ELEMENTARY PHYSIOLOGICAL CHEMISTRY 163 



pitate formed just redissolves) in a test tube and add to it about ten 

 drops of formalin. Boil. Reduction takes place, and the metallic 

 silver is deposited on the wall of the test tube as a mirror. 



Ketoses. As mentioned above, some carbohydrates belong to the 

 group of substances called ketones. A ketose is the oxidation product 

 of a secondary alcohol, and contains the group CO which, instead of 

 being at the end of the chain, is situated in the middle of it. 



Ketones also form compounds with phenyl hydrazine, but only some 

 of them reduce metallic oxides in alkaline solution. Those ketones 

 which belong to the carbohydrates, manifest this reducing power. The 

 only well-known ketose is Laevulose. 



Reactions of Monosaccharides depending on their Chemical Nature. 



/. Their Reducing Power. If, in the above reactions, we take instead 

 of ethylic alcohol, the hexatomic 1 alcohol, sorbite, and carefully 

 oxidise it as above we obtain its aldehyde which is dextrose, 



CH 2 OH - (CHOH) 4 - CH 2 OH + O = CH 2 OH - (CHOH) 4 - CHO + H 2 O. 



Sorbite. Dextrose. 



Since it is an aldehyde it will manifest strong reducing powers on 

 metallic oxides in alkaline solution. 



EXPERIMENT III. Demonstrate the reducing power of a carbo- 

 hydrate, such as dextrose on cupric hydrate. 



Place three drops of a weak solution of cupric hydrate in a test tube ; 

 add about 5 c.c. of a 1% solution of dextrose, and then, drop by drop, a 

 20% solution of caustic potash (KOH) until the precipitate of cupric 

 hydrate at first formed redissolves and a dear blue solution is obtained. 

 Boil. Reduction is effected, a red precipitate of cuprous oxide resulting. 



EXPERIMENT IV. Demonstrate that dextrose also reduces silver 

 nitrate to metallic silver. 



Repeat Expt. II., using a 1% solution of dextrose instead of formalin. 



II. Like other Aldehydes they form Compounds called Osazones, with Phenyl 

 Hydrazine. The compounds are very useful in identifying the various 

 forms of sugars, as each sugar forms a slightly different compound (see 

 p. 417). 



Reactions peculiar to Carbohydrates. 



There are, however, other reactions of carbohydrates which are 

 peculiar to them alone, and do not depend on their chemical con- 

 stitution. The most important of these are : 



*A hexatomic alcohol is one which contains six OH groups. Glycerine is 

 called tri-atomic because it contains three such groups. Ethylic alcohol is 

 mon-atomic because it contains one. 



