Kinetics of thermal decomposition of polyfunctional substituted azido-1,2,4-triazoles

Article abstract of DOI:10.1134/S1070363212090216

Abstract-The structural and kinetics regularities and the mechanism of the limiting step of thermal decomposition of the polyfunctional substituted 1,2,4-triazoles in the melt and in solutions of inert solvents were established. The activation parameters

Full text of DOI:10.1134/S1070363212090216

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 9, pp. 1577–1582. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © L.A. Kruglyakova, R.S. Stepanov, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 9, pp. 1542–1548.
Kinetics of Thermal Decomposition of Polyfunctional Substituted
Azido-1,2,4-triazoles
L. A. Kruglyakova and R. S. Stepanov
Siberian State Technological University, pr. Mira 82, Krasnoyarsk, 660049 Russia
e-mail: lakrugl@sibstu.kts.ru
Received July 14, 2011
Abstract—The structural and kinetics regularities and the mechanism of the limiting step of thermal
decomposition of the polyfunctional substituted 1,2,4-triazoles in the melt and in solutions of inert solvents
were established. The activation parameters of the limiting step were determined. Polyfunctional substituents
are found to show little effect on the rate of thermal decomposition of azide group in the azole ring.
DOI: 10.1134/S1070363212090216
Azido-1,2,4-triazoles are the compounds of scientific
and practical interest as promising components of the
rocket solid propellants [1], but their safe use is
impossible without knowledge of their thermal
stability, which, unfortunately, has been studied only
fragmentary. Filling partially the information gap, in
this study we investigated the kinetics of thermal
decomposition in order to determine the effect of
functional substituents on the rate and mechanism of
decomposition of azido-1,2,4-triazoles.
dibutyl phthalate (ε = 6.44) and 1,3-dinitrobenzene
(ε = 20.6), as well as the concentration of substances in
solution (1–20 wt %) do not change the reaction law
and rate. At the decomposition of the melt the ratio of
the substance weight to the reaction volume does not
affect the rate of decomposition of compounds II–XV
in the range of m/V from 10–4 to 10–2 g cm–3. The same
is valid for the 2–4 times change in the ratio of the
surface area of the reaction vessel to its volume, S/V.
All this testifies to the absence of chain and hetero-
geneous processes, and points to the homogeneity of
the course of the thermal decomposition reaction. Up
to 40–50% conversion, the thermal decomposition is
described by first-order reaction equation.
R3
N
R1
R2
N
N
At the same time, the variation of S/V in the case of
compound I from 1.8 to 7.4 cm–1 does not affect the
decomposition law in the melt (Fig. 1), but leads to 4-
fold increase in the rate constant. This is probably due
to the heterogeneous reactions occurring on the walls
of the reaction vessel at the increase in the reaction
surface area. Therefore, the study of the kinetics of
thermal decomposition of I in the melt was carried out
at S/V = 0.5–1.0 cm–1, when the potential contribution
of heterogeneous reactions to the overall rate of the
process was very small. In addition, for compound I
the kinetics of accumulation of molecular nitrogen
during the thermal decomposition was also studied.
Kinetic curves for the release of molecular nitrogen to
the conversion of about 50% were practically the same
as the curves of the total gas accumulation, and the
values of the rate constants calculated from the curves
R1 = H, R2 = N3, R3 = H (I); R1 = CH2N3, R2 = NO2, R3 =
H (II); R1 = CH2CH2N3, R2 = H, R3 = H (III); R1 =
CH2CH2N3, R2 = N3, R3 = H (IV); R1 = CH2N(NO2)CH3,
R2 = N3, R3 = H (V); R1 = CH2CH2ONO2, R2 = N3, R3 = H
(VI); R1 = CH3, R2 = N3, R3 = C6H5 (VII); R1 = CH3, R2 =
NO2, R3 = N3 (VIII); R1 = 5-NO2-tetrazol-2-yl-CH2 CH2,
R2 = NO2, R3 = H (IX); R1 = C(NO2)(CH2N3)2, R2 = NO2,
R3 = H (X); R1 = CH2CCl2NO2, R2 = N3, R3 = H (XI); R1 =
C(NO2)2Br, R2 = N3, R3 = H (XII); R1 = CCl2NO2, R2 =
NO2, R3 = H (XIII); R1 = CH2COOCH2C(NO2)3, R2 = N3,
R3 = H (XIV); R1 = CCl2NO2, R2 = N3, R3 = H (XV), R1 =
C(NO2)3, R2 = N3, R3 = H (XVI).
