50
B.L. Tumanskii et al. / Journal of Fluorine Chemistry 96 (1999) 47±50
2.1. Kinetic measurements of abstraction of the hydrogen
atom from hydrocarbons by radical III
Ketoradical III (5 vol%) was added into an ampoule
containing a hydrogen donor. The solution was stirred
and placed in the thermostatting system of the ESR spectro-
meter equipped with a Unipan electronic temperature con-
troller. Since the solubility of per¯uoroorganic compounds
in hydrocarbons is low, an appreciable portion of the
ketoradical remained at the bottom of the ampoule. The
ampoule was set in such a way that the ketoradical is outside
the resonance cavity. According to the ESR data obtained,
the concentration of the ketoradical III solution in hydro-
carbons listed in Table 1 was 10 3±10 4 mol l 1. Further,
dependencies of the intensity of the ESR spectra of keto-
radical III on time were recorded at different temperatures.
2.2. Kinetic measurements of Va and Vc dimerization
Fig. 2. (a) ESR spectrum of radical Vc at 300 K and (b) multiplet outside
component of ESR spectrum of radical Vc (1 G cm 1).
To obtain kinetic curves, the magnitudes of magnetic ®eld
were set equal to those corresponding to the maxima of the
absorption signals, and after switching off the light the
intensities of the ESR signals with time were recorded.
The concentrations were determined analogously to the
be associated with the fact that addition of the bulky
silicon-centred radical increases the steric hindrance of
the carbon radical centre. This assumption is con®rmed
by the values of the radical±dimer equilibrium enthalpy
(ÁH5.5 kcalmol 1).
procedure described previously [7]; a toluene solution of
1
2,2,6,6-tetramethylpiperidine-N-oxyl (1.5Â10 3 mol l
)
was used as a standard. a-Diketones V, VI and VII were
synthesized according to known procedures [8].
In the presence of (CF3)3COH (concentration 1 M), the
values of effective rate constants of the dimerization were:
2k240 K2, 2k260 K6.5, and 2k280 K10.6 l mol 1 s 1. A
sharper decrease in the dimerization rate compared to Va is
observed in the case of radical Vc. From these facts, it
follows that co-ordination of the hydroxyl hydrogen of the
alcohol molecule with the unshared electron pair of the
carbonyl oxygen is a major factor that decreases the rate of
radical dimerization.
Acknowledgements
This work was ®nancially supported by the Russian
Foundation for Basic Research (Project no. 95-03-08795).
References
Thus, we have shown that a-ketoradicals of the ¯uoro-
aliphatic series can enter the reaction of hydrogen abstrac-
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involved in the reaction of hydrogen abstraction in the form
of O-centred radicals. The rate of dimerization of a-
hydroxy-a-ketoradicals is determined to a great extent by
the formation of intermolecular hydrogen bonds with the
oxygen of the carbonyl group.
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