COLOR AND CHEMICAL CONSTITUTION 125 



spond in constitution to the two distinct forms, ketonic and enolic, of 



the free ethyl aceto-acetate. They may be graphically represented as 

 follows : 



Ethyl ethyl-aceto-acetate Ethyl /3-ethoxy-crotonate 

 (ketonic) (enolic) 



H 



3 — C— C— ( 



O C 2 H 5 



CH 3 — C— C— C0 2 C 2 H B CH 3 — C=CH— O0>C 2 H 5 



C 2 H B OC 2 H B 



Upon examination of their absorption spectra, it was observed that the 

 enolic compound exerts only a general absorption without a band, 

 while the ketonic compound was practically free from absorption. This 

 is exactly what might be anticipated from the results of Hartley, who 

 had already shown that no difference exists between the absorption 

 spectrum of the compound and that of its alkyl (here ethyl) de- 

 rivative. Even a mixture of these two alkyl derivatives fails to 

 show the presence of absorption bands in the spectrum. We may 

 conclude, therefore, that an absorption band is not to be attributed 

 ,.to either the one or the other form of a tautomeric substance, but 

 rather to the changing of one form into the other — a dynamical 

 isomerism. If this intramolecular transformation is the source of the 

 disturbance which produces the absorption bands, then an acceleration 

 of this transformation should show itself in the increased persistence 

 of the band, while retardation of the same should diminish this per- 

 sistence. For some time it has been known that alkalies exert a 

 marked positive influence upon the velocity of tautomeric changes and, 

 as may be naturally inferred, acids retard this change. On the addi- 

 tion of a small amount of sodium hydroxide to a solution of ethyl 

 aceto-acetate, the form of the absorption-curve changes at once and a 

 band appears. On the further addition of alkali, the depth of this 

 curve, that is the persistence, increases until it reaches a maximum 

 corresponding to the presence of a large excess of alkali. The absorp- 

 tion-curve of the aluminium derivative of this ester has not the per- 

 sistence of that of the sodium salt when the sodium hydroxide is 

 present in excess of one molecular equivalent. "With the addition of 

 hydrochloric acid a retarded action is developed and even the absorp- 

 tion curve for the free ester is seen to diminish slightly in its persist- 

 ence when an excess of acid is present, indicating, therefore, that the 

 free ester is not entirely ketonic, but is in equilibrium with a very 

 small quantity of its enolic modification. Spectroscopic evidence 

 points out that the persistence of the absorption bands over concentra- 

 tion changes is directly proportional to the number of molecules in the 

 state of oscillation or, in other words, is a measure of the dynamical 

 isomerism between substances in equilibrium with each other. 



From these considerations it is evident that this dynamical isomer- 