We studied the kinetics of thermal decomposition
of compounds I–XV in the melt and as solutions in
dibutyl phthalate and 1,3-dinitrobenzene. Preliminary
experiments showed that the dielectric constant of
1577

1578
KRUGLYAKOVA, STEPANOV
The study of the gaseous products at the decomposi-
V, cm3 g–1
tion of some azides showed that the main product was
the molecular nitrogen. In the case of compounds
containing, in addition to the azido function, a nitrate,
bromodinitro- or -trinitromethyl group, even at the
reaction beginning or at a relatively low degree of
conversion a decomposition of this group is observed,
so that carbon oxides appear together with the
molecular nitrogen. For example, at the decomposition
of compound V in dichloroethane at 150°C, upon
increase in the degree of conversion the intensity of the
absorption band of azido group (2160 cm–1) in the IR
spectrum decreases, while the intensity of the bands of
N–NO2 group (1545, 1300 cm–1) remains almost
constant, and the nitrogen oxides begin to appear later,
as the conversion grows (18–20%). The data on this ques-
tion for some azido compounds are given in Table 2.
τ, min
Fig. 1. Effect of S/V on thermal decomposition of com-
pound I at 150°C, cm–1: (1) 1.80, (2) 2.32, (3) 4.10, (4) 5.74.
of nitrogen evolution coincided with those found from
the curves of the total gas production within the error
of their determination. Kinetic parameters of thermal decom-
position of the compounds studied are listed in Table 1.
In the IR spectrum of the condensed residue formed
at the decomposition of compound I (Fig. 2) the azido
group absorption band disappears (2160 cm–1), while
Table 1. Activation parameters of thermal decomposition of polyfunctional azido-1,2,4-triazoles in the melt and in solutions
of dibutyl phthalate and m-dinitrobenzene
Substituent
Comp.
no.
Conditions of
decomposition
k160°C×105,
Еa,
ΔS≠160°C
,
ΔТ, °С
log A
s–1
kJ mol–1
J mol–1 K–1
R1
H
R2
N3
R3
H
Melt
Melt, measuringN2
Melt,
130–170
150–170
140–170
4.5
4.3
2.0
139.5
140.7
160.1
13.49
13.59
15.61
I
1.8
3.8
S/V=(0.5–1) см–1
Dibutylphthalate
solution
42.4
130–170
150–190
2.2
159.4
162.7
15.56
14.59
41.5
22.9
CH2N3
NO2
H
m-Dinitrobenzene
solution
0.09
II
CH2CH2N3
CH2CH2N3
H
H
H
Melt
160–190
140–170
0.08
2.4
163.6
156.3
14.65
15.23
III
IV
24.0
35.2
N3
m-Dinitrobenzene
solution
CH2N(NO2)CH3
CH2CH2ONO2
N3
N3
H
H
Dibutylphthalate
solution
Melt
Dibutylphthalate
solution
140–170
2.1
154.4
14.92
V
29.2
130–170
130–180
3.4
2.9
158.4
158.5
15.64
15.58
VI
43.0
41.9
CH3
CH3
N3
C6H5
Melt
m-Dinitrobenzene
solution
Melt
m-Dinitrobenzene
solution
130–170
130–170
4.3
4.5
156.5
158.1
15.21
15.72
VII
VIII
IX
34.8
44.5
NO2 N3
130–170
130–170
2.6
2.8
156.2
159.2
15.26
15.65
35.7
43.2
NO2
NO2
H
H
Dibutylphthalate
solution
170–200
0.06
163.3
13.46
N
N
1.3
NO2
(H2C)2
N
N
m-Dinitrobenzene
150–180
0.5
161.7
15.18
X
CH2N3
C
CH2N3
34.2
solution
O2N
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 9 2012

KINETICS OF THERMAL DECOMPOSITION
1579
Table 1. (Contd.)
