869 



a light-screen, thus cansiiig I lie absorption effect to be greater than 

 when the ketone had been present in alcoholic solution. 



In each case a ciicnnistance occnis owing to which the phenomenon 

 becomes more complicated, so that from these last experiments we 

 maj at most draw the conclusion that rays of light are indeed always 

 absorbed by the one ketone, which the other required for the oxidation 

 of alcohols. 



The most powerful absorbing ketones appear mostly — but not 

 always — to oxidise the alcohol slowly or not at all, so that w^e 

 gain the impression that a liberal absorption does take place, but 

 that the possibility of the setting in of a reaction and its velocity 

 does not only depend on the alcohol, but in a great measure on 

 the ketone. 



It speaks for it'^elf that the exjjeriments on this almost quite 

 nnexploi"ed region can only bear a veiy provisional character; still 

 we believe we have attained, with very simple means and methods, 

 some i-esults which will prove of importance for the insight into 

 the photochemical reactions. 



For the moment, howe\ er, we wish to refrain from an attempt to 

 explain the phenomena observed until more accurate spectrophoto- 

 metric data are at our disposal. 



SUMMARY. 



I From aromatic ketones and alcohol are formed, in the light, 



exclusively pinacones ; these latter are, therefore, the products 

 to be first isolated in the i-eduction. Hydrols are, in the reduct- 

 ion of the aromatic ketones, always formed secondaiily (see 

 Proc. XVI 9i and 962) either from the pinacones or from 

 the primarily formed half pinacone molecules. 

 II 1. The photo-reduction of the ketone by alcohols was studied 

 by exposing simultaneously to the light a set of tubes of 

 equal dimensions and filled w-ith equal quantities of liquid, 

 thus cansing the light-quantity (i . t) for each object of a 

 serial experiment to be equal. 

 2. The velocity of the pinacone formation appeared to be inde- 

 pendent of the concentration of the benzophenone and propor- 

 tional to the concentration of the alcohol. Hence, it satisfied 

 the equation : 



d pinacone r^, r i -i i i /• 



=: IvL. alcohol and, therefore, the reaction scheme: 



dt ^ ^ 



2 ketone -|- alcohol := pinacone -\- aldehyde. 



