278 



PRACTICAL PHYSIOLOGY 



the corresponding alcohols or ly reduction of the corresponding acids. There 

 are three hexatomic alcohols differing from one another in their 

 constitutional formulae. From each of these a different aldose (or 

 ketose) can be produced by oxidation, and the aldoses can be further 

 oxidised to form three different mono-basic acids, or further still, to 

 form three di-basic acids, thus : 



Another monobasic acid is glycuronic CHO. (CHOH 4 ).COOH which 

 differs from gluconic, etc., in the fact that oxidation has occurred at the 

 C atom furthest removed from the aldehyde group. Its physiological 

 importance will be discussed in the chapter on urine. 



Ketoses. As mentioned above, some carbohydrates belong to the 

 group of substances called ketones. A ketone is the oxidation pro- 

 duct of a secondary alcohol and it contains the group CO which 

 is situated somewhere in the chain between other groups and not 

 at the end of it as in the case of the - CHO group of the aldehydes. 

 The simplest ketone is acetone CH 3 - CO - CH 3 which may be ob- 

 tained by oxidation of secondary propyl alcohol, 



CH 3 - CHOH - CH 3 + = CH 3 - CO - CH 3 + H 2 0. 



Secondary propyl alcohol. . acetone. 



Ketones 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. There are several reactions 

 characteristic of ketoses, of these the following is important. 



EXPERIMENT IX. Seliwanofs Test. Mix a few cubic centimetres 

 of a solution of laevulose with half its volume of concentrated HC1. 

 Add a few crystals of resorcin and heat the mixture. A deep red 

 colour develops and later a brown precipitate. The colour can be 

 extracted by shaking with amyl alcohol. 



Repeat this experiment with pure dextrose solution instead of 

 laevulose. A slight red colour develops but no precipitate. 



1 The formula for these monobasic acids is CH 2 OH - (CHOH) 4 - COOH. 



2 The dibasic COOH - (CHOH) 4 - COOH. 



3 Laevulose when oxidised does not form the same products as mannose or 

 dextrose but breaks down into products with fewer carbon atoms than itself. 

 This is because it is a ketose. 



