Catalytic effect of cuprous ions on the thermal decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in methanol solution

Article abstract of DOI:10.1134/S1070363208060303

Thermal decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane was examined in methanol solution (1.69×10^-2 M) containing cuprous ions (5.05×10^-7 M) in the temperature range from 130 to 166°C using UV spectroscopy as analytical method. The ion-catalyzed reaction follows first-order kinetics with respect to the peroxide and added cuprous ions. The temperature effect on the rate of thermal decomposition of the title compound was described by the corresponding Arrhenius equations, and its stability in solution was estimated on a quantitative level. The activation parameters of the initial step of decomposition of 3,3,6,6-tetramethyl-1,2,4,5- tetraoxane were determined (ΔH^≠ = 14.7±0.8 kcal mol^-1; ΔS^≠ = -38.9±1.4 cal mol ^-1 K^-1; ΔG ^≠ = 31.0±0.8 kcal mol^-1). Electron-transfer mechanism was proposed for the reaction under study.

Full text of DOI:10.1134/S1070363208060303

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 6, pp. 1273–1276. © Pleiades Publishing, Ltd., 2008.
Catalytic Effect of Cuprous Ions on the Thermal Decomposition
of 3,3,6,6-Tetramethyl-1,2,4,5-tetraoxane in Methanol Solution1
L. I. Giméneza, J. M. Romeroa, S. Bustilloa, N. L. Jorgea,
M. E. Gómez Varaa, and E. A. Castrob
a Cátedra de Química Física Ic, Facultad de Ciencias Exactas y Naturales y Agrimensura,
Universidad Nacional del Nordeste, Av. Libertad 5460, (3400) Corrientes, Argentina
phone/fax: (03783) 457996; e-mail: lidianj@exa.unne.edu.ar
b Theoretical Chemistry Division, INIFTA, Suc. 4, C.C. 16, La Plata 19900, Buenos Aires, Argentina
e-mail: eacast@gmail.com; castro@quimica.unlp.edu.ar
Received December 13, 2007
Abstract—Thermal decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane was examined in methanol
solution (1.69×10–2 M) containing cuprous ions (5.05×10–7 M) in the temperature range from 130 to 166°C
using UV spectroscopy as analytical method. The ion-catalyzed reaction follows first-order kinetics with
respect to the peroxide and added cuprous ions. The temperature effect on the rate of thermal decomposition of
the title compound was described by the corresponding Arrhenius equations, and its stability in solution was
estimated on a quantitative level. The activation parameters of the initial step of decomposition of 3,3,6,6-
tetramethyl-1,2,4,5-tetraoxane were determined (ΔH≠ = 14.7 0.8 kcal mol–1; ΔS≠ = –38.9 1.4 cal mol–1 K–1;
ΔG≠ = 31.0 0.8 kcal mol–1). Electron-transfer mechanism was proposed for the reaction under study.
DOI: 10.1134/S1070363208060303
Scheme 1.
Some 1,2,4,5-tetraoxanes and 1,2,4-trioxanes attract
great interest due to their potential as antimalarial
agents [1, 2]. Such compounds are prone to undergo
decomposition in the presence of catalytic amounts of
transition metal ions through initial complex formation
at the peroxide bond [3]. Previous studies have shown
that the unusually high rate of decomposition of
tetramethyl-1,2-dioxetane in methanol is due to
catalysis by metallic impurities present in the solvent.
[4]. The addition of cuprous chloride considerably
accelerates decomposition of 3,3,6,6-tetramethyl-
1,2,4,5-tetraoxane (TMT) in propan-2-ol. It was found
that cuprous salts hardly affect the thermal decom-
position in acetic acid solution, for cupric ions exert no
catalytic effect on peroxide compounds [5]. We
recently demonstrated that cupric ions have catalytic
effect on peroxides [6]. Three mechanistic possibilities
can be considered for the observed catalysis [2, 3]
(Scheme 1): (1) coordination or Lewis acid, (2) inser-
tion, and (3) electron transfer.
CH3
O
O
O
CH3
+ Me2+
H3C
O
CH3
Complex
O
+ Me2+ + O2
2
H3C
CH3
oxane, in methanol in the presence of copper(I) ions to
estimate the catalytic effect of the latter in
homogeneous medium. UV spectrophotometry was
used as analytical method.
The thermal decomposition of 3,3,6,6-tetramethyl-
1,2,4,5-tetraoxane was studied in methanol solution
with an initial concentration of 1.69×10–2 M in the
temperature range from 130 to 166ºC in the presence
of copper(I) chloride (5.05×10–7 M); the reaction was
found to follow first-order kinetic laws until ~60%
In this work we examined thermal decomposition
of a cyclic peroxide, 3,3,6,6-tetramethyl-1,2,4,5-tetra-
1
The text was submitted by the authors in English.
1273

