BENZENE SERIES. AROMATIC COMPOUNDS. 365 



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 6 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. 



The constitution of benzene is best explained by assuming that of 

 the 4 X 6 = 24 affinities of the 6 carbon atoms, 18 affinities are lost 

 by uniting the carbon atoms into a closed chain, while but 6 affinities 

 are left unprovided for and may be saturated by other elements or 

 groups of elements. 



The carbon chain of aromatic compounds and benzene may be 

 graphically represented thus : 



1 H 



6 \ C A C / 2 H \ C A C / H 



d, 



6 0^ \3 H/ \C^ \H 



i A 



It has been found that whenever one atom or one radical replaces 

 hydrogen in benzene, the product obtained is the same, no matter by 

 what method the change is brought about. Thus we know but one 

 mono-brom-benzene, C 6 H 5 Br, one nitro-benzene, C 6 H 5 NO 2 , etc. 



It is different when two or more atoms or radicals (of the same 

 kind) replace hydrogen in benzene, since it has been found that in 

 this case often isomeric compounds are formed. 



For instance, we know three different substances which have been 

 obtained by replacement of two hydrogen atoms in benzene by two 

 hydroxyl groups. This would indicate that it makes a difference, as 

 far as the properties of a compound are concerned, in which relative 



