56 THE CARBOHYDRATES 



It should be noted that hexoses * will also produce this 

 reaction, though to a much smaller extent, since the quantity 

 of furfural produced from them is much less (not more than 

 0-2 per cent) than in the case of the pentoses. The chief 

 product of the action of concentrated hydrochloric acid in 

 hexoses is levulinic acid, 



3. Warmed with concentrated hydrochloric acid (sp. gr. i'2) 

 and a little orcinol, they produce a greenish-yellow colour which 

 is soluble in amyl alcohol to a clear green solution having a 

 characteristic absorption spectrum with bands between the C 

 and D lines. 



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 orcin, 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 

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

 lignocellulose contains a pentose or furfural-yielding com- 

 plex (see p. 142). 



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 a-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 glucosides^ and 

 proteins which contain a carbohydrate radicle. 



6. They form osazones. 



7. The pentoses reduce Fehling's solution. 



* Cf. Tollens : " J. f. Landw.," 1901, 39. 



