The influence of substituents position on products of dinitrotoluene isomers initial thermal decomposition
J Therm Anal Calorim
The influence of substituents position on products of dinitrotoluene isomers initial thermal decomposition
T. Gołofit 0
T. Zielenkiewicz 0
0 Faculty of Wood Technology, Warsaw University of Life Sciences , Nowoursynowska 166, 02-787 Warsaw , Poland
Thermal decomposition of dinitrotoluene (DNT) isomers and formed products were studied. Initial stage of thermal decomposition was the field of interests because decomposition of even a few percent of a substance may cause hazard. The generation of impurities at the initial stage of decomposition was examined on the basis of the decrease of the original substance melting enthalpy and the increase of impurities fraction determined with cryometric method. MALDI TOF was used as the independent method of formed compounds molar masses determination. Samples of DNT isomers at raised temperature (up till 480 K) undergo aggregation leading to macromolecular substances. Regardless the substituents position, the initial phase of decomposition leads to same intermediate products. Mass spectra of DNT isomers thermally conditioned contain strong signals m/z, among others, 688, 1063, 1137, 1211. Thermogravimetric analysis revealed that observed difficulties with studying the decomposition initial stage are related to high vapor pressure below decomposition temperature, what especially concerns 2,6-DNT isomer.
Dinitrotoluene; DSC; MALDI TOF; Nitro-compounds aggregation; Thermal decomposition
Faculty of Chemistry, Warsaw University of Technology,
Noakowskiego 3, 00-664 Warsaw, Poland
Industrial manufacturing of dinitro-derivatives of toluene
(DNT) is related to their wide application not only as high
energetic mixtures, but also as other semi-products in the
chemical industry, e.g., polymerization reactions [
knowledge about parameters determining thermal stability
of DNT allows safety maintenance during the performance
of technological operations with their participation.
Uncontrolled decomposition of nitro-compounds may lead
to the accident . That is why studies of hazard related to
high energetic compounds thermal decomposition are
]. Thermal explosion is the phenomenon which
causes the strongest hazard during technological processes,
storage and exploitation of nitro-compounds. Thermal
explosion occurs as the result of cumulation of heat released
in the decomposition reaction , and it may take place even
with low conversion degree [
]. That is why detailed studies
of the initial stage of thermal decomposition are significant.
The fact, that it occurs below the temperature when
exothermic effect of decomposition is visible, is its special
feature. From this reason, one of the methods of the initial
stage of decomposition process examination is liquid–
crystal equilibrium studying, and especially determination
of impurities content influence on this equilibrium
temperature shift [
]. The equilibrium is very sensitive to
impurities content and allows studying the initial stage of
decomposition. The aggregation of partially degraded
2,3DNT molecules to macromolecules with molecular mass
even above 1000 Da is also suggested [
]. The thermal
stability of these compounds is significantly different in
comparison with original substance.
Thermal decomposition of dinitrotoluenes was often
examined with DSC analysis [
reactionary calorimeter (ARC) was also used , and
decomposition products were determined with the
application of mass spectrometer [
]. The first stage of
nitrocompounds decomposition differs depending on the
substituents position in the aromatic ring. Nitro-group without
substituents in ortho position may undergo the homolytic
separation from the ring and following regrouping to nitrite
]. Two nitro-groups in ortho position may react
with each other forming benzofurazan and benzofuroxane
]. Nitro-group in the neighborhood of methyl group
leads to anthranil [
]. It is accepted that in case of
ortho-nitrotoluenes, homolytic cleavage of C–H bond in
the methyl group is the first stage of decomposition and in
the second step hydrogen attaches oxygen atom in
]. Then number of subsequent reaction take
place which forms gaseous products. Case’s studies 
indicate that at the temperature below 573 K, stable solid
phase product of dinitrotoluenes decomposition is formed
which immediately decomposes to gaseous products above
mentioned temperature. Maksimov [
] studied the thermal
decomposition of 2,4-DNT, 2,6-DNT and 3,5-DNT
analyzing gaseous products. He calculated decomposition rate
constants at 500 K, the highest for 2,4-DNT and the lowest
The relevant literature does not contain any systematic
studies about DNT isomers decomposition products and
influence of substituents on the thermal stability and
decomposition products. No complex studies containing
particular analysis of all DNT isomers with the same
analytical technique were performed. It is unfavorable
because changing of the method may influence the thermal
decomposition process [
]. That is why thermal stability
parameters and decomposition products cannot be
compared on the basis of the existing literature. The aim of this
paper is to perform complex examination of the influence
of nitro-groups position on all of the DNT isomers
decomposition products and the influence of conditioning
parameters on the decomposition process.