1274
ln A
GIMÉNEZ et al.
much more effective than the decomposition catalyzed
by copper(II) ions [6].
0.68
The catalytic effect was verified as a marked
increase of kexp and low activation energy. Complexa-
tion of 3,3,6,6-tetramethyl-1,2,4,5-tetra-oxane by a
metal ion should favor decomposition by removing
orbital symmetry restrictions or by lending positive
character to one or both peroxide oxygen atoms, thus
destabilizing the peroxide bond and facilitating ring
cleavage. Copper(I) chloride in methanol undergoes
dissociation to give solvated ion Cu+(MeOH)x. Two
necessary conditions for catalysis are dissociation of
the metal salt and fast exchange of ligands around the
metal ion.
0.64
0.60
0.56
0.52
0.48
1
2
Temperature effect on the rate constant (kexp) for
unimolecular reaction can be represented by the
Arrhenius equation where the errors are standard
deviations determined by the least-squares treatment of
the kinetic data [8, 9].
3
4
lnk (s–1 ) = (11.22 0.8) – (15518.39 1000)/T[CuCl]
= 5.05×10–7 M.
0.00
1.00
2.00
3.00
4.00
5.00
t, min
Fig 1. First-order kinetic plots for the thermal decom-
position of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in metha-
nol in the presence of CuCl at (1) 130, (2) 140, (3) 150,
and (4) 166°C.
The corresponding plot is almost linear (r = 0.993)
within a relatively large temperature range (ΔT ≈
36ºC). This suggests that the calculated activation
parameters for the decomposition of 3,3,6,6-tetra-
methyl-1,2,4,5-tetraoxane in methanol solution belong
to a single process. The low activation barrier, about
15.5 kcal, confirms the catalytic effect.
conversion of the substrate (Fig. 1). The apparent rate
constants (kexp) for the decomposition of 3,3,6,6-
tetramethyl-1,2,4,5-tetraoxane in methanol in the
presence of CuCl were much greater than those
obtained using commercial methanol as solvent [7]
(Table 1). Thus the Cu(I)-catalyzed decomposition is
The second-order rate constants k2 for the catalytic
decomposition were calculated by the following
equation:
kap − k0
k2 = ______________
[metal salt]
_____k__ca_t______
=
,
Table 1. First-order rate constants for the decomposition of
3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in methanol contain-
ing CuCla
[metal salt]
where k0 is the rate constant for the uncatalyzed
decomposition, and kap is the apparent rate constant for
the catalyzed decomposition. The second-order rate
constants k2 for the catalytic decomposition and
experimental conditions are summarized in Table 2.
Temperature, °C [TMT]×102, M k0×105, s–1 kap×105, s–1
130
140
150
166
1.69
1.69
1.69
1.69
0.40
1.00
2.44
6.59
027.19
049.44
073.64
136.61
No counterion effect [3] was observed in the
reaction catalyzed by cuprous ions (Table 3). The data
in Table 3 indicate that stronger complexation of the
metal ion makes the catalysis less efficient and that the
presence of uncomplexed metal ions is necessary to
achieve strong catalytic effect. The latter increases in
parallel with the concentration of cuprous ions.
a
k0 is the experimental rate constant for the decomposition of
3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in methanol at an initial
concentration of 1.69×10–2 M; kap is the apparent rate constant
for the decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in
methanol (cTMT = 2.34×10–2 M) in the presence of CuCl (cCuCl
=
5.05×10–7 M).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 6 2008

