Recombination of 1,1-dimethylpropyl peroxy radicals in polar solvents

Article abstract of DOI:10.1134/S0023158416010031

The kinetics of 1,1-dimethylpropyl peroxy radicals recombination in polar solvents - water, methanol, and their mixtures - was studied by EPR spectroscopy in combination with the stopped-flow method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing solutions of Ce⁴⁺ sulfate and 1,1-dimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radical is a singlet with a g-factor of 2.015 ± 0.001, and a line width of ΔH = (1.36 ± 0.02) × 10^-3 T for methanol and ΔH = (9.7 ± 0.2) × 10^-4 T for water. The measured rate constants of (CH₃)₂C(O₂ ^·)CH₂CH₃ radical recombination at 298 K are 2k _t = (3.9 ± 0.4) × 10⁴ L mol^-1 s^-1 for water and 2k _t = (5.2 ± 0.5) × 10³ L mol^-1 s^-1 for methanol. A linear relationship between ln(2k_t ) and the Kirkwood function (?-1)/(2? + 1), where e is the dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation in the recombination of tertiary peroxyl radicals.

Full text of DOI:10.1134/S0023158416010031

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2016, Vol. 57, No. 1, pp. 22–25. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © T.G. Denisova, V.F. Shuvalov, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 1, pp. 25–28.
Recombination of 1,1ꢀDimethylpropyl Peroxy Radicals
in Polar Solvents
T. G. Denisova* and V. F. Shuvalov†
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
*
eꢀmail: denisova@icp.ac.ru
Received March 11, 2015
Abstract—The kinetics of 1,1ꢀdimethylpropyl peroxy radicals recombination in polar solvents—water,
methanol, and their mixtures—was studied by EPR spectroscopy in combination with the stoppedꢀflow
method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing
4+
solutions of Ce sulfate and 1,1ꢀdimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radꢀ
–
3
ical is a singlet with a
and = (9.7 0.2)
bination at 298 K are
methanol. A linear relationship between ln(2 t
g
ꢀfactor of 2.015
±
0.001, and a line width of 10 T for methanol
O
10 T for water. The measured rate constants of (CH ) C( )CH CH3 radical recomꢀ
2
Δ
H
= (1.36
±
0.02)
×
–4
i
Δ
H
±
×
3
2
2
2
kt = (3.9
±
0.4)
×
104 L mol–1 s–1 for water and
2
kt = (5.2
ε
±
0.5)
– 1)/(2
×
103 L mol–1 s–1 for
+ 1), where is the
k
)
and the Kirkwood function (
ε
ε
dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation
in the recombination of tertiary peroxyl radicals.
Keywords: 1,1ꢀdimethylpropyl peroxy radical, rate constant, electron paramagnetic resonance, stoppedꢀflow
method, nonspecific solvation, Kirkwood function
DOI: 10.1134/S0023158416010031
INTRODUCTION
media. The issue of how the polarity of the medium
affects the recombination of tertꢀalkyl peroxy radicals
remains unclear, but this issue is very important,
because, in hydrocarbon oxidation, polar oxygenated
products accumulate and the medium becomes polar.
For the recombination of secondary peroxyl radicals,
this issue was studied and the effect of polar solvents on
Radical reactions involving peroxyl radicals
occupy a central place in the liquidꢀphase oxidation of
hydrocarbons. These radicals lead a chain process, and
chain termination occurs as a result of their recombiꢀ
nation. In the oxidation of branched hydrocarbons
containing a tertiary carbon atom, it is primarily terꢀ
tiary peroxyl radicals that form and lead the chain
reaction. Their recombination was studied, and it was
proved that it takes place in several steps [1, 2].
2kt was elucidated [4]. The aim of this study is to deterꢀ
mine the absolute values of the rate constants for the
recombination of 1,1ꢀdimethylpropyl peroxy radicals
in polar media (water, methanol, and their mixture).
For this purpose we used EPR spectroscopy in combiꢀ
nation with the stoppedꢀflow (SF) method [5, 6]. In
the SF method implemented by Shuvalov [5],
approaches described in a monograph by Caldin were
used [6].
i
2
i
RO + RO ↔ ROOOOR
(
KROOOOR
)
2
i
i
ROOOOR → [RO + ROOO ]
kd
( )
i
i
ROOO → RO + O
(
rapidly
)
)
2
i
i
i
(
kD
[
RO + RO ] → 2RO
i
i
EXPERIMENTAL
[
RO + RO ] → ROOR
.
(
kc)
The experimentally observed reaction rate constant
kt is a combination of constants: kt KROOOOR kd
. The expression for kt was obtained in
Chemicals
2
2
2
=
×
×
1
,1ꢀDimethylpropyl
hydroperoxide
kc/(kD
+
kc
)
(CH ) C(O H)CH CH (DMPH), which was the
3
2
2
2
3
the quasiꢀsteadyꢀstate approximation with respect to
the labile intermediates [2]. For this reaction, there is
a large data array on the recombination of tertꢀbutyl
and cumyl peroxide radicals [1, 3]. However, this reacꢀ
tion was studied mainly in nonpolar (hydrocarbon)
source of 1,1ꢀdimethylpropyl peroxy radicals
i
(
CH ) C( )CH CH3, was synthesized by the reacꢀ
O
3 2 2
2
tion of hydrogen peroxide with isoamylsulfonic acid
C, 1 : 1) [7]. Isoamylsulfonic acid was prepared by
adding a sevenfold excess of 65% H SO4 to a cooled
(0°
2
†
Deceased.
(–10 to 5°C) of 2ꢀmethylbutꢀ2ꢀene. Crude DMPH
22

