558 CONSIDERATION OF CARBON COMPOUNDS. 



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methane derivatives differ considerably in many respects, and, as a 

 general rule, aromatic compounds cannot be converted into fatty 

 compounds, or the latter into aromatic compounds, without suffering 

 the most vital decomposition of the molecule, and in many cases this 

 transformation cannot be accomplished at all. 



On the average, aromatic compounds are richer in carbon than fatty 

 compounds, containing of this element at least G atoms ; when decom- 

 posed by various methods, aromatic compounds, in many cases, yield 

 benzene as one of the products ; most aromatic substances have anti- 

 septic properties, and none of them serves as animal food, although 

 quite a number are taken into the system in small quantities, as, for 

 instance, some essential oils, caffeine, etc. 



While some aromatic compounds are products of vegetable life, 

 many of them (like benzene itself) are obtained by destructive distil- 

 lation, and are, therefore, contained in coal-tar, from which quite a 

 number are separated by fractional distillation. 



Constitution. There is not known any benzene compound which 

 has less than six atoms of carbon. In all of the various decomposi- 

 tions and replacements which occur in the formation of benzene 

 derivatives, the six carbon atoms persist, like a unit. These condi- 

 tions have led chemists to look upon the six carbon atoms as being 

 joined together, forming a nucleus to which other atoms or groups 

 are attached, in all of the known aromatic compounds. Thus, in 

 benzene, C 6 H 6 , which is the fundamental or mother-substance of 

 these compounds, the six carbon atoms are joined to six atoms of 

 hydrogen. 



If benzene were of the nature of a fatty compound, we should 

 expect to find its structure correspond to a formula of this kind : 



H H 



This representation would indicate that benzene ought to behave 

 like a highly unsaturated compound. Moreover, we should expect 

 to obtain two isomeric compounds by replacing either a centrally 

 located hydrogen atom or one occupying a terminal position. 



As a matter of fact, benzene does not behave like an unsaturated 

 chain compound (although it can be caused to unite directly with 

 some elements), and by replacement of a hydrogen atom but one kind 

 of substitution product has ever been obtained. These facts lead us 

 to believe that benzene is not an unsaturated chain compound, and 



