tion that the formation of an enolate anion results in metal
chelate formation and stability, the present results imply that
changes in pressure within the “normalÏ extraction range
would not drastically change extraction efficiencies. However,
changes in temperature should a†ect the extraction efficiency
and the lowest possible temperature should be used to opti-
mize metal extractions with the b-diketones, ACAC and TFA.
Among the three metal chelating agents, the extraction
efficiencies are expected to decrease in the series
HFA [ TFA [ ACAC.
signiÐcantly di†erent from the excluded volume of the keto
form.
The results for the supercritical Ñuid solutions are qualit-
atively the same as those for the pure liquids with the keto
population increasing as temperature is increased. For ACAC
*H increases by ca. 0.4 kcal mol~1 relative to the neat liquid
at the lower pressures investigated, while at the higher-
pressure of 2168 bar *H decreases by ca. 1.4 kcal mol~1. The
primary di†erence in the higher-pressure system is the
increased density relative to the lower-pressure systems. Also,
as the temperature is increased the density changes for this
higher-pressure system are not as great as those of the lower-
pressure systems. These facts and the results suggest that
molecular packing does play a role, however, from the present
study only a hint of this is observed. The only analysable data
Work at the PaciÐc Northwest Laboratory was supported by
the Office of Energy Research, Office of Basic Energy Sciences,
Chemical Sciences Division of the U.S. Department of Energy,
under Contract DE-AC06-76RLO 1830. Instrumentation
development was supported through the Advanced Processing
Technology Initiative, PaciÐc Northwest Laboratory. Work at
the University of Idaho was supported by a Department of
Energy DOE-EPSCoR traineeship and IdahoÏs NSF-EPSCoR
Program.
for a CO ÈTFA solution indicate that the *H increases by 0.8
2
kcal mol~1 relative to the neat liquid. The disappearance of
the keto wCH w with increased temperature and increased
2
pressure, as well as the appearance of a new peak in this
region, suggests that an exchange process is occurring. Two
forms of the enol (cis and trans) form of TFA are certainly
possible, however, based on the present experimental e†ort we
cannot deÐnitively address this issue. Further investigations
are of interest.
References
The supercritical Ñuid solution data imply that the enol
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1
2
3
4
K. E. Laintz, C. M. Wai, C. R. Yonker and R. D. Smith, J. Super-
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2
K. E. Laintz, C. M. Wai, C. R. Yonker and R. D. Smith, Anal.
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2
would be secondary considerations in determining the concen-
5
6
7
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reduced torsional motion of TFA relative to ACAC. In the
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experimental uncertainty.
8
9
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Conclusions
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The stability of the enol tautomer increases with the degree of
Ñuorination. The earlier reported e†ect of pressure shifting the
equilibrium toward the keto tautomer in ACAC is not
observed. It was seen that the keto tautomers have a greater
entropy than the enol tautomers in both neat ACAC and TFA
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Paper 7/01851G; Received 17th March, 1997
2394
J. Chem. Soc., Faraday T rans., 1997, V ol. 93