Substituent
Comp.
no.
Conditions of
decomposition
k
160°C×105,
s–1
Еa,
ΔS≠160°C
,
ΔТ, °С
log A
kJ mol–1
J mol–1 K–1
R1
R2
N3
R3
H
CH2CCl2NO2
Melt
Dibutylphthalate
solution
140–170
140–170
2.1
1.9
156.7
157.4
15.18
15.31
34.2
36.7
XI
C(NO2)2Br
CCl2NO2
N3
H
H
H
H
H
Dibutylphthalate
solution
Dibutylphthalate
solution
Dibutylphthalate
solution
Dibutylphthalate
solution
120–150
145–165
160–180
140–160
327.3
1.8
130.4
157.3
159.9
156.4
14.24
15.23
15.63
15.41
16.2
35.2
42.8
38.6
XII
NO2
N3
XIII
XIV
XV
CH2COOCH2C(NO2)3
CCl2NO2
2.2
N3
3.5
C(NO2)3 [3]
N3
Melt
Dibutylphthalate
solution
100–140
100–150
66.0
63.3
139.7
140.6
14.67
14.76
24.4
26.2
XVI
of phenylazide, 157.7 kJ mol–1 [4]. Compound I (melt),
XII, and XIII are the exception whose activation
energy is Ea = 161.6–163.7 kJ mol–1. For compounds
II, III, IX, and X, Ea = 161.7–163.7 kJ mol–1, as for
the substituted alkylazides [4–6], and their thermal
decomposition, as well as that of phenylazide,
proceeds with the primary break of the N1–N2 bond in
the azido group, the decomposition of compounds I,
IV– VIII, XI, XIV, and XV follows reaction (1) and
of compounds II, III and X, reaction (2).
the absorption bands characteristic of the triazole ring
(1535, 1460, 1380, 870, 830 cm–1) are retained. New
absorption bands at 1620–1630 cm–1 and at 3400–
3600 cm–1 appear, which can be attributed to the 3,3'- azo-
1,2,4-triazole, and 3-amino-1,2,4-triazole, respectively.
The first thing that attracts attention when
considering the data in Table 1 is virtually the same
activation energy (within the error of its determination)
for most of the polyfunctional substituted 3(5)-azido-
1,2,4-triazoles, which is close to the activation energy
R1,R3
[R1,R3
N
+ N2
(1)
[R1,R3
Tr
N
Tr N3
Tr
N
N]
Tr is 1,2,4-triazol-5-yl.
[R2,R3 Tr
(CH2)nN
R2,R3 Tr
(CH2)nN3
[R2,R3 Tr
(CH2)nN
N
N]
+ N2
(2)
n = 1, 2.
Taking into account the activation energy and
entropy of the thermal decomposition (Table 1) the
reactions (1) and (2) should be regarded as proceeding
through a semi-rigid activated complex [4, 7].
The nitrenes arising in the reactions (1) and (2)
have short lifetimes and, given the composition of the
condensed phase formed at the decomposition of
compounds I, IV–VIII, XI, XIV, and XV, are
Table 2. The composition of gaseous products of thermal decomposition of azido compounds I, VII, VIII and XIII at 140°C
Composition (%), under a conversion
Comp. no.
5
20
100
N2 (100)
N2 (100)
N2 (100)
N2 (70.7), CO, and NO (11.3),
CO2 (8.7), N2O (9.3)
N2 (100)
–
–
I
VI
N2 (68.1), CO, and NO (11.3),
CO2 (13.1), N2O (12.4)
N2 (100)
N2 (14.8), NO (28.4),
N2O (3.5), CO2 (53.3)
N2 (100)
N2 (14.5), NO (31.2),
N2O (2.7), CO2 (51.6)
VIII
XVI
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 9 2012

1580
KRUGLYAKOVA, STEPANOV
ν, cm–1
Fig. 2. IR spectrum of condensed products of thermal decomposition of compound I in the melt: (1) initial, (2) 80% conversion.
stabilized through a dimerization [reaction (3)], and in
the case of compounds II, III, and X, through an
isomerization of nitrene [reaction (4)].
accepting substituents in the azole ring (NO2, C6H5,
CH3) have little effect on the rate of thermal
decomposition of the azide group, since there is no
resonance conjugation between them and the reaction
centre. For this reason, the substituents R1 = CH2N·
(NO2)CH3, CH2CH2N3, CH2CH2ONO2, CH2CCl2NO2,
and CH2COOCH2C(NO2)3 virtually “do no work.”
