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Page:Popular Science Monthly Volume 72.djvu/129

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spond in constitution to the two distinct forms, ketonic and enolic, of the free ethyl aceto-acetate. They may be graphically represented as follows:

PSM V72 D129 Chemical formula.png

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) derivative. 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 persistence. 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 addition 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 absorption-curve of the aluminium derivative of this ester has not the persistence 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 absorption curve for the free ester is seen to diminish slightly in its persistence 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 concentration 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-