Thermolysis of methanolic solutions of acetyl propionyl peroxide

Article abstract of DOI:10.1023/A:1012399605722

Kinetic measurements by ¹H NMR spectroscopy and chemically induced dynamic nuclear polarization (CIDNP) effects were used to study thermolysis of solutions of acetyl propionyl peroxide in methanol-d₄. It is found that the thermolysis may occur both by radical and by nonradical mechanisms. A scheme of the thermolysis is proposed. Parameters of the Arrhenius equation for the rate constants of the decomposition of the peroxide and the decarboxylation of acyloxyl radicals are obtained.

Full text of DOI:10.1023/A:1012399605722

Russian Journal of General Chemistry, Vol. 71, No. 4, 2001, pp. 614 622. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 4, 2001,
pp. 659 667.
Original Russian Text Copyright
2001 by Skakovskii, Stankevich, Lamotkin, Tychinskaya, Rykov.
Thermolysis of Methanolic Solutions
of Acetyl Propionyl Peroxide
E. D. Skakovskii, A. I. Stankevich, S. A. Lamotkin, L. Yu. Tychinskaya, and S. V. Rykov
Institute of Physical Organic Chemistry, Academy of Sciences of Belarus, Minsk, Belarus
Received June 2, 1999
Abstract Kinetic measurements by 1H NMR spectroscopy and chemically induced dynamic nuclear
polarization (CIDNP) effects were used to study thermolysis of solutions of acetyl propionyl peroxide in
methanol-d4. It is found that the thermolysis may occur both by radical and by nonradical mechanisms. A
scheme of the thermolysis is proposed. Parameters of the Arrhenius equation for the rate constants of the
decomposition of the peroxide and the decarboxylation of acyloxyl radicals are obtained.
Thermolysis of diacyl peroxides has been consi-
dered in sufficient detail in [1 3]. The most thorough-
ly explored is their thermolysis in inert hydrocarbon
solvents. While alcohols are available and convenient-
in-operation solvents, there have only a few works
[2 6] that deal with possible mechanisms of the re-
actions of diacyl peroxides. Thermolysis of benzoyl
peroxide in ethanol, isopropanol, and tert-butanol
[2 5], of acetyl peroxide in isopropanol, isobutanol,
and tert-butanol [4], and of acetyl benzoyl peroxide
in methanol, isopropanol, and butanol [6] have been
studied.
quartet of ethyl ethanoate (CH2 4.29 ppm), and
propane multiplets with mixed type of polarization
(CH2 1.53 ppm, E + E/A; CH3 1.10 ppm, A + E/A;
here A is enhanced absorption, E is emission, E/A is
(a)
The cited works all showed that peroxides are
thermolyzed by a radical mechanism to give al-
dehydes and ketones. No attention in these works was
paid to nonradical mechanisms. At the same time,
some information [7, 8] on possible nonradical trans-
formations of peroxides in polar solvents is available.
The aim of the present work was a detailed study
of thermolysis of a diacyl peroxide in an alcohol. As
the object for study we chose one of the simplest un-
symmetrical diacyl peroxides, acetyl propionyl per-
oxide (I) whose thermolysis gives rise to compounds
II XVIII which can readily be identified by NMR
spectroscopy.
(b)
As the solvent we took methanol-d4, the simplest
alcohol, in which, according to our expectations, a
narrower range of radical reaction products would
form compared with other normal alcohols, thus
facilitating analysis.
5
4
3
2
1
, ppm
1
1
Figure 1a depicts the H NMR spectrum of com-
Fig. 1. The H NMR spectra of 0.1 M solutions of acetyl
pound I, obtained in the course of thermolysis at
353 K. The spectrum shows an ethene signal
(5.57 ppm) with enhanced absorption, an emission
propionyl peroxide in (a) methanol-d and (b) carbon
tetrachloride at 353 K, measured in the course of
thermolysis.
4
1070-3632/01/7104-0614$25.00 2001 MAIK Nauka/Interperiodica

THERMOLYSIS OF METHANOLIC SOLUTIONS OF ACETYL PROPIONYL PEROXIDE
615
Table 1. Yields (%) of thermolysis products of 0.1 M solutions of peroxide I in methanol-d4 as a function of thermolysis
time at 323 K
Time, h
Comp.
Compound
no.
