Page:Popular Science Monthly Volume 72.djvu/128

124 observations made it was discovered that an acid and its alkyl ester (esters formed by such, groups as methyl, CH3, ethyl, C2H5, 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 substances 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:

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 condition 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 derivatives of ethyl aceto-acetate were examined. These compounds made by entirely different processes have different properties and