22 



THE ABSORPTION SPECTRA OF SOLUTIONS. 



others where the theory of steric hindrance would lead to conclusions directly 

 contrary to the facts. According to this theory acetacetic ester exists as 



CH 3 C = CH COOC 2 H 5 

 OH 



CH 3 C C H COOC 2 H 5 

 O H 



During this dynamic equilibrium there are periods during which a nas- 

 cent carbonyl group exists. From analogy to the action of other nascent 

 substances, this would occasion a very much greater reactivity. 



Stewart and Baly, on further investigation, found that the absorption 

 band in this case has a persistence which decreases proportionately to the 

 decrease of reactivity of the ketone's carbonyl group. It has been noticed by 

 chemists that the velocity of tautomeric change depends on the solvent. 

 Stewart and Baly also found that the persistence of the absorption band was 

 very different in aqueous and alcoholic solutions. In the case of pyruvic 

 ester they show that the facts may be represented by the scheme 



CH 3 C C OC,H 6 



II II 



o o 



CH 3 C = C OC 2 H 

 00 



In tautomerism we have a wandering of the hydrogen atom, so Stewart and 

 Baly propose to call this process isorropesis (from the Greek word toopponela, 

 equipoise), or an oscillation in the carbonyl grouping. By the persistence of 

 absorption bands it is possible to measure the activity of chemical compounds. 

 The band produced by the isorropesis is also much nearer the red than that 

 produced by the process of enol-keto tautomerism. An example of isorropesis 

 would be the quinone band l/\ = 2480, which would be due to the following 

 change : 



O 



C- 

 \ 



-0 



C 



c 



c 



ic 



cl 



-0 



c 



II 



o 



In general, then, according to the theory of Baly, Desch, Stewart, and 

 Collie, any absorption by organic compounds is due to the conditions that 

 occur during isomeric changes. Benzene, for example, appears in two forms, 

 and the absorption spectrum is due to a condition of benzene while it is chang- 

 ing from one form to the other. This theory explains quite well the action of 

 chromophores, since, as we have seen, every chromophore contains at least 

 one double bond. 