5
10
20
30
50
70
I
II
III
IV
V
CH3CH2C(O)OOC(O)CH3 91.4 2.7
82.7 1.2
2.2 0.2
4.9 0.5
0.9 0.1
0.1 0.1
1.9 0.2
1.5 0.2
1.9 0.2
1.4 0.1
0.6 0.1
2.8 0.3
1.8 0.2
4.3 0.4
1.9 0.2
1.2 0.1
3.4 0.3
0.6 0.1
1.0 0.1
69.1 2.0
3.9 0.2
8.8 0.9
0.9 0.1
0.2 0.1
3.2 0.3
3.9 0.4
4.7 0.5
3.4 0.3
1.3 0.1
4.7 0.3
3.0 0.3
7.4 0.6
3.2 0.3
2.0 0.2
6.9 0.7
1.0 0.1
1.6 0.2
57.1 1.9
5.3 0.3
12.2 1.0
0.9 0.1
0.3 0.1
4.3 0.4
6.5 0.7
6.5 0.7
5.1 0.5
1.8 0.2
6.1 0.6
3.9 0.4
10.0 0.8
4.2 0.4
2.5 0.3
9.6 0.9
1.3 0.1
2.2 0.2
39.4 1.5
7.2 0.7
17.3 0.9
0.8 0.1
0.4 0.1
5.8 0.5
9.1 0.9
9.2 0.9
7.4 0.7
2.5 0.3
7.6 0.8
4.8 0.5
13.3 1.0
5.5 0.4
3.0 0.3
13.3 1.3
1.6 0.2
2.9 0.3
27.3 2.8
8.2 0.8
20.7 0.8
0.7 0.1
0.4 0.1
6.7 0.7
10.3 1.0
11.1 1.0
8.7 0.8
3.1 0.3
8.7 0.9
5.2 0.5
15.0 1.4
6.2 0.6
3.1 0.3
15.4 1.5
1.8 0.2
3.4 0.3
CH3CH2OC(O)CH3
CH3C(O)OD
C2H4
1.1 0.1
2.5 0.2
0.9 0.1
0.1 0.1
1.0 0.1
0.3 0.1
0.4 0.1
0.3 0.1
0.2 0.1
1.5 0.1
0.9 0.1
2.2 0.2
0.9 0.1
0.6 0.1
0.9 0.1
0.3 0.1
0.5 0.1
CH3CH2C(O)OCH3
C3H8
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
CH4
CH3D
C2H6
C2H5D
CH3C(O)OCD3
CH3CH2C(O)OCD3
CH3CH2C(O)OD
CH3CH2OD
CH3OD
CH3CH(OCD3)C(O)OD
XVII CH3CH2OCD3
XVIII C4H10
Table 1. (Contd.)
multiplet polarization, where the first letter relates to
polarization of the dowlfield part of the multiplet, and
the second, to the upfield part of the multiplet).
Similar spectrum is observed at 343 K, but the lines
of products with polarized protons are 3 times
weaker. Analysis of the CIDNP effects led us to con-
clude that the polarization arises in acetyloxyl ethyl
and methyl ethyl radical pairs (RP), which is nicely
consistent with our previous data for thermolysis of
the same peroxide in carbon tetrachloride [9].
Time, h
150
Comp. no.
100
250
I
15.6 1.6
8.9 0.9
24.1 2.5
0.5 0.1
0.5 0.1
7.5 0.7
11.1 1.1
12.8 1.3
9.8 1.0
3.5 0.4
9.7 1.0
5.3 0.5
16.0 1.5
6.4 0.6
3.0 0.3
17.0 1.6
1.9 0.2
3.8 0.4
6.1 0.6
8.8 0.9
26.8 1.5
0.3 0.1
0.6 0.1
8.5 0.7
11.8 1.0
14.3 1.3
10.0 1.0
3.8 0.4
11.2 1.2
5.2 0.5
15.7 1.5
6.2 0.7
2.9 0.3
17.0 0.4
2.0 0.2
4.3 0.4
0.9 0.1
8.2 0.9
28.2 2.9
0.2 0.1
0.6 0.1
9.9 1.0
12.0 1.2
15.1 1.5
9.3 1.0
3.8 0.4
14.0 1.5
5.1 0.5
14.3 1.2
5.7 0.3
3.0 0.3
14.3 1.4
2.1 0.2
5.0 0.5
II
III
IV
V
To gain a deeper insight into the mechanism of the
thermolysis of compound I, we performed kinetic
measurements. The resulting data are presented in
Tables 1 3.
VI
VII
VIII
IX
Tables 1 3, along with the concentrations of per-
oxide I at various time, show data on the accumula-
tion of principal identified products. The main product
of thermolysis of peroxide I in methanol-d3 are
ethanoic (III), propanoic (XIII), and 2-(methoxy-d3)-
propanoic (XVI) acids, ethyl ethanoate (II), (methyl-
d3) ethanoate (XI), methane (VII), methane-d (VIII),
and ethane (IX). The final yields of these compounds
are 10% or higher (macro products). The identified
microproducts (final yield lower 10%) include
(methyl-d3) propanoate (XII), methanol-d (XV),
ethanol-d (XIV), (methyl-d3) ethyl ether (XVII),
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

616
SKAKOVSKII et al.
Table 2. Yields (%) of thermolysis products of 0.1 M solutions of peroxide I in methanol-d4 as a function of thermolysis
time at 333 K
Time, h
25
68.4 4.0 46.8 0.3 32.0 0.2 21.7 0.2 15.0 0.5 10.2 0.2
Comp.
no.
