PENTOSES 71 



The reaction may be modified by adding a couple of drops 

 of ferric chloride to the solution after it has been heated with 

 hydrochloric acid and orcinol, when a bright green colour is 

 produced. 



N.B. This test is characteristic for pentoses. 



4. By substituting phloroglucinol for orcinol in the above 

 test, a red colour is produced, which changes to a brown pre- 

 cipitate; the latter is soluble in amyl alcohol, the solution 

 having an absorption band between the D and E lines. This 

 is the same reaction that is employed for the detection of 

 Hgnified tissues ; its use in this case depends on the fact that 

 lignocellulose contains a pentose or furfural-yielding com- 

 plex (see p. 165). 



Dextrose and levulose when subjected to this test produce 

 a yellow or brown colour. 



5. Pentoses answer Molisch's test for carbohydrates. This 

 test, which is also dependent on the formation of furfural from 

 the sugar, consists in adding 2 c.c. of concentrated sulphuric 

 acid to a mixture of the sugar solution with 2 drops of 15 

 per cent alcoholic solution of o--naphthol, which must be free 

 from acetone. At the junction of the two liquids a green 

 ring is produced, and over this a red zone ; on cooling and 

 shaking the colour changes to purple. 



This test is given by all carbohydrates and glucoside^ and 

 proteins which contain a carbohydrate radicle. 



6. They form osazones. 



7. The pentoses reduce Fehling's solution. 



PROPERTIES OF INDIVIDUAL PENTOSES. 

 Arabinose. 



Arabinose is best obtained by the hydrolysis of cherry gum 

 with 4 per cent sulphuric acid ; it can also be obtained by the 

 hydrolysis of gum arabic and of peach gum. Arabinose has a 

 very sweet taste, is dextro-rotatory, a D in 10 per cent solu- 

 tion = + 105, crystallizes in prisms, and melts at 160; it 

 reduces Fehling's solution, and yields with diphenylhydrazine 

 a characteristic diphenylhydrazone, melting at 218.* 



*Neuberg: " Ber. deut. chem. Gesells.," 1900, 33, 2243. 



