of the Aromatic Nucleus. 



413 



olefinoid linkages in these bodies, is that the two intermediate 

 substances are much more readily disintegrated by oxidizing 

 agents than the others. I may quote Baeyer's formulas : — 



CH.COOH 



Hexaliydro-acid (stable). 



CH.COOH 



Tetrahydro-acid (unstable). 



C COOH 



C.COOH 



CH 



CH COOH C.COOH 



A 1,5 dihydro-acid (unstable). Terephthalic acid (stable). 



The behaviour of the intermediate bodies towards bromine 

 and hydrobromic acid, compared with that of terephthalic 

 acid itself, shows that they are (in a different sense) unsatu- 

 rated bodies. 



We should rather expect a gradational change of properties, 

 if one, two, and three para-linkages were successively formed. 

 Whatever may be the view entertained of this question, it is 

 nevertheless true that the results of Baeyer's work are incon- 

 sistent with Kekule's symbol for benzene. Baeyer states that 

 the obvious conclusion, from the reduction in one stage of the 

 dibromide of the A 1,5 dihydro-acid to terephthalic acid — viz. 

 that terephthalic contains para- or meta -linkages — would 

 involve him in serious inconsistency. It seems to me that in 

 this respect he is illogical. Experiment justifies the belief 

 that double, and not para-linkages, are set up in the interme- 

 diate compounds ; but we are not thereby warranted in the 

 assumption that para-linkages are not formed in the end reac- 

 tion, which is admittedly of a different character. (In the 

 preceding year he stated that benzene has a double-bond, 

 because tetrahydro-terephthalic acid was thought to have 

 one, Ber. xix. p. 1797.) The ultimate formation of the 

 aromatic nucleus is brought about by a comparatively compli- 

 cated change. Three para-linkages are simultaneously set up, 

 and the atoms are drawn more closely together by the result- 

 ing pull towards the centre of the molecule. 



The symbol which he proposed, and which I have already 

 discussed, has been somewhat improved by Marsh (Phil. Mag. 