5
10
15
20
30
40
50
60
100
I
4.8 0.2 2.2 0.2 1.1 0.1 0.5 0.1
II
III
IV
V
3.3 0.2 5.5 0.1 7.0 0.3 8.1 0.8 8.8 0.5 9.2 0.2 9.8 0.7 10.1 0.5 10.2 0.4 10.2 0.2
10.5 0.9 16.7 0.5 20.7 0.1 23.3 0.6 25.1 0.7 26.2 0.5 27.4 0.6 27.8 0.5 27.7 0.9 27.5 1.0
0.6 0.1 0.9 0.1 1.0 0.1 1.1 0.1 1.1 0.1 1.1 0.2 1.0 0.1 0.9 0.1 0.8 0.1 0.5 0.1
0.2 0.1 0.3 0.1 0.3 0.1 0.4 0.1 0.4 0.1 0.4 0.1 0.5 0.1 0.5 0.1 0.5 0.1 0.5 0.1
3.2 0.3 5.3 0.5 6.8 0.1 7.8 0.2 8.5 0.7 9.0 0.5 9.5 0.7 9.8 0.5 9.9 0.3 10.0 0.4
4.5 0.5 7.6 0.8 9.7 0.7 11.2 0.4 12.2 1.1 12.8 1.0 13.6 1.4 14.0 1.1 14.1 1.4 14.2 0.3
6.1 0.6 10.3 0.9 13.1 0.3 15.1 0.4 16.4 1.0 17.3 1.4 18.4 1.8 18.9 1.4 19.1 0.6 19.2 0.5
2.5 0.3 5.1 0.1 6.9 0.4 8.1 0.4 8.7 0.5 9.1 0.5 9.4 0.4 9.4 0.3 9.3 0.3 9.2 0.6
1.2 0.1 2.0 0.1 2.6 0.1 3.0 0.1 3.2 0.1 3.4 0.1 3.6 0.2 3.7 0.3 3.8 0.2 3.8 0.2
3.6 0.3 5.7 0.1 7.1 0.5 8.1 0.5 8.4 0.6 8.7 0.4 9.0 0.4 9.2 0.5 9.3 0.9 9.4 0.5
1.7 0.2 2.9 0.2 3.6 0.4 4.0 0.4 4.3 0.2 4.4 0.3 4.5 0.3 4.4 0.3 4.2 0.4 4.0 0.4
7.1 0.7 10.1 1.0 12.2 0.7 13.3 0.5 13.9 0.5 14.1 1.4 14.1 1.1 13.9 0.8 13.8 1.3 13.7 1.3
3.6 0.4 3.8 0.4 4.3 0.1 4.5 0.5 4.7 0.5 4.8 0.5 4.8 0.5 4.8 0.4 4.7 0.2 4.7 0.2
2.1 0.2 3.2 0.1 3.7 0.1 3.9 0.2 4.0 0.3 4.0 0.3 3.9 0.2 3.7 0.3 3.6 0.2 3.3 0.1
11.2 1.4 16.0 0.9 18.2 0.2 19.2 0.9 19.4 0.6 19.4 0.7 18.8 1.2 18.4 0.6 17.6 1.8 17.0 0.6
0.6 0.1 1.0 0.1 1.3 0.1 1.4 0.2 1.5 0.2 1.6 0.2 1.6 0.2 1.6 0.2 1.5 0.2 1.3 0.1
1.6 0.2 2.7 0.3 3.4 0.4 3.9 0.4 4.3 0.4 4.5 0.4 4.8 0.5 4.9 0.4 4.9 0.4 5.0 0.5
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
Table 3. Yields (%) of thermolysis products of 0.1 M solutions of peroxide I in methanol-d4 as a function of thermolysis
time at 343 K
Time, h
Comp.
no.
