Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

262 DBSMOTKOPISM
In the case of ethyl acetoacetate we thus deduce, by reference to the
table, that in water the keto-ester should be the more soluble, and in
petrol ether the enol-ester ; this is actually the case.
Liquid ethyl acetoacetate consists of 92-5 percent of the keto-form
and 7 -5 per cent of the enol-form. The freshly distilled substance contains
considerably more enol since the enol-ester, because of its lower
boiling point, distils first and then more is formed in the distillation flask.
Experiment.—Ethyl acetoacetate (2-5 g.) is dissolved in 20 c.c. of
iV-alkali hydroxide solution, the solution is cooled in ice to 0° and
20 c.c. of cooled iV-hydrochloric acid are added in one lot, with
shaking. A turbid milky solution is formed which, however,
becomes clear in a few seconds. The enol, which is less soluble
in water than the keto-form, at first separates, but changes very
rapidly and almost completely into the more soluble keto-form, as
the conditions of the equilibrium in water require.
K. H. Meyer's " bromine method " x makes it possible to determine
the enol content in almost all solutions of tautomeric substances.
It has also been possible, in various ways which cannot be detailed
here, to prepare both the keto- and enol-forms of ethyl acetoacetate in
the pure state (Knorr, K. H. Meyer). Their physical constants are
altogether different. The refractive index, for example, is 1-4225
(D10o) for the keto-form and 1-4480 for the enol-form. Prom determinations
of the refractive indices of equilibrium mixtures the content
of both forms can be calculated by interpolation (Knorr, 1911),
and these results have been confirmed spectroscopically (Hantzach,
1910).
Whether or no both forms of a tautomeric substance are capable
of isolation in the free state depends chiefly on the velocity of rearrangement
of the more labile form. The isolation of keto- and
enol-forms in a permanent crystalline state was first carried out with
unsymmetrical dibenzoylacetone (Claisen, 1896) :
(C6H5.CO)2:CH.CO.CH3 and (C6H5CO)2:C=C—CH3 .
OH
For such cases in which the two forms have been brought to light
solely as a result of improved experimental methods, the term " tautomerism
" has been modified to " desmotropism ". In desmotropic
substances the tautomeric relations are therefore particularly clear and
well defined ; numerous examples have become known, which now also
include aceto-acetic ester. The case of acetylacetone is quite similar,
but here the formation of the enol-form is much more favoured. The
liquid substance contains up to 80 per cent of enol.
1 Annalen, 1911, 380, 212.