LECTURE XIV.] HISTORY OF CHEMISTRY. 273 



The aromatic compounds are obtained by the replacement 

 of these hydrogen atoms in benzene. But it follows from the 

 equivalence of all six hydrogen atoms that when only one of 

 them is replaced, it must be a matter of indifference which of 

 them it is ; or, in other words, only one variety of any of the 

 mono-substitution products of benzene can exist. A view of 

 this kind was only possible after it had been shown that 

 methyl-benzene is identical with toluene, and benzoic acid 

 with salylic acid (compare p. 271). 



When two or more hydrogen atoms in benzene are replaced, 

 Kekule's hypothesis predicts the existence of numerous isomers, 

 occasioned by differences in the relative positions of the atoms 

 or groups that enter the molecule ; and the number of these 

 isomers can be determined. Thus there are three isomers 

 possible when two hydrogen atoms in benzene are replaced by 

 other atoms or by radicals ; and it is immaterial whether the 

 atoms or radicals which enter are identical or different. Of 

 tri-substitution derivatives of benzene there are three possible 

 isomeric forms when the three substituting atoms or groups 

 are the same, but six when two of them are different from the 

 third. Further the hypothesis foretells the existence of three 

 isomeric tetra-substituted benzene derivatives but only one 

 penta- and one hexa-substituted derivative when all the sub- 

 stituting atoms or groups, in each case, are the same. In 

 accordance with the hypothesis, by the replacement of one or 

 more of the hydrogen atoms in benzene by any given element 

 or group of atoms, twelve substances can be obtained, and this 

 has been actually accomplished in at least one case. Thus 

 Beilstein was able to show that exactly twelve chlorinated ben- 

 zenes exist, 2 after it had been proved that the alleged existence 

 of two isomeric pentachlorobenzenes 3 was a mistake. 



It follows, moreover, from the constitution of benzene, that 

 ethylbenzene must be different from the three possible dimethyl- 

 benzenes ; and, further, that by the action of chlorine or of 



2 Beilstein and Kurbatow, Annalen. 192, 228. u Ladenburg, 



ibid. 172, 331. 



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