1
2
3
4
6
9
12
15
18
25
I
78.7 1.3 62.5 2.9 49.7 0.4 39.4 0.2 24.9 0.2 12.4 0.2
6.1 0.4 3.0 0.1 1.5 0.1 0.3 0.1
II
III
IV
V
2.2 0.2 3.8 0.3 5.1 0.6 6.1 0.4 7.6 0.4 8.8 0.9 9.5 0.5 9.8 0.9 9.9 0.4 10.1 0.9
5.7 0.6 10.0 1.0 13.4 0.9 16.2 0.9 20.1 2.0 23.4 1.6 25.1 2.1 25.9 1.0 26.3 0.7 26.6 1.3
0.9 0.1 1.2 0.1 1.3 0.1 1.2 0.1 1.0 0.1 0.8 0.1 0.8 0.1 0.7 0.1 0.6 0.1 0.5 0.1
0.1 0.1 0.2 0.1 0.3 0.1 0.3 0.1 0.4 0.1 0.4 0.1 0.5 0.1 0.5 0.1 0.5 0.1 0.5 0.1
2.5 0.3 4.6 0.4 6.3 0.2 7.8 0.4 9.8 0.6 11.1 0.4 11.2 0.7 10.8 0.5 10.3 0.4 10.0 0.5
2.3 0.3 4.7 0.5 6.7 0.3 8.2 0.4 10.0 0.3 11.4 0.4 12.0 0.5 12.1 1.0 12.2 0.4 12.3 0.7
3.3 0.3 6.5 0.7 9.2 0.2 11.2 0.8 13.9 1.4 15.9 1.0 16.7 0.4 17.0 1.1 17.0 1.5 17.1 0.6
1.5 0.2 3.2 0.3 4.7 0.1 5.8 0.3 7.0 0.6 7.7 0.5 7.7 0.4 7.6 0.4 7.4 0.7 7.1 0.9
0.9 0.1 1.6 0.2 2.2 0.1 2.5 0.1 3.1 0.3 3.4 0.2 3.6 0.2 3.7 0.3 3.6 0.3 3.7 0.4
3.1 0.3 5.2 0.5 6.8 0.7 8.0 0.8 9.7 0.9 11.0 1.0 11.7 1.2 12.1 1.2 12.3 1.0 12.5 1.0
1.3 0.1 2.3 0.2 2.9 0.2 3.3 0.3 3.8 0.2 4.0 0.2 4.0 0.4 4.0 0.4 3.9 0.4 3.9 0.4
1.8 0.2 3.5 0.4 5.0 0.2 6.4 0.3 8.5 0.9 10.7 1.0 11.8 0.6 12.2 0.8 12.2 1.0 11.8 0.4
1.6 0.2 2.6 0.1 3.3 0.2 3.8 0.2 4.2 0.3 4.6 0.2 4.7 0.3 4.9 0.5 4.9 0.5 5.0 0.4
1.3 0.1 2.1 0.1 2.7 0.2 3.1 0.2 3.4 0.2 3.2 0.2 2.8 0.1 2.4 0.2 2.2 0.2 2.1 0.2
4.7 0.5 8.0 0.4 10.5 0.4 12.2 0.5 14.4 0.3 15.9 1.0 16.3 0.6 16.5 0.9 16.6 0.8 16.7 0.9
0.7 0.1 1.1 0.1 1.4 0.2 1.5 0.2 1.7 0.2 1.7 0.2 1.6 0.1 1.6 0.1 1.5 0.1 1.5 0.1
1.3 0.1 2.3 0.2 3.2 0.3 3.9 0.4 4.9 0.4 5.6 0.5 5.6 0.5 5.4 0.5 5.2 0.4 5.0 0.4
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
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THERMOLYSIS OF METHANOLIC SOLUTIONS OF ACETYL PROPIONYL PEROXIDE
617
(2)
propane (VI), butane (XVIII), ethane-d (X), ethene
(IV), and methyl propanoate (V).
k = (1.4 0.4) 1014exp[ (123 2)/RT].
Comparing Eqs. (1) and (2), we can state that the
solvent takes an active part in thermolysis of peroxide
I in methanol, thus accelerating the process and
reducing its activation energy.
The time dependences of the concentration of per-
oxide I in the coordinates ln(c0/ct) t, where c0 is the
initial concentration of peroxide I and ct is the con-
centration of compound I at time t, are close to linear
for all the three temperatures. Consequently, the de-
composition of compound I is an irreversible reaction
of the first or pseudo-first order. The decomposition
Calculations by Eq. (1) showed that compound I
would decompose at an appreciable rate already at
275 K. The solution of peroxide I was kept for a year
at 275 K. The resulting data are given below. Only
5 compounds could be detected: peroxide I, ethanoic
(III) and propanoic (XIII) acids, and their methyl-d3
esters XI, XII.
6
rate constants k are (5.19 0.02) 10 , (2.10 0.01)
5
5
1
10 , (6.46 0.01) 10
s
for 323, 333, and 343 K,
respectively. Using the Arrhenius equation, we ob-
tained Eq. (1) for the apparent rate constant of per-
oxide I decomposition in methanol:
Comp. no.
I
III
XI
XII
XIII
Yield, %
72.4 6.9
11.7 1.3
16.0 1.5
12.2 1.3
16.8 1.7
k = 5.0 1013exp[ (116 12)/RT].
(1)
Here the activation energy is given in kJ/mol. The
resulting values for the parameters of the Arrhenius
equation are in a good agreement with respective
values for aliphatic diacyl peroxides [1].
Earlier we studied thermolysis of 0.5 M solutions
of compound I in carbon tetrachloride [9]. With the
rate constants obtained in that work, the Arrhenius
equation can be written in form (2):
Thus, the CIDNP effects and the data in Tables 1 3
establish that peroxide I in methanol-d4 is partially
thermolyzed by the radical mechanism described
in [1 5].
