224 REPORTS ON THE STATE OF SCIENCE. — 1920. 



/CH, 



■\ 



shows 

 CO 



exhibit strong selective absorption. For example camphor ' CgHj 



a marked band, as also doss the disubstituted compound * 



/CH3 



C8Hh<' \Br 



in which no tautomeric equilibrium seems possible. Again, azo-iso-butyronitrile 

 shows marked selective absorption. 



CH3\. yCHj 



\c-N = N-C<' 

 ^ I ^CH, 



CH. 



CN NO 



The most interesting example of a compound which exhibits an absorption band is 

 chloropicrin, CCl^NO,, which does not contain any hydrogen atoms at all. It 

 may be noted that Hantzsch has taken up the position that there is a definite 

 correlation between constitution and absorption, and he has published very many 

 papers in support of his theory. The starting-point of the theory is the 

 derivatives of ethyl acetoacetate which have already been referred to. He 

 showed that ethyl dimethylacetoacetate, which is an absolutely definite ketonic 

 compound, exhibits only slight general absorption. The enolic derivative ethyl 

 yS-ethoxycrotonate at equal molecular concentration exhibits more strongly marked 

 general absorption. Hantzsch assumes « that the absorption curves are truly 

 characteristic of the ketonic and enolic forms respectively. He then assumes 

 that the absorption band shown by the metallic derivatives of ethyl acetoacetate 

 is due to the constitution where M stands for a monovalent metal. The novelty 



CH3 



I 



/\ 



H2C O 



I I 



yC M 



O 



of the conception lies in the mutual influence of the secondary valencies or 

 residual affinities of the metal and oxygen atoms, this influence being denoted 

 by the dotted line in the formula. It will be seen that this explanation of 

 selective absorption does not involve any liable atoms but _ attributes the 

 phenomenon to secondary valencies. Starting from this original assumption 

 Hantzsch has built up a complete theory of a direct correlation between absorp- 

 tion and constitution which states that if a substance exhibits different absorption 

 curves under different conditions of solvent, &c.. this is due to a definite 

 change in constitution. It is not worth while to describe in detail the conclusions 

 which Hantzsch arrives at as regards the specific compounds examined by him,!" 

 such, for instance, as the variety of absorption bands shown by compounds of 

 an acid type when dissolved in different basic solvents, each different absorption 

 band being attributed to a different structure of the compound. It is perhaps 

 worthy of mention that Hantzsch finds it necessary to confess that in some cases 

 the variations in absorption shown by certain compounds are more numerous 

 than can be accounted for by changes in constitution. 



It may be stated at once that there are several very grave objections to 