R1,R3–Tr–N: → R1,R3–Tr–N=N–Tr–R1,R3,
(3)
R2,R3–Tr–(CH2)nN: → R2,R3–Tr–CH=NH
(R2,R3–Tr–CH2CH–NH).
(4)
The mechanisms 1–4 explain well the effect of
substituents on the rate of decomposition of the azido
group in the azole ring. If we take into account that the
reactivity of the azido group depends not only on the
strength of the broken N1–N2 bond but also on the
thermodynamic stability of the resulting electron-
deficient nitrene, then the conjugation of nitrene
nitrogen with the double bond should increase the
thermodynamic stability of nitrene and the reactivity of
the azido group of the original azole. Therefore, the
electron-donating substituents in the azole ring should
promote the thermal decomposition. However, Table 1
shows that both electron-donating and electron-
The decrease in the activation energy of the thermal
decomposition of compound I in the melt (first two
lines of Table 1) can be attributed to the ionization of
the 3-azido-1,2,4-triazole, which is a weak NH-acid
[8]. The reaction of ionization of N–H bond, as noted
above, is heterogeneous, it occurs on the walls of the
reaction vessel, and shows a peculiar effect on the
reactivity of the azido group. Thus, according to two
octet resonance structures of azido group (A and B),
the order of breaking N1–N2 bond is equal to 1.5 [9].
This double-bonding in the anion is less than in the 3-
azido-1,2,4-triazole molecule.
N
N
A
N
N
N
B
N
N
N
N
_
H
HN
N
N
N
N
N
N
N
HN
N3
N
N
N
C
D
and, consequently, to the change in the mechanism of
the decomposition involving the homolytic cleavage of
the C–NO2 bond as the limiting step.
Therefore, the thermal decomposition of the ionized
form of 3-azido-1,2,4-triazole D proceeds with lower
activation energy than the non-ionized form C. At the
same time, the ionized form D is a stronger electron-
donating substituent compared with the molecular one
and favors the decomposition.
This is consistent with the analysis of the gas phase
of the decomposition products (Table 2), as well as
with the IR-spectroscopic study of the parent
compound XVI, which showed gradual disappearance
of the most intense bands at 800, 1300, and 1600 cm–1
in the course of the C(NO2)3 group decomposition.
Note that the shifts of these bands depending on the
Compared to other substituted azido-1,2,4-triazoles,
the activation energy of compounds XII, XVI is lower.
It may be due to the shift of the reaction site to the
carbon atom of the substituted dinitromethyl group
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 9 2012

KINETICS OF THERMAL DECOMPOSITION
NO2
1581
(5)
N3 Tr_C(NO2)2R
NO2
N3 Tr_C(NO2)R
NO2
+
_
_
N3 Tr_C
_
R
R is Br or NO2.
chemical environment at the trinitromethyl groups and
the aggregative state are usually negligible [10], which
is important for their identification. As for the azido
function in the azoles, the intensity of the absorption
band at 2160 cm–1 (solution in methylcyclohexane)
remains practically unchanged in the IR spectrum of
compound XVI up to 10–12% conversion.
decomposition (Table 1) compared with methylazide
(Ea = 170.7 kJ mol–1, log A = 14.45, k160°C = 7.0×
10–7 s–1 [5]) due to the steric effect of substituents.
If the azole ring is in the β-position, its effect is
almost unseen in the rate of thermal decomposition of
compound III. At the same time, as in the compounds
with the nitrogen–nitrogen bonds, for example, the
nitramines (TrCH2)2N–NO2 and (TrCH2CH2)2N–NO2,
the ratio of the rate constants of the thermal
decomposition of α- and β-substituted derivatives at
160°C is 98 [11].
