1614
RYBACHENKO et al.
son of Eqs. (13) and (15) shows that the acetyl trans-
fer in reaction (1) is controlled not only by the ther-
modynamic factor as it could be concluded from
Eq. (13) considered separately, but also from the in-
ACKNOWLEDGMENTS
The authors are grateful to the Mianowski Founda-
tion for Science Support (Poland) for assistance.
ternal activation barrier
= 1/2(logkii + logkjj).
This reactivity parameter was introduced for the first
time by Marcus in analysis of the simplest one-
stage reactions [9], and its role and significance in
nucleophilic substitution are only outlined [22, 26],
especially for reactions at the carbonyl center [27, 28],
and require further study.
REFERENCES
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New York: McGraw-Hill, 1969.
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cheskikh reaktsii (Principles of the Quantitative
Theory of Organic Reactions), Leningrad: Khimiya,
1977.
EXPERIMENTAL
3. Litvinenko, L.M. and Oleinik, N.M., Mekhanizmy
deistviya organicheskikh katalizatorov (Mechanisms
of the Action of Organic Catalysts), Kiev: Naukova
Dumka, 1984.
All the salts were prepared and purified as de-
scribed in [13, 29]. 4-(4 -N,N-Dimethylaminostyryl)-
pyridine N-oxide was prepared by the procedure given
in [30]. The nucleophiles were distilled or recrystal-
lized before use. Acetonitrile of anh grade (Aldrich)
4. Trushkov, I.V., Chuvylkin, N.D., Koz’min, A.S., and
Zefirov, N.S., Izv. Ross. Akad. Nauk, Ser. Khim., 1995,
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was stored over 3
molecular sieves before use.
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and Homogeneous Catalysis), Kiev: Naukova Dumka,
1981.
The reaction rate constants were determined with
an Applied Photophysics stopped-flow apparatus
equipped with a temperature-controlled unit ( 0.1 K).
The observed pseudo-first-order rate constants were
calculated by the Guggenheim procedure from the
time dependence of the UV absorption as described
in [8, 31]. The rates of the symmetrical reactions were
determined from the dynamic NMR spectra, which
were taken on a Gemini-200 spectrometer (200 MHz).
As indicator signals we used those of the -protons
of the pyridinium ring in the nucleophile and its acetyl
salt; their shifts ( , ppm) for the Nui AcNu+i pairs
were as follows: i = 1 (8.05, 8.75), i = 2 (8.02, 8.66),
i = 3 (7.98, 8.58), i = 4 (7.97, 8.56), i = 7 (7.90,
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The electronic absorption spectra of the participants
of reaction (1) were taken on a Specord UV Vis spec-
trometer; the optical densities were measured with an
SF-26 spectrophotometer in a temperature-controlled
cell at 298 0.1 K. When calculating the constants, we
used the concentration quantities; the activity coeffi-
cients were not introduced because of the symmetry
of reaction (1). The reproducibility of the measured
equilibrium constants was no worse than 3%; that of
the rates of the identical reactions, better than 15%;
and that of the rates determined by the stopped-flow
procedure, 5 7% for fast (logk2 > 5) and 3 5% for
the other processes.
no. 1, pp. 1 3.
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15. Titov, E.V., Rybachenko, V.I., and Chotiy, C.Ju.,
298 K; solutions of CH3COCl in chloroform (c 5
3
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10 M) were titrated with solutions of N-oxides of
similar concentration.
16. Semenova, R.G., Gol’dshtein, I.P., Grebenyuk, L.V.,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001