PENTOSES 91 



time and watch for the appearance of a reddish-violet colour ; 

 when this appears, remove from the water bath, cool, and 

 shake up with amyl alcohol ; the solution in the alcohol has 

 an absorption band between the D and E lines. 



4. When boiled with 12 per cent of hydrochloric acid or 

 sulphuric acid, pentoses give rise to furfural which is carried 

 off by the escaping steam ; if this is allowed to impinge 

 upon a filter paper moistened with a drop of aniline acetate 

 a bright pink colour is formed. 



CHOH— CHOH CH— CH 



II I! II 



CHj CHOH . CHO — 3H2O = CH C . CHO 



\ \/ 



OH O 



Methyl pentoses under these conditions give methyl fur- 

 fural, while hexoses give small quantities of hydroxy-methyl- 

 furfural. 



CHOH— CHOH CH— CH 



11 " I' 



CH2OH— CHOH CHOH CHO — 3H2O = CH^OH C C . CHO 



O 



All furfural derivatives give similar colour reactions to fur- 

 fural both with aniline acetate and with phloroglucinol and 

 orcinol (see below) ; were it not for the fact that hydroxy- 

 methylfurfural is itself readily decomposed further into levu- 

 linic acid and formic acid, 



CeHgOa + aH^O = HCOOH -f CH3 COCH^ CH, COOH 



neither of which give the above colour reactions, the test 

 described would not be specific for pentoses. 



In carrying out the above test it must therefore be borne 

 in mind that a very faint positive reaction should not be taken 

 as evidence of the presence of pentose without further evidence. 

 This reaction has also been made the basis of a method for 

 the quantitative estimation of pentoses (see p. 137). 



5. Pentoses reduce Fehling's solution and yield osazones 

 but are not fermentable. 