In compounds II, III, and X the azido group is in
the alkyl fragment, so they should be regarded as
alkylazides with substituted 1,2,4-triazolyl functions.
Compound IX containing a “bent” azido group may
also be assigned to this group. The presence of 3-
nitroazolyl ring in the α-position to the azido group in
compound II tenfold increases the rate of the thermal
Compound IX decomposes homolytically, with the
primary break N2–N3 bond in the tetrazole ring by
another route than in compound III.
N
N
N N
N
N
O2N
O2N
N
N
N
N (CH2)2
NO2
N
(CH2)2
N
NO2
N
N
N
N
N
N
_N2
O2N
O2N
N
(CH2)2
N
N (CH2)2
O2N
N
C N
N
NO2
NO2
N
N
N
Reaction products.
N
C
N
(CH2)2
N
NO2
(6)
N
It is noteworthy that the activation energy and the
rate of thermal decomposition of the bent azido group
agree well with the corresponding parameters for the
linear form of azido group (compound III, Table 1).
However, in the latter case, the activation entropy of
the transition state is greater due to releasing of
internal rotation of the broken N2–N3 bond, which is
inhibited in the ground state of the molecule, and due
to a decrease in the frequency of deformation
vibrations of fragments associated with this bond [4].
the N1–N2 bond, but reduce the thermodynamic stability
of the formed nitrene.
Activation parameters of thermal decomposition of
compounds XII, XIV, and XVI differ considerably,
due to the different steric effect of α-substituents in the
gem-dinitromethyl function [3]. The reduced steric
influence on the gem-dinitromethyl group in
compound XIV (the sum of the steric constants of the
α-substituents CH2CH2 and NO2 ΣEs = –2.15)
compared to XVI (ΣEs = –5.64) increases the thermal
stability (the rate constant decreases ~29 times) of the
gem-dinitromethyl group. Therefore, the thermal
decomposition is initiated at the nitrogen atom N1 of
the azido group. Using IR spectroscopy, it was found
that when the degree of conversion of compound XIV
was 5–7% the intensity of the absorption band of azido
group in the ring (2160 cm–1) decreased, and in the gas
phase the only detected product was the molecular
In contrast to compounds III and IX with β-
substituents decomposing with almost equal rates, the
thermal decomposition of compound X, having a β-
branched substituent, proceeds with the rate 6.3–8.3
times higher. This increase in the rate, in our opinion,
is due to the induction effect of three electron-
accepting β-substituents, which, judging by the
activation energy, have little effect on the strength of
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 9 2012

1582
KRUGLYAKOVA, STEPANOV
nitrogen, therewith the most intense absorption bands
of gem-trinitromethyl group at 800, 1300 and 1000 cm–1
remained unchanged. Nitrogen and carbon oxides
appear in the later stages of decomposition (~12–15%).
Similarly, the thermal decomposition of compound XI
occurs through the primary homolysis of the N1–N2
bond in the azido function (Table 1).
(published: 1.4926). 1,3-Dinitrobenzene was recrystal-
lized twice from ethanol–chloroform (1:1), mp 89.5°C
(published: 89.7°C).
The analysis of gaseous decomposition products
was carried out on a LKhM-72 chromatograph
(temperature of the column with charcoal 140°C) and
IR spectrometer UR-10 with the gas cell. Spectra of
condensed products of thermal decomposition were ob-
tained in solutions of chloroform and dichloroethane.
The thermal decomposition of compound XV to
5% conversion proceeds with the formation of molecular
nitrogen and nitrogen dioxide, and therefore we can
assume that it takes two independent parallel routes,
through homolysis of C–NO2 and N1–N2 bonds, with a
slight predominance of the second route. In our case,
the infrared spectroscopy cannot distinguish between
these ways of decomposition, but a comparison of
compounds XIII and XV favors the second route.
The rate constants were calculated with the
equation of the first order reaction, as well as by the
Guggenheim method [16]. The error in determining the
rate constants did not exceed 6.5% in both methods.
