neces- 

 sary for a positive test for levulose are as follows : The concentration 

 of the hydrochloric acid must not be more than 1 2 per cent. The reac- 

 tion (red color) and the precipitate must be observed after not more 

 than 20-30 seconds boiling. Glucose must not be present in amounts 

 exceeding 2 per cent. The precipitate must be soluble in alcohol with 

 a bright red color. 



8. Borchardt's Reaction. To about 5 c.c. of a solution of fructose in a test- 

 tube add an equal volume of 25 per cent hydrochloric acid and a few crystals of 

 resorcinol. Heat to boiling and after the production of a red color, cool the tube 

 under running water and transfer to an evaporating dish or beaker. Make the 

 mixture slightly alkaline with solid potassium hydroxide, return it to a test-tube, 

 add 2-3 c.c. of acetic ether and shake the tube vigorously. In the presence of 

 levulose, the acetic ether is colored yellow. (For further discussion of the test see 

 Chapter XXIII.) 



9. Formation of Methylphenylfructosazone. To a solution of 1.8 grams of 

 levulose in 10 c.c. of water add 4 grams 3 of methylphenylhydrazine and enough 

 alcohol to clarify the solution. Introduce 4 c.c. of 50 per cent acetic acid and heat 

 the mixture for 5-10 minutes on a boiling water-bath. 4 On standing 15 minutes 



1 SeliwanofT's reagent may be prepared by dissolving 0.05 gram of resorcinol in 100 c.c. 

 of dilute (1:2) hydrochloric acid. 



2 Koenigsfeld: Block. Zeit., 38, 311, 1912. 



3 3.66 grams if absolutely pure. 



4 Longer heating is to be avoided. 



