450 CONSIDERATION OF CARBON COMPOUNDS. 



Finally, it is assumed that the carbon atoms are united partially 

 by double and partially by single union, as, for instance, in the so- 

 called closed chain of C 6 , capable of forming the hydrocarbon benzene, 

 C 6 H 6 : 



H 



\ C A C / H \c/ 



A C A H A C A H 



' i 



A chain has also been termed a skeleton, because it is that part of an organic 

 compound around which the other elements or radicals arrange themselves, 

 filling up, as it were, the unsaturated affinities. 



Homologous series. This term is applied to any series of organic 

 compounds the terms or members of which, preceding or following 

 each other, differ by CH 2 , Moreover, the general character, the con- 

 stitution, and the general properties of the members of an homologous 

 series are similar. 



The explanation regarding the formation of an homologous series 

 is to be found in the above-described property of carbon to form 

 chains. By saturating, for instance, the affinities in the open carbon 

 chains mentioned above, we obtain the compounds CH 4 , C 2 H 6 , C 3 H 8 , 

 C 4 H 10 , etc. 



H HH HHH HHHH 



H-C-H, H-C-C-H, H-C-C-C-H, H-C-C-C-C-H, 



H HH HHH HHHH 



Many homologous series of various organic compounds are known, 

 as, for instance : 



C H 3 Cl, C H 4 O, C H 2 2 . 



C 2 H 5 Cl, C 2 H 6 O, C 2 H 4 2 . 



QA Cl, C 3 H 8 O, C 3 H 6 2 . 



C 4 H 9 C1, C 4 H 10 0, C 4 H 8 2 . 



c 5 H n ci, cjr.A cjw 



etc. etc. etc- 



Substitution is a term used for those reactions or chemical changes 

 which depend on the replacement of an atom or a group of atoms by 

 other atoms or groups of atoms. Substitution takes place in organic 

 or inorganic substances, and its nature may be illustrated by the fol- 

 lowing instances : 



etc. 