CATALYTIC EFFECT OF CUPROUS IONS ON THE THERMAL DECOMPOSITION
1275
Table 2. Second-order rate constant k2 for the catalyzed
decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in
methanol in the presence of metal salt at different
temperatures
The negative value of entropy variation implicates
diminution of the degree of freedom of the reactive
molecules in passing to a rigid transition state,
assuming that the thermolysis in methanol involves
rupture of the peroxide bond in the 3,3,6,6-tetramethyl-
1,2,4,5-tetraoxane molecule, which is assisted by the
solvent and metal ion forming a complex with the
solvent and the diperoxide (Table 4). The kinetic
parameters given in Table 4 show that the behavior in
methanol in the presence of metal ions is similar to the
behavior observed in acetic acid.
Temperature, ºC
[TMT]×102, M
k2, M–1 s–1
0530.5
0959.2
1409.9
2574.6
130
140
150
166
1.69
1.69
1.69
1.69
Table 3. Cuprous ion-catalyzed decomposition of 3,3,6,6-
tetramethyl-1,2,4,5-tetraoxane in treated methanol
The following conclusions can be drawn. The
thermal decomposition of 3,3,6,6-tetramethyl-1,2,4,5-
tetraoxane in methanol in the presence of copper(I)
ions follows first-order kinetics. Analysis of the
products and activation parameters led us to postulate a
thermolysis mechanism typical of tetraoxanes. The
process begins with homolytic dissociation of the
peroxide bond to give intermediate diradical and
subsequent cleavage of the C–O bond to produce
acetone and oxygen as final products.
Anion or salt
None
[Anion]/[Cu+]
–
Kcat×105, s–1
49.44
Na2EDTA
1.2
49.74
Table 4. Activation parameters for the thermal decom-
position of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in
different solvents
An increased catalytic effect was observed at a
metal ion concentration that is only ten times larger
than its concentration in commercial reagent. Thermal
decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetra-
oxane is strongly accelerated by addition of traces of
single-electron reducing agents such as copper(I) salts.
The reaction involves electron transfer to the lowest
ΔS≠, cal×
Solvent
ΔH≠, kcal mol–1
Reference
[10]
mol–1 K–1
Methanol
25.8 0.3
14.7 0.8
19.4 1.3
13.3 4.4
–17.9 0.60
Methanol/Cu(I)
Propan-2-ol
Acetic acid
–38.9 1.40 This paper
–31.4 3.50
–43.7 10.5
0[5]
0[4]
ln k/T
–12.40
Insertion mechanism requires formal two-electron
oxidation of the metal ion, while electron-transfer
mechanism involves one-electron oxidation (transfer
of an electron to the lowest antibonding orbital of the
peroxide oxygen, followed by cleavage of the O–O
bond). Coordination mechanism where the metal ion
acts as a Lewis acid is completely consistent with the
counterion effect and the linear free energy
relationship. Complexation of 3,3,6,6-tetramethyl-
1,2,4,5-tetraoxane by a metal ion might facilitate
decomposition by removing positive character to one
or both oxygen atoms destabilizing the peroxy bond
and allowing for more facile ring cleavage [3].
–12.80
–13.20
–13.60
–14.00
–14.40
The activation parameters for the unimolecular
reaction under study (enthalpy variation ΔH≠ and
activation entropy variation, ΔS≠) were determined
using the Eyring equation (Table 4, Fig. 2). The error
limits were calculated using an appropriate literature
method [10].
0.002250 0.002300 0.002350 0.002400 0.002450
T, K
Fig. 2. Eyring equation plot for the thermal decomposition of
3,3,6,6-tetramethyl-1,2,4,5-tetraoxane in methanol contain-
ing CuCl (cCuCl = 5.05×10–7 M).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 6 2008