RECOMBINATION OF 1,1ꢀDIMETHYLPROPYL PEROXY RADICALS
23
was isolated as a sodium salt, by treating the hydroperꢀ
oxide with 31% NaOH at . The resulting crystals
0°C
were washed with acetone and dissolved in distilled
water, and the hydroperoxide was isolated by passing
CO2 through the solution. The resulting DMPH was
separated and was vacuumꢀdistilled three times at 11–
1
2 Torr and 46–47°C. For quantitative measurements,
fractions containing ~85–95% hydroperoxide were
used. The main impurity was tertꢀamyl alcohol.
Δ
H = 1.36 mT
Ammonium cerium nitrate [Ce(NH ) (NO ) · 2H O]
4
2
3 6 2
(
analytical grade) was used as the oxidizer. Sulfuric
acid was reagentꢀgrade. The Solvents: methanol (reagent
grade) was subjected to distillation, and water was twice
distilled. After purification, the solvents had constants
coinciding with the corresponding reference data: water,
2
0
3
bp = 100.0
00.00°C (760 Torr),
anol, bp = 64.7°C (758 Torr),
°C
(758 Torr),
d
= 0.998 g/cm (bp =
4
3
1
d
= 1.000000 g/cm [8]); methꢀ
20
3
d
= 0.7925 g/cm
20
3
(
bp = 64.6°C (760 Torr), d = 0.7923 g/cm [8]).
Kinetic Measurements
A setup consisting of a modified EPR spectrometer
Fig. 1. The EPR signal under the SF conditions from the
,1ꢀdimethylpropyl peroxy radical in methanol: ꢀfactor of
.015 0.001, line width of = (1.36 0.02)
DMPH] = 5.04 10 mol/L; oxidizer concentration of
1
2
[
g
×
–3
±
Δ
H
±
10 T,
EPRꢀ2ꢀIKhF (REꢀ1301) with a modulation freꢀ
quency of 117 kHz, equipped with a system stabilizing
the ratio of the microwave field frequency to the magꢀ
i
–4
×
0
4+
[Ce ]0 = 0.02 mol/L,
T = 298 K.
netic field strength, was used to measure [RO ]. The
2
i
2
the (CH ) C(
O
)CH CH3 radical was observed. This
microwave path of the spectrometer was based on a cylinꢀ
3 2
2
drical transmissionꢀline ТЕ011ꢀmode resonator. The radical formed as a result of the fast reaction
15
i
2
lower sensitivity limit of the spectrometer was 10 spin/T
5]. All experiments were performed under turbulent
flow conditions at = 298 0.5 K.
4+
Ce + RO + H+.
3+
Ce + ROOH
→
[
The EPR signal for the 1,1ꢀdimethylpropyl peroxy
T
±
radical at 298 K is a symmetric singlet with a
of 2.015
and a line width of
methanol and = (9.7
g
ꢀfactor
The decay kinetics of 1.1ꢀdimethylpropyl peroxy
radicals was studied by the SF method [5]. Reactant
solutions (0.15 mL) were injected using a 2ꢀmL
syringe. For a quick stop of the flow, the setup was
equipped with a specialꢀpurpose syringe device. This
made it possible to monitor the reaction progress startꢀ
ing at 0.01–0.02 s after mixing the reactants and to
study the kinetics of the reactions taking place within
±
0.001, which is typical of peroxyl radicals,
–
3
ΔH
= (1.36
0.2)
±
0.02)
×
10 T for
–
4
ΔH
±
×
10 T for water. A
typical EPR spectrum of the peroxyl radical is shown
in Fig. 1.
Determination of 2kt for the Recombination
of 1,1ꢀDimethylpropyl Peroxy Radicals
1
min. The changes in the amplitude of the signal were
recorded with an Nꢀ320 highꢀspeed recorder.
i
2
To determine the rate constant
2
kt, the RO disapꢀ
The starting solution contained 2.0
×
×
10–4 to
10 mol/L DMPH and 0.02–0.05 mol/L tetꢀ
ravalent cerium ammonium nitrate Ce(NH ) (NO )
pearance kinetics was fitted to the bimolecular reacꢀ
tion equation
–3
1.0
·
4
2
3 6
i
2
i −1
2 0
–1
[
RO ] =[RO ] + 2ktt
.
(1)
2
H O. Sulfuric acid at a concentration of 0.07 mol/L
2
i
2
was added to the aqueous solutions to avoid the
hydrolysis of the salt. The oxidizer was introduced in
excess so that the hydroperoxide was almost instantly
i
RO ]–1 versus time
Typical plots of
[
t
for the
recombination of 1,1ꢀdimethylpropyl peroxy radicals
in water and methanol are presented in Fig. 2.
and stoichiometrically converted into RO .
2
RESULTS AND DISCUSSION
Primary Data Processing
Effect of the Medium on the Recombination
of Tertiary Peroxyl Radicals
After a Ce4+ nitrate solution was mixed with an
aqueous or methanolic solution of DMPH under SF
The recombination rate constants 2k of 1,1ꢀdimeꢀ
t
conditions in a EPR resonator, the EPR spectrum of thylpropyl peroxy radicals in water, methanol, and
KINETICS AND CATALYSIS Vol. 57
No. 1
2016

