ISOMERIC CHANGE 633 



648), was based on an observation by Zincke {Bcr. 1884, 17, 

 3030) which had shown that the phenyl hydrazone derived 

 from naphthoquinone was identical with the phenylazo-derivative 

 of a-naphthol, with which it should have been isomeric. 



C.H 5 . NH . N = Ci H a = O C 6 H 5 . N = N . C, H 6 . OH 



Hydrazone Azo-compound 



These observations, as well as the analogous experiments which 

 had shown the identity of quinone monoxime and nitrosophenol, 



O = C 6 H 4 = NOH HO . C 6 H 4 . NO 



might have been explained either by Butlerow's theory, or 

 by the theory of labile isomerism made use of by Baeyer f 

 but Laar preferred to develop a new hypothesis, derived in 

 part from the benzene-theory of Kekule. It will be remembered 

 that in order to account for the identity of the 1 : 2 and 

 1 : 6 di-derivatives of benzene, Kekule was led to advocate a 

 kinetic conception of atomic linkages according to which 

 a double bond merely represented a double number of 

 collisions in unit time between the two atoms so united 

 {Ann. 1872, 162, 86); as a result of this view it followed that 

 the single and double bonds in a benzene molecule could be 

 represented as occupying the 1:2 or the 1 : 6 positions 

 indifferently, according to the interval over which the collisions 

 were counted. This loose view of the nature of atomic 

 linkages was boldly transferred by Laar to the whole range 

 of cases in which attempts to prepare isomeric substances 

 had given rise to identical products. Many of these involved 

 the transference of a hydrogen atom in addition to a shifting 

 of the bonds, and it was therefore necessary to assume that 

 the atoms as well as their linkages were in a constant state 

 of flux. Compounds which in consequence of this incessant 

 motion of bonds or atoms could never be properly represented 

 by a single formula, Laar proposed to call " tautomeric." 



It was apparently Butlerow himself who called Laar's atten- 

 tion to the alternative explanation which he had suggested 

 eight years previously, and so elicited from Laar a further 

 explanation of his hypothesis (" Ueber die Hypothese der wech- 

 selnden Bindung," Ber. 1886, 19, 730), in which special attention 

 was directed to points of contrast. These proved to be of 

 fundamental importance. Butlerow had postulated the existence 



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