HETEROCYCLIC COMPOUNDS. 



Numerous compounds exist containing rings or nuclei composed 

 of carbon atoms and atoms of other elements, especially oxygen, 

 sulphur and nitrogen. These ring compounds are grouped together as 

 the heterocyclic compounds. 



Some of the heterocyclic compounds are closely connected with the 

 aliphatic series of compounds, e.g. the anhydrides of dibasic acids, such 

 as succinic anhydride ; the 7-lactones and other lactones ; the imides 

 from ammonium salts of dibasic acids, such as succinimide ; the 

 polymers of the aldehydes, such as trioxymethylene, paraldehyde ; 

 and of cyanic acid, namely, cyanuric acid. Creatinine may also be 

 placed in this group. In these compounds the ring is formed easily 

 and it is easily ruptured. They are therefore usually considered as 

 aliphatic cdmpounds. Other heterocyclic compounds possess a more 

 stable ring and they resemble the aromatic substances very closely in 

 their properties. They include pyridine, quinoline and their deriv- 

 atives, the alkaloids. 



Intermediately between these two classes there are other heterocyclic 

 ring compounds which do not possess the chief properties of aromatic 

 compounds in forming nitro- and sulphonic acid derivatives, but they 

 possess a ring which is comparatively stable and is not easily broken 

 down. In this group are included the cyclic ureides, pyrimidine and 

 purine and their derivatives, pyrrole, thiophene, furfurane. 



As in the carbocyclic compounds the rings containing 5 atoms 

 and 6 atoms are the most stable. 