CH3C(O)OOC(O)CH2CH3
k
S
CH3C(O)O
OC(O)CH2CH3 (RP1)
k1
k2
CO2
CO2
S
S
CH3C(O)O
OC(O)CH2CH3 (RP2) CH3C(O)O
OC(O)CH2CH3 (RP2 )
k2
k1
CO2
CO2
S
CH3
CH2CH3 (RP3)
According to the above mechanism, under the
action of temperature peroxide I decomposes to give
reaction products; and third, decarboxylation to give
a secondary radical pair (RP2, RP2 ). The radicals in
acetoxyl and propionyloxyl radicals which form a RP2 recombine, affording methyl propanoate, and
primary geminate radical pair (RP1). The radicals in
this pair can undergo various transformations: First,
recombination to the starting peroxide; second, con-
version to an acid, after leaving the solvent cage and
proton or deuterium abstraction from metanol-d4 or
abstract hydrogen or deuterium from reaction products
or methanol-d4, yielding methane and propanoic acid
after leaving the solvent cage. The radicals in RP2,
apart from recombination to ethyl ethanoate, can dis-
proportionate, affording ethanoic acid and ethene.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

618
SKAKOVSKII et al.
Moreover, after leaving the solvent cage and hydro-
gen and deuterium abstraction they form ethanoic acid
and ethane, both deuterated and not.
peroxide I in carbon tetrachloride are equal to respec-
tive values for thermolysis of I in methanol-d4. The
same should be true of the yield ratios of cage
products, i.e., the yield of propane from recombination
of methyl and ethyl radicals should be 3% (Tables
1 3). Consequently, 7% of propane are formed by
another reaction (see below).
The CIDNP effects on ethyl ethanoate and propane
protons, the appreciable yields of ethyl ethanoate
and the low yields of methyl propanoate suggest that
a route involving convertion of RP1 to RP2 prevails.
In the scheme, the decarboxylation rate constant for
propionyloxyl radicals is labeled k1, and that for
acetyloxyl radicals, k2.
Thus, by the procedure described in [10] we, taking
account of the aforesaid, estimated parameters of the
Arrhenius equation for decarboxylation of propionyl-
oxyl (k1) and acetyloxyl (k2) radicals.
Complete decarboxylation of the acyloxyl radicals
gives rise to a methyl ethyl radical pair (RP3). The
radicals in this pair recombine to give propane, dis-
proportionate, yielding methane and ethene, and, after
leaving the cage, methane and ethane, both deutera-
ted and not.
k1 = 5 1011exp[ (7 3)/RT],
k2 = 2 1013exp[ (27 6)/RT].
(3)
(4)
Analysis of the products of the thermolysis of com-
pound I (Tables 1 3) shows that the formation of
some of them cannot be explained in terms of a clas-
sical scheme of radical homolysis. It is reasonable to
suggest that methyl-d3 ethanoate and methyl-d3
propanoate are formed by methanolysis of the starting
peroxide, i.e., by a nonradical reaction.
Earlier we proposed a procedure for estimating the
decarboxylation rate constants of acyloxyl radicals
[10]. According to this procedure, k1 and k2 are
determined, based on the radical pair theory [11, 12],
from a comparison of a theoretical NMR spectrum of
the recombination product of RP3 with polarized
nuclei with an experimental spectrum of the same
product, obtained in the course of the reaction, with
account for spin-lattice relaxation times. In addition,
for more reliable results, theoretical and experimental
yield ratios for cage products formed from RP2,
RP2 , and RP3 are compared.
CH3C(O)OOC(O)CH2CH3 + CD3OD
CH3C(O)OCD3 + CH3CH2C(O)OOD,
CH3C(O)OOD + CH3CH2C(O)OCD3.
In [13], we obtained k1 and k2 for thermolysis of
0.5 M solutions of peroxide I in carbon tetrachloride.
Comparing the yields of cage products of thermo-
lyses of compound I in carbon tetrachloride [13] and
methanol-d4 we noted that the ratios of the total yields
of recombination products for RP2 (ethyl ethanoate
and 2-chloroethyl ethanoate) and RP3 (propane and
2-chloropropane) are 5 and 1.3, respectively.
Therewith, methyl-d3 ethanoate prevails at all the
temperatures. At low temperatures its yield is
1.3 times higher than that of methyl-d3 propanoate,
whereas in the range 323 343 K this ratio increases
to 3. These results point to a primarily nucleophilic
substitution by the acetyl carbonyl.
Peroxypropanoic and peroxyethanoic acids could
not be detected at any thermolysis stage, apparently
because of their thermal instability. However, the
possibility of formation of these products can be
deduced from the fact that in the course of thermolysis
of peroxide I in water-d2 we found a mixture of the
corresponding acids and peroxy acids, which further
decomposed with acid formation.
To find out the reasons for this difference, we
1
measured the H NMR spectra of 0.1 M solutions of
peroxide I in carbon tetrachloride at 353 K (Fig 1b).
1
Note that the H CIDNP NMR spectra of CCl4 solu-
tions with various concentrations of compound I (0.1
and 0.5 M) are qualitatively similar to each other,
except for an inconsiderable difference in the intensi-
ties of propane and chloroethane lines. As follows
from a comparison of Figs. 1a and 1b, the methyl
proton signals of propane are almost identical.
Moreover, the yield ratios of ethyl ethanoate and
methyl propanoate for thermolyses of 0.1 M solutions
of compound I in CCl4 and CD3OD, are almost equal
to each other ( 18).
