126 Professor Julius Wilhelm Brilkl [May 25, 



ten years without anybody's surprise being excited. When his short 

 note came into my hands, I was fortunate enough to begin wondering 

 at the right time. 



It seemed to me really extraordinarily remarkable that all optically 

 abnormal substances, without exception, gave a too high molecular 

 refraction. It was no less astonishing to me that the saturated 

 hydrocarbons were optically normal, but became more and more 

 abnormal at successive withdrawals of hydrogen— while pure carbon, 

 uncombined with hydrogen, is again completely normal. 



But I was most particularly struck by the quantitative amount of 

 the abnormality in the case of benzene compounds, especially their 

 refractive increment of six units. The number 6 fascinated me. I 

 could not help thinking that therein lay the key to the mystery, and 

 I lost no time in making use of it. 



§ 12. According to Kekule's ingenious hypothesis we can imagine 

 benzene, GqRq, to have arisen from the saturated hydrocarbon hexane, 

 C6Hi4, by successive removal of hydrogen. 



H H H 



H H \ /H \ / \ 



HCH HCH HCH HCH HC OH 



I I -> ! I -> II I 



HCH HCH HCH HCH HC CH 



\^/ \?/ \c^ 



H H H 



(Hexane) (Hexamethylene) (Benzenes 



CeHi, CeH,, OeH« 



Thus altogether four pairs of hydrogen atoms have been removed. 

 The elimination of the first pair was made the occasion to form 

 another simple carbon bond, like those already present in hexane, and 

 with it the ring was closed. The splitting-off of the other three 

 pairs of hydrogen atoms, on the other hand, resulted in the formation 

 of three double bonds of carbon atoms— a kind of bond which does 

 not occur in the optically normal hexane. 



Now Griadstone had found that benzene exhibits a refractive in- 

 crement of G units. Reading this I was strack in a moment by the 

 tliought : might not this abnormal refractive increment of benzene be 

 due to its double carbon bonds, which are absent in the optically 

 normal hexane ? 



And if this were so, I went on to reason, since three double bonds 

 in benzene correspond to a refractive increment of 6 units, therefore 

 one double bond must entail the increment of 2. 



These ideas received no support whatever from the then known 

 facts. For Gladstone had stated expressly that the defines, i.e. open- 

 chain hydrocarbons, containing one double carbon bond, were optically 

 normal. 



