40 PHYSIOLOGICAL CHEMISTRY 



CH 3 . C CH 2 . CH 2 . COOH 



II 

 



Oxidation of Carbohydrates. Oxidation Tests. Since the 

 simple carbohydrates contain either an aldehyde or a ketone 

 group they are easily oxidized, even by mild oxidizing agents. 

 The products of mild oxidation are acids having the same num- 

 ber of carbon atoms as the original material. If the oxidation is 

 more vigorous the molecule may break into fragments as was de- 

 scribed under the action of alkalies. Some of the most important 

 carbohydrate tests depend upon .oxidation processes. Among 

 the oxidation tests are the Fehling, Almen-Nylander, and Bar- 

 f oed tests. 



Fehling Test. If solutions of copper sulphate and sodium 

 hydroxide are mixed, a light blue or whitish precipitate is 

 formed. This is cupric hydroxide 



CuS0 4 +2 NaOH-+Cu(OH) 2 +Na 2 S0 4 



If this mixture is boiled, the precipitate is converted into 

 black cupric oxide. 



Cu(OH) 2 ->CuO+H 2 0. 



If a sugar is present in the solution, however, the copper is 

 reduced by the sugar which is converted into an acid. If a sugar 

 is added to a mixture of copper sulphate and sodium hydrate the 

 liquid will turn a very deep blue, and a much smaller precipitate 

 will form as most of the Cu(OH) 2 is held in solution by com- 

 bining with the sugar. If the mixture is allowed to stand, or 

 if it is boiled, the color becomes perhaps yellow at first, and 

 ultimately a red precipitate forms. The sugar has reduced th* 1 

 cupric hydroxide to yellow cuprous hydroxide, which, on boiling, 

 decomposes into the red cuprous oxide. 



2 Cu(OH) 2 +C 5 H 11 0,.CH0^2 CuOH+ 



CsH^O, . COOH+H 2 

 2 CuOH^Cu 2 O+H 2 



As a matter of fact, the sugar undoubtedly undergoes further 

 change, as we already have seen in considering the action of al- 