It is known that peroxy acids can decompose by a
nonradical mechanism, yielding acids and oxygen [1].
CH3C(O)OOD
CH3C(O)OD + 1/2O2,
CH3CH2C(O)OOD
CH3CH2C(O)OD + 1/2O2.
This route is prevailing both in a strongly polar
liquid (D2O) and in methanol-d4 at low temperatures
and, partially, at 323 343 K. Simultaneously, non-
It is thus reasonable to suggest that k1 and k2 for
the acyloxyl radicals resulting from thermolysis of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

THERMOLYSIS OF METHANOLIC SOLUTIONS OF ACETYL PROPIONYL PEROXIDE
radical decomposition of peroxy acids can also take
619
30
place [1].
25
S,F
II
CH3CH2C(O)OOD
CH3CH2C(O)O OD (RP4)
III
20
15
10
5
VI
S,F
VII
VIII
IX
CH3CH2 OD (RP5)
CH3C(O)OOD
CH3 OD (RP7)
CH3CH2OD,
S,F
XI
CH3C(O)O OD (RP6)
XIII
XVI
S,F
CH3OD.
0
20
40
60
80
100
, %
Evidence in favor of this route is provided by the
presence among final products at 323 343 K of
ethanol-d and methanol-d. No CIDNP effects were
observed with the alcohols, which can be explained
by the low concentration of the peroxy acids and the
short time of electron relaxation for hydroxyl radical
[14], which, in its turn, results in a loss of spin cor-
relation in radical pairs RP4 RP7, thus reducing the
yield of the alcohols.
12
10
8
IV
V
6
X
XII
4
XIV
XV
However, we could observe nuclear polarization in
alcohols in the course of photolysis of methanolic so-
lutions of peroxy acids and acids, which is in itself in-
teresting in terms of magnetic and spin effect studies.
2
XVII
XVIII
0
20
40
60
80
100
, %
Finally, one more group of compounds exists,
formed by radical reactions with thermolysis products
of compound I. They include 2-(methoxy-d3)-
propanoic acid, methyl-d3 ethyl ether, butanes, and
most propane. The first of the listed compounds is
formed by -proton abstraction by different radicals
from propanoic acid, followed by recombination of
the resulting radical with methoxy-d3 radical. The
ether and alkanes are formed by addition be the ethene
double bond of methoxy-d3-ethyl and methyl radicals,
respectively, followed by hydrogen abstraction.
Fig. 2. Yields (reduced to 100% peroxide decomposition)
of products (%) of thermolysis of a 0.1 M solution of
peroxide I in methanol-d as a function of the fraction of
4
decomposed peroxide ( , %) at 323 K. The curve
numbers are the same as the compound numbers in
Tables 1 3.
the yields of thermolysis products give only a general
idea of the overall process. A more interesting infor-
mation on the relative contributions of the reaction
routes can be gained from the ratios of the concentra-
tions of the products to the concentration of decom-
posed peroxide (yield reduced to 100% peroxide de-
composition) for all the time points.
It should be noted that methanol-d4 gives rise
primarily to methoxyl-d3, rather than hydroxymethyl-
d3 radicals, which is consistent with data of Shannon
and Harrison [15], who showed that methyl radicals
more readily abstract from methanol the hydroxyl
proton compared with methyl protons.
Figures 2 and 3 depict the plots of the reduced
yields of thermolysis products as a function of the
As seen from data in Tables 1 3, when the tempera- fraction of decomposed peroxide ( ) at 323 and 343 K.
ture is raised from 323 to 343 K, the final yields of
the products do not change considerably, even though
the low-temperature thermolysis is no more than
methanolysis and, therefore, has a much simpler
mechanism.
For clarity sake, the yields of macro and micro-
products are given in different scales.
The first observation is that the yield of ethene
(IX) decreases with increasing . This fact provides
evidence for our assumption that part of propane and,
probably, all butane are formed by addition of methyl
and ethyl radicals to ethene. The resulting propyl and
butyl radicals are not very active and abstract hydro-
gen rather than deuterium. The main source of hydro-
Thus, for instance, the total yield of deuterated and
undeuterated ethane is slightly reduced, like the total
yields of alcohols and ethanoic and propanoic acids,
whereas the total yield of propanoic and 2-(methoxy-
d3)propanoic acids is almost unchanged. However,
gen are, in our opinion, the least strongly bound
-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

620
SKAKOVSKII et al.
depth of thermolysis, implying increasing contribution
27
22
of the nonradical mechanism of thermolysis with
increasing peroxide concentration.
II
The aforesaid also applies to alcohols, since their
yield depends on the contribution of nonradical de-
composition. The latter processes prevail at low tem-
peratures, which corresponds to their lower activation
energies compared with radical processes.
