i2 4 POPULAR SCIENCE MONTHLY 



observations made it was discovered that an acid and its alkyl ester 

 (esters formed by such, groups as methyl, CH 3 , ethyl, C 2 H 5 , etc.) gave 

 identical absorption bands; a fact that pointed conclusively to the 

 identity in molecular constitution existing between the two compounds. 

 But among the most interesting cases bearing upon the relation of 

 these absorption bands to chemical constitution stand the two sub- 

 stances acetyl acetone and ethyl aceto-acetate. We assume that each 

 of these compounds can exist in either of two forms — one in which 

 an oxygen atom is doubly linked to a carbon atom which bears in turn 

 two carbon radicals and thus forms a so-called ketone ; the other, where 

 this same oxygen atom is singly linked to the carbon atom in question 

 and has its second affinity absorbed in a hydrogen atom, thus forming 

 a so-called hydroxyl derivative, or one usually designated by the term 

 enolic. The two forms may be graphically represented thus : 



Ethyl Aceto-acetate. 



CH 3 — C— CH 2 — C=0 CH 3 — C=CH— C=0 



OC 2 H B OC 2 H 5 



(Ketonic) H (Enolic) 



■ 



Acetyl Acetone. 



CH 3 — C— CH 3 — CO— CH, OH 3 — C=CH— CO— CH 3 



B J. 



I 



(Ketonic) H (Enolic) 



Such compounds are described as tautomeric, i. e., they contain a 

 labile atom, hydrogen, which in its wandering or change of position 

 brings into existence two distinct modifications of a compound without 

 altering its general structure. As this change is not complete at any 

 one instant and may vary with change of conditions, we have a condi- 

 tion of equilibrium always existing between the two forms. In the 

 compounds just cited the labile hydrogen atom may be replaced by the 

 atom of a metal and thus give what are called metallic derivatives, 

 which from chemical evidence are supposed to exist entirely in the 

 enolic form. Upon examination of the absorption spectra of these 

 compounds, acetyl acetone itself, as well as its aluminium derivative, 

 was found to give similarly banded absorption, but with that of the 

 aluminium salt showing a greater persistence. Now ethyl aceto- 

 acetate gives only a slight general absorption without trace of a band. 

 Its aluminium derivative, however, gives a banded spectrum which 

 bears a great similarity to the spectrum of acetyl acetone. Therefore, 

 if the metallic salts are enolic, as chemical evidence strongly favors, 

 the free ethyl aceto-acetate certainly must be ketonic. 



In order to investigate this matter more closely the two ethyl de- 

 rivatives of ethyl aceto-acetate were examined. These compounds 

 made by entirely different processes have different properties and corre- 



