ALKALOIDS 265 



From the above examples it will be seen that the presence 

 of a trivalent nitrogen atom in a compound, whether in a ring 

 or attached to a straight chain, will, as a rule, confer on that 

 compound basic properties, owing to the tendency of that 

 nitrogen to become pentavalent by combining with an acid 

 and producing a salt. It is this property which gives rise to 

 the term Nitrogen base, 



The discovery and isolation from natural sources of a 

 number of nitrogen bases, such as cinchonine, quinine, bru- 

 cine, strychnine, morphine, etc., having properties analogous 

 to those of the alkalis in being able to form salts with 

 acids, led to their designation as alkaloids or alkali-resembling 

 substances. As the number of such substances increased, a 

 distinction began to be made between animal and vegetable 

 alkaloids. The term alkaloid is, however, better reserved 

 for nitrogen bases of vegetable origin ; it was at one time 

 suggested that the term should include only derivatives of 

 pyridine, quinoline, and isoquinoline, 



CH CH CH CH CH 



/ \ S \/ \ S \ / \ 



CH CH CH C CH CH C CH 



II I I II I I H I 

 CH CH CH C CH CH C N 



\ ^ \'/\ S \ / \ S 



N CH N CH CH 



Pyridine Quinoline Isoquinoline 



but this definition excludes such compounds as stachydrine 

 and hygrine, etc., which are pyrrolidine derivatives, and also 

 the purine bases which, according to most authors, should be 

 included among the alkaloids. 



This difficulty is, however, overcome by defining alkaloids 

 as nitrogen bases of vegetable origin whose nitrogen atom 

 forms part of a ring. 



Even this definition is not entirely satisfactory, as it would 

 include substances which, owing to their properties, could 

 hardly be classed as alkaloids, and excludes others, such, for 

 example, as hordenine. 



ALKALOIDS. 

 Occurrence. 



The alkaloids do not appear to have a wide distribution in 

 the vegetable kingdom. Amongst the Angiosperms, the Apo- 

 cynaceae, Leguminosae, Papaveraceae, Ranunculaceae, Rubi- 