III
VI
VII
VIII
17
12
7
XI
XIII
XVI
The results of the present study allow the me-
chanism of thermolysis of peroxide I in methanol-d4
to be described by reactions (5) (37):
20 30 40 50 60 70 80 90 100
, %
S
10
8
Peroxide I
CH3CH2C(O)O OC(O)CH3 (RP1), (5)
CH3CH2C(O)O
CH3CH2 + CO2,
CH3 + CO2,
(6)
(7)
CH3C(O)O
IV
V
S
6
IX
CH3CH2 OC(O)CH3 (RP2)
CH3CH2OC(O)CH3, (8)
S
X
XII
4
XIV
XV
CH3CH2 OC(O)CH3 (RP2)
(9)
XVII
XVIII
CH2=CH2 + CH3C(O)OH,
S
2
0
CH3 OC(O)CH2CH3 (RP2 )
CH3OC(O)CH2CH3, (10)
20 30 40 50 60 70 80 90 100
, %
S
CH3CH2 CH3 (RP3)
CH3CH2CH3,
(11)
(12)
(13)
S
Fig. 3. Yields (reduced to 100% peroxide decomposition)
of products (%) of thermolysis of a 0.1 M solution of
peroxide I in methanol-d as a function of the fraction of
decomposed peroxide ( , %) at 343 K. The curve
numbers are the same as the compound numbers in
Tables 1 3.
CH3CH2 CH3 (RP3)
CH2=CH2 + CH4,
CH3CH2C(O)O + CD3OD
CH3CH2C(O)OD + CD3O ,
4
CH3C(O)O + CD3OD
CH3CH2C(O)O + RH
CH3C(O)O + RH
CH3CH2 + CD3OD
CH3 + CD3OD
CH3C(O)OD + CD3O, (14)
CH3CH2C(O)OH + R, (15)
CH3C(O)OH + R, (16)
CH3CH2D + CD3O , (17)
CH2 protons of propanoic acid (much this acid is
formed by thermolysis). The more reactive ethyl and,
especially, methyl radicals are less selective and can
abstract deuterium from methanol-d4 to form ethane-d
and methane-d.
CD3O + CH3D,
CH3CH3 + R,
CH4 + R,
(18)
(19)
(20)
CH3CH2 + RH
We determined the yield ratios for protonation and
monodeuteration of these radicals. The ratio [CH4]/
[CH3D] varies from 0.86 0.06 to 0.72 0.01, and the
ratio [C2H6]/[C2H5D], from 2.68 0.14 to 2.13 0.08,
as the temperature is raised from 323 to 343 K. Thus,
the fraction of deuterated products increases with
temperature, implying that the activation energy of
duterium abstraction from the solvent is higher than
that of hydrogen abstraction from various compounds.
Therewith, the yield ratio is independent of the depth
of thermolysis and, consequently, of the concentration
of the starting peroxide.
CH3 + RH
CH2=CH2 + CH3CH2
CH3CH2CH2CH2 + RH
CH3CH2CH2CH2 , (21)
CH3CH2CH2CH3 + R, (22)
CH2=CH + CH3
2
CH3CH2CH2,
(23)
CH3CH2CH2 + RH
CH3CH2CH3 + R, (24)
CH2=CH + CD3O
2
CD3OCH2CH2 ,
CD3OCH2CH3 + R,
CH3CH2C(O)OOD
(25)
(26)
CD3OCH2CH2 + RH
Peroxide I + CD3OD
Unlike the above case, the relative total yield
of methyl-deuterated esters decreases with inreasing
+ CH3C(O)OCD3,
(27)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

THERMOLYSIS OF METHANOLIC SOLUTIONS OF ACETYL PROPIONYL PEROXIDE
621
Peroxide I + CD3OD
+ CH3CH2C(O)OCD3,
CH3CH2C(O)OD + 1/2O2, (29)
CH3C(O)OOD
position, as we expected, is higher than that of the
(28)
nonradical decomposition and almost equals the activa-
tion energy of thermolysis of peroxide I in carbon
tetrachloride.
CH3CH2C(O)OOD
CH3C(O)OOD
CH3C(O)OD + 1/2O2,
(30)
Our study showed that thermolysis of peroxide I
in methanol-d4 is an intricate process involving both
radical and nonradical reactions, whose contribution
depends on the temperature and the concentration of
the peroxide.
S,F
CH3CH2C(O)OOD
CH3CH2C(O)O OD (RP4), (31)
S,F
CH3CH2 OD (RP5)
CH3CH2OD,
(32)
(33)
(34)
(35)
S,F
CH3CH2 OD (RP5)
CH2=CH2 + HOD,
EXPERIMENTAL
S,F
1
CH3C(O)OOD
CH3C(O)O OD (RP6),
The H NMR spectra were obtained on BS-567A
and AM-360 spectrometers at operating frequencies of
100 and 360 MHz, respectively.
S,F
CH3 OD (RP7)
CH3OD,
CH3CH2C(O)OD + R
CH3CHC(O)OD + RH, (36)
Acetyl propionyl peroxide (I) was prepared by the
procedure in [16]. The active oxygen content, as
determined by iodometric titration, was 99.5%.
