G Model
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I. Fechete, V. Jouikov / C. R. Chimie xxx (2014) xxx–xxx
Fig. 4. Dielectric function (e–1)/(2e + 1) for the binary mixtures THF-AN
and EC-AN with different compositions.
Fig. 3. Dependence of the
DEp/Dlog(n) value on the molar fraction of
acetonitrile in binary solvents THF-AN and AN-EC. (8) 1; (7) 2; (–) 3; (,) 4;
(4) 6; (8) 7. GC electrode, T = 298 K.
substrate. With this assumption (corroborated by the
interactions is quite complex [3] and the ‘‘polarity’’ of a
solvent can be described through a number of parameters
[41,42]. In this context, certainly not the most selective [43],
though having a simple physical sense, the solventdielectric
permittivity eis a convenient characteristicof the polarity of
the used medium.
continuity of the Ep vs. log(v) graph upon switching THF-
AN to EC-AN mixture), any change of vibrational modes can
be considered as arising from dielectric-sensitive intramo-
lecular interactions. At 1 mmol Lꢀ1, solubility changes and
aggregation phenomena are neglected. For this model of
solvent polarity, the dielectric function
developed for a diatomic oscillator in a dielectric continuum
with macroscopic dielectric constant , was used (Fig. 4)
[47]. Dielectric permittivities of solvents in binary mixtures
being additive, the resulting has been calculated using the
simplified Shakhparonov equation [48]:
(e–1)/(2e + 1),
The evolution of the slope of the characteristic function
Ep–log(
dielectric permittivity of the binary mixture is shown in
Fig. 3. All selenides, for which transannular Se. . .
v
) for the oxidation of selenides 1–5 with the
e
O
e
interaction is important, exhibit a change of this parameter
from 20 mV to 30 mV upon increasing the medium’s
2
polarity, attesting to
potential-determining reaction of the cations radicals
1+ꢆ–5+ꢆ from disproportionation (kinetic order
= 2) to a
first-order reaction ( = 1). The test compound 7, in which
a progressive evolution of the
ð
eAN ꢀ esolvÞ
e
¼ xeAN þ ð1 ꢀ xÞesolv þ 0:043
(2)
x
eAN þ ð1 ꢀ xÞesolv
a
a
(where x is molar fraction of acetonitrile in the binary
such interactions are absent, shows no dependence of the
nature of the follow-up reaction on solvent polarity.
Selenide 6, whose cation radicals exhibit monomolecular
reaction in AN (Table 1), also shows the dichotomy of
reactivity (Fig. 3), as observed for 1–5; however, this
mixture, AN and solv are low-frequency dielectric permit-
e
e
tivities of acetonitrile and of the co-solvent, respectively).
Corresponding shifts of C5O and N–H vibrations with
solvent polarity are shown in Fig. 5. In IR spectra of neat
compounds, the amide I band of the stretching vibrations
of C5O group in N-(2-(phenylselenyl)hexyl)acetamide 2 is
shifted to 1625 cmꢀ1 compared to N-hexyl acetamide 7 in
change occurs in the media less polar than AN, at
e ffi 16.
The solvatochromism of electronic (UV/Vis) or molecular
(IR) modes is often considered for studying specific
interactions of substrates with the solvent [44]; obviously,
intramolecular interactions show similar trends of optical
properties. However, UV/Vis spectroscopy is precluded for
the case of selenides 1–5 because of strong aromatic
absorption of the PhSe moiety. In place, solution IR
spectroscopy was used [42], because THF, AN, and EC are
all optically transparent in the window of characteristic
amide I and II vibrations [45]. Since it is difficult to separate
specific solute–solvent interactions from purely dielectric
effects [46], a simple model was adopted. In binary solvents,
given a large excess of solvent compared to the selenide (at a
substrate concentration of 1 mmol Lꢀ1, for each molecule of
the selenide there are approximately 104–102 molecules of
solvent, depending on whether it is pure or added as a co-
solvent at ꢅ 1%), solvent–solute interactions (solvation of
C5O and N–H groups and Se) in the whole range of co-
solvent concentrations are supposed to be constant, since
only few solvent molecules can coordinate at once with the
which Se. . .O interactions are absent, C=O = 1645 cmꢀ1. In
v
dilute acetonitrile solutions, this vibration is freed from
solid-state associations and appears at higher frequency
(1670 cmꢀ1), and is progressively shifted to higher
frequencies with a change of polarity of the solvent
(Fig. 5). For amide II band vibration vN–H, an analogous
trend was found, which was even more pronounced. The
influence of Se on amide II absorption (1530 cmꢀ1 in
acetonitrile) is complex, resulting from the complex nature
of this vibration due to the interaction between C–N
stretching and N–H bending modes of the C–N–H
fragment. The fact that vN–H vibration mode is more
sensitive to solvent polarity than vC=O attests to the fact
that Se. . .O interaction prevails over Se. . .H(N) in solution
for neutral selenides. This feature reflects the fact that
Se. . .O interaction in a non-oxidized molecule 2 has a
repulsive and destabilizing character, contrary to stabiliz-
ing Se(+). . .O interaction between positively charged Se and
electron-rich carbonyl oxygen in the cation radical
Please cite this article in press as: Fechete I, Jouikov V. Solvent-induced dichotomy in the oxidation mechanism of alkyl
Fechete, Ioana