The r.m.s. error of activation energy was
6.2 kJ mol–1, and of the logarithm of preexponential
factor, 0.53 logarithmic units.
Thus, we conclude that the rate and activation
parameters of thermal decomposition of polyfunctional
substituted 3(5)-azido-1,2,4-triazole are affected
significantly by the resonance conjugation of azido
group with the azole ring, while the nature of the
substituent in the azoles has little effect when the
substituent does not include a reaction centre to initiate
the thermal decomposition reaction. A similar pattern
of the influence of nature of substituents on the rate of
thermal decomposition has been observed previously
[13] for m-substituted phenylazides.
REFERENCES
1. Denault, G.C., Marx, P.C., and Takimoto, H.H., US
Gov. Res. Develop. Rep., 1967, vol. 67, no. 21, p. 66;
C. A., 1968, vol. 68, 77589.
2. Pevzner, M.S., Ross. Khim. Zh., 1997, vol. 41, no. 2,
p. 73.
3. Stepanov, R.S., Kruglyakova, L.A., and Astakhov, A.M.,
Zh. Obshch. Khim., 2007, vol. 77, no. 11, p. 1881.
4. Stepanov, R.S. and Kruglyakova, L.A., Kinetika i
Kataliz, 1986, vol. 27, no. 2, p. 479.
5. O’Dell, M. S. and de Darwent, B., Can. J. Chem., 1970,
vol. 48, no. 7, p. 1140.
EXPERIMENTAL
6. Geiseler, G. and König, W., Z. Phys. Chem. (DDR),
1964, vol. 227, nos. 1–2, p. 81.
7. Nazin, G.M. and Manelis, G.B., Izv. Akad. Nauk SSSR.
Ser. Khim., 1972, no. 4, p. 881.
The compounds I–XVI were synthesized and
purified by known methods [2, 14], they were
chromatographically pure and contained 99.4–99.8%
of the basic substance.
8. Pevzner, M.S., Martynova, M.N., and Timofeeva, T.N.,
Khim. Geterotsikl. Soed., 1974, no. 9, p. 1288.
9. L’Abbe, G., Chem. Rev., 1969, vol. 69, no. 3, p. 345.
10. Slovetskii, V.I., Usp. Khim., 1971, vol. 40, no. 4, p. 740.
11. Stepanov, R.S., Astakhov, A.M., Kruglyakova, L.A.,
and Nefedov, A.A., Zh. Obshch. Khim., 2002, vol. 72,
no. 8, p. 1375.
12. Prokudin, V.G., Poplavskii, V.S., and Ostrovskii, V.A.,
Izv. Akad. Nauk, Ser. Khim., 1996, no. 9, p. 2209.
13. Smith, P.A.S. and Hall, J.H., J. Am. Chem. Soc., 1962,
vol. 84, no. 3, p. 480.
14. Kofman, T.P., Trubitsyn, A.E., Dmitrienko, I.V.,
Glazkova, E.Yu., and Tselinskii, I.V., Zh. Org. Khim.,
2007, vol. 43, no. 5, p. 760.
15. Stepanov, R.S., Fiziko-khimicheskie ispytaniya vzryv-
chatykh veshchestv (Physico-Chemical Testing of
Explosives), Krasnoyarsk: Izd. Krasnoyarsk. Politekh.
Inst., ch. I., 1989.
Thermal decomposition of compounds I–XVI was
studied by the manometric method using a Bourdon
type glass gauge [15] at a residual pressure of air in the
reaction volume 10–2 to 10–3 mm Hg. To determine the
effect of S/V on the rate of decomposition of
compounds in the melt we used the Bourdon’s
manometers with the reaction volume in the form of a
globe, which allowed us to determine reliably the
surface of the reaction vessel.
Along with manometry, we used volumetric
method, by measuring the accumulation of molecular
nitrogen. Both methods yielded the same results in the
calculation of the rate constants.
Dibutyl phthalate was washed with 5 wt % sodium
carbonate and water, dried with magnesium sulfate and
then fractionally distilled in a vacuum: nD20 1.4931
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 9 2012