Examined substances were synthesized in Military
University of Technology in Warsaw. Table 1 presents
cryometrically determined purity and physicochemical
parameters of studied substances. Values from the
literature were also included for comparison. Purity was also
determined with gas chromatography; its value was 99.9%
for all substances.
Distinctions in determined samples purity are related to
the measuring methods specifics. Content of compounds
soluble in liquid phase are measured in cryometric method,
while in GC method—substances volatile in increased
temperature. It may lead to the conclusion that compounds
with high molecular mass are the impurities. Differences
between determined parameters (melting enthalpy and
temperature) and literature values may be related to
different purity of analyzed compounds.
Micro-calorimeter UNIPAN DSC 605 was used for studies.
It was calibrated with indium, cadmium, naphthalene, tin
and benzoic acid. Purity of metals used for calibration was
above 99.999%, and purity of organic compounds—above
99.95. Sample of analyzed substance was closed in
aluminum pan under reduced pressure of ca. 1.3 kPa.
Measurements were taken with 2 K min-1 rate.
Measurements of the melting process were terminated after the
formation of base line. Non-isothermal conditioning was
lead to the assumed temperature or visible exothermic
effect of decomposition, when the increase of calorimetric
signal indicated for high thermal power, experimentally
determined, which may lead to the device destruction.
Isothermal conditioning was conducted at the temperature
in the middle of melting point and decomposition
Liquid–crystal equilibrium was applied to analyze
decomposition products. This equilibrium is very sensitive
for impurities content in liquid phase. Small amount of
impurities causes significant decrease of substance melting
temperature. Following assumptions were introduced:
Components are totally miscible in the liquid phase and
totally immiscible in the solid phase; melting enthalpy of
components in considered temperature range is
approximately independent on the temperature; the solution is
diluted enough to be described with ideal solution model.
Molar concentration of impurities—purity of analyzed
compound may be calculated from cryometric equation
8, 9, 24, 25
where F—sample melting degree, x—real molar fraction of
impurities, Tm0—melting temperature of perfectly pure
substance, Tm—sample melting temperature,
DHm—melting enthalpy. On the basis of impurities molar fraction,
purity of the sample may be determined from following
P ¼ ð1
Sample becomes contaminated with decomposition
products during thermal analysis performance. Leading the
measurement to arbitrarily chosen final temperature Tf, the
Examination of strong exothermic decomposition of DNT
isomers was problematic because of high vapor pressure
and gaseous products of decomposition. High pressure
inside hermetic measuring pan made of aluminum foil
caused deformation and unsealing at high temperatures.
Timely termination of the measurement was the only way
to analyze decomposition products. Examining baseline of
thermo-analytical curve before low temperature
decomposition peak, specific deviation from linear character may
be observed. It is related to exothermic decomposition of
the compound. Figure 1 presents exemplary curve of
melting and decomposition of 14 mg 2,4-DNT sample (two
runs on the same sample).
The first run shows sample melting process and
hightemperature deviation of DSC curve indicating for the
beginning of compound exothermic decomposition.
Melting process in the second run begins at the lower
temperature, and its thermal effect is lower, what is caused by
initial substance decomposition and impurities production
in the first run. Formation of low molecular compounds as
the result of high-temperature conditioning was initially
expected. In that case, impurities increase denoted in mol%
should be higher than the decrease of the melting enthalpy.
increase of impurities takes place (Dx), which is equal to
the difference between molar fraction of impurities in the
sample taken for analysis (x0) and in the sample after
decomposition (xf). The increase of impurities fraction
equals the real molar fraction of impurities (x) related to
the thermal decomposition.
where xi—molar fraction of ‘‘i’’ decomposition product.