Methanol-d4 of brand A was used as received. The
deuteration degree and the weight fraction of the main
substance were 99.5%. 0.1 M solutions of compound
I were used. For analysis of the reaction products, the
peroxide solutions were sealed in glass inserts
(2.6 mm i.d.) so that the volume of the gas phase was
10% the volume of the liquid. Under such conditions
all gaseous products almost completely remained in
the solution and thus could be analyzed. Thermolysis
was performed in an oil thermostat at 323, 333, and
343 K. In addition, decomposition of the peroxide
solutions at a refrigerator temperature ( 275 K) was
followed over the course of a year. Periodically, the
inserts (by 3 for each measument) were taken off from
F
CH3CHC(O)OD OCD3 (RP8)
(37)
CH3CH(OCD3)C(O)OD.
The apparent rate constant of the decomposition
can be represented by Eq. (38):
k = krad + knonrad[CD3OD].
(38)
Here krad and knonrad are the rate constants of radical
and nonradical peroxide decomposition.
As follows from the thermolysis scheme, the non-
radical route is described by the second-order equa-
tions (27) and (28); therewith, knonrad is given by
Eq. (39):
1
the thermostat, placed in an NMR tube, and H NMR
knonrad = W/[I][CD3OD].
(39)
spectra were measured in the quantitative mode.
To determine the yields of thermolysis products,
the H NMR spectra of the initial solutions were first
Here W is the sum of the accumulation rates of methyl-
deuterated esters (Tables 1 3), measured at the onset
of decomposition of compound I.
1
measured and integral intensity ratios of signals of
compound I, reduced per one proton, to residual
proton (OH, CHD2) signals of methanol-d4 were
determined (correction factors). Further for each time
point the integral intensitities of signals of products,
reduced per one proton, were related to the integral
intensities of residual proton signals of the solvent,
and, with account for the correction factors, the yields
of the corresponding products were estimated. For
control, the yields of compounds formed from dif-
The resulting knonrad (mol 1 s ) values were (6.65
0.09) 10 , (1.59 0.07) 10 , (6.05 0.08) 10
1
8
7
7
for 323, 333, and 343 K, respectively.
1
Using Eq. (38), the corresponding krad (s ) were
6
5
calculated: (3.73 0.09) 10 , (1.75 0.07) 10 , and
(5.13 0.08) 10 .
5
By the Arrhenius equation we estimated the
apparent rate constants of thermolysis of 0.1 M solu-
tions of peroxide I in methanol-d4.
ferent fragments of peroxide
I
[CH3C(O)O ,
CH3CH2C(O)O ] were summarized with the amount
of peroxide I. This sum was always only slightly less
than 100%, thus attesting the analytical procedure.
knonrad = 1 109exp[ (102 15)/RT],
krad = 3 1014exp[ (120 16)/RT].
(40)
(41)
Unfortunately, the above procedure was unsuitable
for analysis of carbon dioxide. However, since the
knowledge of its total content carried, by our opinion,
Thus, the activation energy of the radical decom-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

622
SKAKOVSKII et al.
no important mechanistic information, we made no
additional efforts to determine the yield of this
gaseous product.
Khim., 1956, vol. 26, no. 7, pp. 1986 1991.
7. Dutka, V.S., Pankevich, R.V., and Koval’skii, Ya.P.,
Abstracts of Papers, Vsesoyuznoe Soveshchanie Ki-
netika radikal’nykh reaktsii v zhidkoi faze (All-
Union Conf. Kinetics of Liquid-Phase Radical Reac-
tions ), Gorki, 1987, p. 108.
The relative error in the yields, averaged over
3 measurements, was 10%.
Further insight into the thermolysis mechanism
8. Dutka, V.S., Pankevich, R.V., and Soltys, M.N.,
Abstracts of Papers, IX Vsesoyuznaya konferentsiya
po khimii organicheskikh i elementoorganicheskikh
peroksidov (All-Union Conf. on Chemistry of Organic
and Organoelement Peroxides), Gorki, 1990, p. 133.
1
was obtained by H CIDNP NMR studies on a BS-
567A spectrometer using as the internal reference
CHD2OD which is present as the main admixture in
1
CD3OD, 3.50 ppm. The H NMR spectra showed
characteristic signals of peroxide I, , ppm: 2.36 s
[CH3C(O)O], 2.65 q (CH2), 1.41 t (CH3). The spin-
lattice relaxation times for the propane line were
9. Skakovskii, E.D., Stankevich, A.I., Lamotkin, S.A.,
Tychinskaya, L.Yu., and Rykov, S.V., Zh. Obshch.
Khim., 1996, vol. 66, no. 6, pp. 881 887.
measured using 180
90 pulse sequences. The
10. Turetskaya, E.A., Skakovskii, E.D., Rykov, S.V., Glaz-
kov, Yu.V., and Ol’dekop, Yu.A., Dokl. Akad. Nauk
BSSR, 1980, vol. 24, no. 1, pp. 57 59.
relaxation times differed only slightly.
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