24
DENISOVA, SHUVALOV
•
–1
–4
ity of the medium strongly influences the recombinaꢀ
[
RO ] × 10 , mol/L
2
i
tion of tertiary RO radicals. For methanol (
ε
= 32.6),
kt
2
4
2kt = 0.52
×
10 L/(mol s); for water (
ε
= 78.5), the
2
value is almost one order of magnitude larger and is
4
3
2
1
0
4
3
.9
the literature. According to Howard [10], in nonpolar
CCl ( = 2.23) 1,1ꢀdimethylpropyl peroxy radicals
×
10 L/(mol s) [9]. This trend is in agreement with
ε
4
2
recombine with
= 1.79), with
2kt = 4.5
×
10 L/(mol s); in CCl2F2
102 L/(mol s) (303 K).
(ε
2kt = 2.0
×
Thus, the polarity of the medium is obviously a key
i
factor in the rate of recombination of tertiary RO radꢀ
2
i
2
icals. Both RO and the intermediate peroxides forming
i
2
from it (see the above kinetic scheme of RO recombiꢀ
nation) are polar species, and, in a polar solvent, they
are solvated. According to the Kirkwood model, the
Gibbs energy of solvation of the species is [11]:
2
1
2
3
t
, s
RO ]–1 versus time
i
Fig. 2. Typical plots of
nation of 1,1ꢀdimethylpropyl peroxy radicals in polar solꢀ
vents:
[ t for the recombiꢀ
2
N μ ε −1
A
ΔGsolv = −
,
(2)
–
4
3
᭹, water;
᭺
, methanol;
[
RO2]0 = 5.04
×
10 mol/L;
4πε r 2ε + 1
0
[
Ce4+]0 = 0.02 mol/L;
T
= 298 K.
where NA is the Avogadro constant,
moment of the species,
the medium (which is considered as a continuum),
μ
is the dipole
ε
is the dielectric constant of
ε
0
ln(2kt)
is the vacuum permittivity, and r is the radius of the
species. With this approach, the rate constant for polar
1
0.5
0.0
species can be represented as
2
NA ε −1 Σμ
i
⎛
k
c
⎞
1
RT ln(2k ) = −
+ RT ln ⎜
,
(3)
t
3
⎟
4
πε 2ε + 1 ri
⎝k + k ⎠
D c
0
where
μ
is the dipole moment of the species (reactant
9.5
9.0
8.5
i
or product) and ri is its radius. The sign of the algebraic
sum in Eq. (3) influences the dependence of ln on
– 1)/(2 + 1), which can have both a positive and a
negative slope. Figure 3 and the table illustrate this
dependence for ln(2 t for 1,1ꢀdimethyl propylperoxy
k
(ε
ε
k
)
radicals in MeOH–H O polar solvent mixtures . The
dielectric constant of the mixtures was calculated via
2
0
.476
0.480
0.484
0.488
– 1)/(2
0.492
+ 1)
(ε
ε
the additivity equation
ε
=
ε
H2OVH2O
values taken from a handbook
MeOH
+
ε
VMeOH
MeOH
[L mol–
1 –1
s
] on (
ε
– 1)/(2
ε
+ 1)
with the and
ε
ε
Fig. 3. Dependence of ln(2kt
for 1,1ꢀdimethylpropyl peroxy radicals in polar solvents:
water, methanol, and their mixtures; = 298 K.
)
H2O
[9]; VH2O and VMeOH are the volume fractions of water
T
and methanol, respectively.
As shown in Fig. 3, for all of the
2
kt values there is
their mixtures measured by the method described a good linear correlation between ln(2 t
k
)
and (
ε
–
above are presented in the table. From a comparison of 1)/(2
ε
+ 1) at
ε
= 33–78, which is given by
2kt values for different media, it follows that the polarꢀ
ln(2kt) ~ (157.3
±
6.3) (ε – 1)/(2ε + 1).
(4)
Recombination rate constants (2kt) for 1,1ꢀdimethylpropyl peroxy radicals in water, methanol and their mixtures, meaꢀ
sured by EPR SF, and the dielectric constants of the media (
ε)
Solvent
kt, L mol–1 s–1
2
ε
(ε
– 1)/(2ε + 1)
H O
2
(3.9 ± 0.4)
(2.7 ± 0.2)
(1.5 ± 0.1)
(5.2 ± 0.5)
×
×
×
×
104
104
104
103
78.54
59.72
46.86
32.63
0.4901
0.4875
0.4842
0.4770
H O/MeOH (59 : 41) vol %
2
H O/MeOH (31 : 69) vol %
2
MeOH
[
DMPH] = (0.2–1.0)
×
10–3 mol/L, [Ce4+]0 = 0.02–0.05 mol/L,
T
= 298 K.
0
KINETICS AND CATALYSIS Vol. 57
No. 1
2016

