62 CARBOHYDRATES MONOSACCHARIDES 



When the Hexoses are heated for prolonged periods with dilute 

 mineral acids (excepting nitric acid) they yield Levulinic Acid (acetyl 

 propionic acid) and formic acid, besides "humin substances" containing 

 a higher proportion of carbon than the carbohydrates. The reaction 

 producing levulinic acid proceeds as follows: 



C 6 Hi 2 O 6 = CH 3 CO.CH 2 .CH 2 .COOH + H.COOH + H 2 O 

 hexose = levulinic acid + formic acid + water. 



When nitric acid is employed Saccharic Acid is produced (see below) . 



Levulinic acid is soluble in water, alcohol and ether and forms a 

 colorless viscous liquid which boils at 250 C. and yields with silver 

 nitrate a crystalline salt with the formula CH3.CO.(CH 2 ) 2 COOAg. 



The Pentoses do not yield levulinic acid on boiling with mineral acids ; 

 on the contrary, they yield Furfurol 



HC CH 



II II 



HC C.CHO 



\ / 



o 



which may be collected by distillation and detected by the aid of 

 aniline acetate paper, which is colored red by furfurol. This reaction 

 may also be used for the estimation of pentose since the yield of fur- 

 furol is quantitative. For this purpose the furfurol is distilled and 

 bisulphite added to the distillate, when the usual bisulphite compound 

 with aldehydes is formed and the unconsumed bisulphite is estimated 

 by titration with iodine. Or the furfurol may be converted into the 

 phloroglucide by addition of phloroglucin and the yield of this com- 

 pound determined gravimetrically. It should be very carefully borne 

 in mind, however, that Glucuronic Acid and its compounds (see below) 

 also yield furfurol on treatment with dilute acids so that before deciding 

 that pentoses are present their identity should be established by the 

 formation of osazones and the absence of glucuronic acid. 



The pentoses also yield the following reactions: 



The Orcin Reaction. To a small amount of the solution is added an 

 equal volume of a solution of orcin in concentrated hydrochloric acid. 

 On heating, the solution turns reddish-blue and then bluish-green. If 

 pentoses are present in abundance, a green precipitate will be obtained 

 on cooling and standing. On shaking up the mixture with amyl 

 alcohol the green coloration passes over into this solvent. This 

 reaction is also yielded by glucuronic acid. 



The Phloroglucin Reaction. This reaction is carried out in the same 

 manner as the above, phloroglucin being used in the place of orcin. 

 The mixture turns red on heating and becomes cloudy on cooling On 

 shaking with amyl alcohol the red color passes over into the amyl. 

 alcohol layer. This reaction is also yielded by glucuronic acid. 



Selivanoff's Reaction. The Ketoses may be distinguished from the 

 Aldoses by Selivanoff's reaction, as follows: 



