TAUTOMERISM 127 



for by Laar 1 in a movement of the hydrogen atom, indicated 

 in the following symbol : 



H.CCH C0 2 C 2 H 5 



' O 



It has of late been suggested 2 that tautomerism corre- 

 sponds to a mixture of two isomers in chemical equilibrium. 

 Fundamentally these two conceptions agree, only the first 

 is the molecular-mechanical view of the fact formulated in 

 the latter. We will therefore recount the facts on which 

 the latter formulation is based. 



In the first place the supposed isomerism has been 

 observed in certain cases in closely allied bodies. Wilhelm 

 Wislicenus 3 observed this, e. g., in formylphenylacetic ether, 

 which is very similar to aceto-acetic ether. Obtained from 

 ethyl formate and phenyl-acetate : 



HC0 2 C 2 H 5 + C C H 5 CH 2 C0 2 C 2 H 5 = HCOC (C 6 H 5 ) 



HC0 2 C 2 H 5 + C 2 H 5 OH, 



the body in question behaves as if containing hydroxyl : 

 HCOH = C (C 6 H 5 ) C0 2 C 2 H 6 , 



e. g. it gives an addition product with phenyl-cyanate, 

 OCNC G H 5 . On melting (indefinitely between 60 and 70) 

 an alteration sets in ; solidification does not take place, and 

 the substance, now liquid at ordinary temperatures, retains 

 indeed the same molecular weight, but colours with ferric 

 chloride, and behaves like an aldehyde. This modification 

 is known as the ' aldo-form ' (the former as ' enol- '), and is 

 represented by the formula 



HCOCH(C 6 H 5 )C0 2 C 2 H 5 . 



The reconversion is accomplished by potassium carbonate, 

 in which the solid compound at once dissolves, but the 



1 Berl Bcr. 18. 648, 19. 730. 



2 8. Traube, 1. c. 29. 1715. 



3 1. c. 28. 767 ; Guthzeit, 1. c. 31. 2753. 



