■THE PRINCIPLES OE ORGANIC CHEMISTRY 51 



position under defined conditions, but they cannot be looked 

 upon as chemical individuals in the same sense as compounds 

 which fulfil one of the above-mentioned requirements. 



The determination of the constitutional formulae of the 

 countless substances met with in the study of organic chemistry 

 shows that they can be classified under three heads. 



1. Alifhatic compounds, viz., all open chain compounds both 

 saturated and unsaturated, viz., the paraffin, define, acetylene, 

 etc., hydrocarbons already referred to, and their derivatives. 



2. Isocyclic compounds. — All compounds containing closed 

 chains formed by the union of carbon atoms only, viz., deri- 

 vatives of polymethylene hydrocarbons, consisting of rings 

 formed by three or more CHg groups ; thus : — 



trim ethylene /\ 



or substances derived from benzene 



CH 



HCf ^CB. 



CH 



and from hydrocarbons containing more than one ring such 

 as naphthalene, anthracene, etc. 



3. Heterocyclic compounds. — All compounds containing 

 closed chains, having other atoms in addition to carbon atoms, 

 viz. : — 



CH 

 HC CH ^\ 



^„ pyridine HC CH 



HC CH 



thiophene -p-p 



S 



N 



e2 



