Chemistry Letters Vol.33, No.11 (2004)
1445
tion medium and applying pressures of several hundred mega-
pascals (MPa), we could convert ‘‘slow’’ reactions with a rela-
tively large activation energy, Ea > 50 kJ molꢂ1 from ‘‘activa-
tion-limited’’ to ‘‘fluctuation-limited.’’ At high viscosities, sol-
vent thermal fluctuations became too slow to maintain the
thermodynamic equilibrium between the initial and the transi-
tion state and solvent reorganization process became rate-limit-
ing. At such conditions, the reaction rate decreased with increas-
ing medium viscosity and the so-called dynamic solvent effects
were observed. If the high-pressure mechanism in the present re-
action involves solvent participation, we will find similar viscos-
ity-induced retardations at high pressures in viscous liquids. The
results obtained in GTA and MPD are compared with those in
nonviscous counterparts in Figures 2 and 3, respectively. Obvi-
ously, the pressure dependence was similar at low pressures,
however both in GTA and MPD, the reaction was rather strongly
retarded by an increase in pressure as observed in our previous
studies. These results provide further support to the solvent par-
ticipation at high pressures in the present tautomerization.
Measurements at different temperatures are in progress and
the results will be reported when they are completed.
0.5
0.0
-0.5
k
-1.0
AcOMe
-1.5
GTA
-2.0
0
100 200 300 400 500 600 700
/MPa
P
Figure 2. A comparison of pressure effects in methyl acetate
and GTA.
0.0
References and Notes
1
2
3
4
A. E. Chichibabin, B. Kuindishi, and S. V. Benewolenskaya,
Ber. Dtsch. Chem. Ges., 58B, 1580 (1925).
R. Hardwick, H. S. Mosher, and P. Passailaigue, Trans.
Faraday Soc., 56, 44 (1960).
H. Takahashi, Y. Kobayashi, N. Kaneko, T. Igarashi, and
I. Yagi, J. Raman Spectrosc., 12, 125 (1982).
A. Corval, K. Kuldova, Y. Eichen, Z. Pikramenou, J. M.
Lehn, and H. P. Trommsdorf, J. Phys. Chem., 100, 19315
(1996).
-0.5
k
-1.0
5
D. Ben-Hur and R. Hardwick, J. Chem. Phys., 57, 2240
(1972); A. G. Turovets and V. I. Danilova, Izv. Vyssh.
Uchebn. Zaved., Fiz., 15, 68 (1972); D. Klemm, E. Klemm,
A. Graness, and J. Kleinschmidt, J. Prakt. Chem., 321, 415
(1979); K. Yokoyama and T. Kobayashi, Chem. Phys. Lett.,
85, 175 (1982); Y. Eichen, J.-M. Lehn, M. Scherl, D. Haarer,
J. Fischer, A. DeCian, A. Corval, and H. P. Trommsdorff,
Angew. Chem., Int. Ed. Engl., 34, 2530 (1995); M. Scherl,
D. Haarer, J. Fischer, A. DeCian, J.-M. Lehn, and Y. Eichen,
J. Phys. Chem., 100, 16175 (1996); I. Frank, S. Grimme, and
S. D. Peyerimhoff, J. Phys. Chem., 100, 16187 (1996); Y.
Eichen, M. Botoshansky, U. Peskin, M. Scherl, D. Haarer,
and S. Khatib, J. Am. Chem. Soc., 119, 7167 (1997); I. Frank,
D. Marx, and M. Parrinello, J. Phys. Chem. A, 103, 7341
(1999); S. Khatib, S. Tal, O. Gidsi, U. Peskin, and Y. Eichen,
Tetrahedron, 56, 6753 (2000).
EtOH
MPD
-1.5
0
100 200 300 400 500 600 700
/MPa
P
Figure 3. A comparison of pressure effects in ethanol and
MPD.
the absence of solvent participation. In ethanol, the reaction was
slightly retarded by an increase in pressure suggesting a higher
polarity of the reactant in this solvent and its decrease in the
activation step. At high pressures, however, the reaction was
accelerated both in AcOMe and EtOH. The activation volumes
were ꢂ2:6 ꢃ 0:1 and ꢂ7:8 ꢃ 0:2 cm3 molꢂ1, respectively. This
change in the sign of the activation volume can most reasonably
be explained by the intervention of the solvent molecule.
Namely, the results in Figure 1 strongly suggest the existence
of two competing reaction routes as discussed above.
In order to obtain additional evidence for this mechanism,
we studied viscosity dependence of the reaction rate. In our pre-
vious studies,9 we measured effects of viscosity on the rate of
various isomerizations. By using viscous liquids such as glycerol
triacetate (GTA) or 2-methylpentane-2,4-diol (MPD) as a reac-
6
R. van Eldik, T. Asano, and W. J. le Noble, Chem. Rev., 89,
549 (1989).
T. Asano and T. Okada, J. Phys. Chem., 88, 238 (1984).
The reaction was followed by the absorption decrease at
570 nm. The standard deviation of the rate constant was less
than 6%.
H. Kono, H. Osako, M. Sasaki, T. Takahashi, Y. Ohga, T.
Asano, M. Hildebrand, and N. N. Weinberg, Phys. Chem.
Chem. Phys., 6, 2260 (2004) and earlier papers cited therein.
7
8
9
Published on the web (Advance View) October 2, 2004; DOI 10.1246/cl.2004.1444