1276
GIMÉNEZ et al.
antibonding orbital of the peroxide oxygen, followed
by cleavage to the O–O bond.
equation, and the errors were determined by the least-
squares procedure [12, 13].
At present, research is in progress to determine
whether the complexation involves one or both oxygen
atoms of 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane. Our
further study will be aimed at analyzing the effect of
larger concentrations of copper(I) salts on the
decomposition of 3,3,6,6-tetramethyl-1,2,4,5-tetra-
oxane in methanol solution.
REFERENCES
1. Free Radicals in Biology, Pryor, W.A., Ed., New York:
Academic, 1976–1984, vols. 1–6.
2. Wen-MinWu, Zhu-Jun Yao, Yu-Lin Wu, Kun Jiang,
Yan-Fang Wang, Hai-Bao Chen, Feng Shan, and Ying Li,
Chem. Commun., 1996, vol. 18, p. 2213.
3. Bartlett, P.D., Baumstark, A.L., and Landis, M.E., J. Am.
Chem. Soc., 1974, vol. 96, p. 5557.
EXPERIMENTAL
4. Wilson, T., Landis, M., Baumstark, A., and Bartlett, P.D.,
3,3,6,6-Tetramethyl-1,2,4,5-tetraoxane was prepared
according to the procedure described in [9], and its
purity was checked by GLC. Methanol was
commercial reagent (Merck) of analytical grade; it was
purified by standard techniques [11].
J. Am. Chem. Soc., 1973, vol. 95, p. 4765.
5. Moryganov, B.N., Kalinin, A.I., and Mikhotova, L.N.,
Zh. Obshch. Khim., 1962, vol. 32, p. 3414.
6. Jorge, N.L., Romero, J.M., Leiva, L.C., Castellanos, M.G.,
Cafferata, L.F.R., and Gómez Vara, M.E., Afinidad,
2003, vol. 60 (506), p. 291.
Pyrex glass tubes (70 mm×4 mm o.d.) filled with
2 ml of a solution of 3,3,6,6-tetramethyl-1,2,4,5-tetra-
oxane in methanol containing cuprous chloride were
thoroughly degassed under vacuum line at –196°C and
then sealed with a flame torch. The ampoules were
immersed into a thermostated silicone oil bath ( 0.1°C)
and withdrawn after definite time, the reaction being
stopped by cooling to 0°C. The concentrations of
unreacted 3,3,6,6-tetramethyl-1,2,4,5-tetraoxane and
its decomposition products in the reaction mixture
were determined by quantitative UV-visible analysis
using a CampSpec M330 spectrophotometer. The
absorbance was measured at λ 209 nm.
7. Leiva, L.C., Castellanos, M.G., Jorge, N.L., Cafferata, L.F.,
and Gómez Vara, M.E., Rev. Soc. Quím. México, 2002,
vol. 46, no. 1, p. 1.
8. Schaleger, L.L. and Long, F.A., Adv. Phys. Org. Chem.,
1963, vol. 1, p. 1.
9. Huyberechts, S., Halleux, A., and Kruys, P., Bull. Soc.
Chim. Belg., 1955, vol. 64, p. 203.
10. Leiva, L.C., Castellanos, M.G., Jorge, N.L., Caffera-
ta, L.F.R., and Gómez Vara, M.E., Rev. Soc. Quím.
México, 1998, vol. 42, no. 5, p. 223.
11. Organic Solvents. Physical Properties and Methods of
Purification, Riddick, J.A. and Bunger, W.B., Eds.,
New York: Wiley, 1970, 3rd ed.
The corresponding first-order rate constant values
were calculated from the slope of the plot of lnA
(where A is the absorbance, Fig. 1) versus reaction
time. In all cases, the progress of the reaction was
followed for at least one half-conversion period. The
activation parameters were calculated using the Eyring
12. Scholz, B., Butzert, H., Neumeister, J., and Nierlich, F.,
Ulmann’s Encyclopedia of Industrial Chemistry,
Weinheim: Wiley, 1985, vol. A16.
13. Machado, C. and Machado, V.G., J. Chem. Educ., 2001,
vol. 78, no. 5, p. 649.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 6 2008