RECOMBINATION OF 1,1ꢀDIMETHYLPROPYL PEROXY RADICALS
25
i
2
in Organic Chemistry, Boca Raton, Fla.: CRC, Taylor
and Francis Group, 2005.
. Emanuel’, N.M., Zaikov, G.E., and Maizus, Z.K., Rol’
sredy v radikal’noꢀtsepnykh reaktsiyakh okisleniya
organicheskikh soedinenii (Role of the Medium in Freeꢀ
Radical Chain Oxidation of Organic Compounds),
Moscow: Nauka, 1973.
Since the recombination of tertiary RO radicals is
a cascade of consecutive reactions, this relationship
cannot be assigned to any elementary step.
Thus, in this EPR study in the SF mode, we meaꢀ
sured the rate constants for the recombination of
4
1
,1ꢀdimethylpropyl peroxy radicals in polar solvents,
namely, water, methanol, and their mixtures and
obtained experimental evidence that nonspecific solꢀ
vation is an important factor in the solvent effect on
this process.
5
. Shuvalov, V.F., Cand. Sci. (Chem.) Dissertation, Cherꢀ
nogolovka, Moscow oblast: Inst. of Chemical Physics,
1977.
6
. Caldin, E.F., Fast Reactions in Solution, Oxford: Blackꢀ
well, 1964.
7
. Degtyareva, T.G., Solyanikov, V.M., and Denisov, E.T.,
Neftekhimiya, 1972, vol. 12, no. 6, p. 854.
ACKNOWLEDGMENTS
We sincerely thank Candidate of Chemistry
N.I. Shuvalova, Doctor of Chemistry E.T. Denisov,
and Candidate of Chemistry S.Ya. Gadomsky for
helpful advice in the preparation of this manuscript.
8. Spravochnik khimika (Chemist’s Handbook), vol. 2:
Osnovnye svoistva neorganicheskikh i organicheskikh
soedinenii (Main Properties of Inorganic and Organic
Compounds), Nikol’skii, B.P., Ed., Leningrad: Goskꢀ
himizdat, 1963, 2nd ed.
9
.
Handbook of Chemistry and Physics, Lide, D.R., Ed.,
Boca Raton, Fla.: CRC, Taylor and Francis Group,
REFERENCES
2004–2005, 85th ed.
1
2
3
. Denisov, E.T. and Afanas’ev, I.B., Oxidation and Antioxꢀ
idants in Organic Chemistry and Biology, Boca Raton,
Fla.: CRC, Taylor and Francis Group, 2005.
10. Howard, J.A., in Advances in Free Radical Chemistry,
Williams, G.H, Ed., London: Logos, 1972, vol. 4, p. 49.
1
1. Denisov, E.T., Sarkisov, O.M., and Likhtenshtein, G.I.,
Khimicheskaya kinetika (Chemical Kinetics), Moscow:
Khimiya, 2000.
. Mill, T. and Hendry, D.G., in Comprehensive Chemical
Kinetics, Bamford, C.H. and Tipper, C.F.H, Eds.,
Amsterdam: Elsevier, 1980, vol. 16, p. 1.
. Denisov, E.T., Denisova, T.G., Trepalin, S.V., and
Drozdova, T.I., Data Base of Oxidation and Antioxidants
Translated by A. Pashigreva
KINETICS AND CATALYSIS Vol. 57
No. 1
2016