Introducing the assumption that one molecule of the liquid
phase impurity is formed during the decomposition of one
molecule of the examined substance, and other vapor
impurities are in the gaseous phase; the value of real molar
fraction of impurities (Dx) should be equal to the initial
substance conversion degree (af). If the value of Dx is
higher than af parameter, then it means that on average,
more than one molecule of the liquid phase impurities is
formed during decomposition of one molecule of initial
substance. However, when parameter Dx is lower than af
parameter, molecules of decomposition products in liquid
phase aggregate to each other forming molecules with
molecular mass higher than initial compound. The size of
these aggregates could be defined with the so-called mean
aggregation number (k) [
], which may be calculated
on the basis of following equation:
Dx ¼ k
MALDI TOF analysis of decomposition products
molecular mass was performed in the Centre of
Macromolecular Research in Lodz, using Kratos Kompact Maldi
4 apparatus with nitrogen laser with the wavelength of
22 nm, impulse of 3 ns, positive polarization, accelerating
voltage of 20 kV, 200 impulses for each spectrum.
Decomposition products were dissolved in acetone to
Thermogravimetric analysis was performed using TA
Instruments SDT Q600 device. Measurements were taken
for samples with mass of 5.2 ± 0.1 mg, in aluminum pans
covered or closed with the cover with 75 lm hole.
Nitrogen flow of 100 mL min-1 and temperature increase rate
b = 5.0 K min-1 were applied.
The opposite effect was denoted in all cases—impurities
increase was lower than enthalpy decrease. Mean
aggregation number was calculated, and results are presented in
Table 2. As it was mentioned above, decomposition often
leads to the measuring pan unsealing because of high
Comparing thermo-analytical curves of melting
processes of samples conditioned at temperatures much higher
than melting point, thermal decomposition of DNT may be
observed. Mean aggregation number k higher than 1
indicates for the isolation of impurities from the liquid phase or
the impurities reaction leading to compounds with higher
molecular mass. In order to confirm the possibility of high
molecular mass compounds formation, spectra of samples
with the application of MALDI TOF device were
performed. To verify MALDI TOF method usefulness for
decomposition products examination, samples before and
after conditioning were measured. Spectra obtained for
2,4DNT are presented in Fig. 2.
Mass spectrum of the standard sample contains low
signals with high m/z values (above 500). This material has
been stored for dozens of years at the ambient temperature.
Aggregation reaction could proceed in these conditions,
leading to macromolecular products formation. Mass
spectrum of thermally conditioned 2,4-DNT sample
contains strong signals with values of 551, 688, 1062, 1137,
1211. These results indicate that molecular mass of initial
decomposition products exceeds several dinitrotoluene
mass. Oligomers with similar molecular masses were
observed in the initial and in thermally conditioned sample.
The intensities of peaks in the initial sample are obviously
significantly lower in relation to conditioned samples.
Similar decomposition products testify that the mechanism
Dx 9 100
Fig. 2 MALDI TOF mass spectra of 2,4-DNT sample before and
after thermal conditioning
of DNT decomposition at increased and ambient
temperature is the same. The same test was carried out for the
other DNT isomers. The obtained spectra are shown in
Mass spectra for all isomers before and after
conditioning were compared. Oligomers with the same
molecular masses were observed in both initial and conditioned
samples. Intensities of peaks for conditioned samples are
higher again. That is why it can be concluded that for all
isomers, mechanism of decomposition at increased and
ambient temperature is the same. Repeatability of these
results despite differences in nitro-groups location is very
interesting. It may mean that regardless the location of
substituents, the initial stage of decomposition runs
similarly, but for sure leads to identical by-products. Only for
2,6-DNT sample, the number of repeated peaks with high
intensities is not visible. It is related to low temperature of
partial decomposition measurement termination.
Measurement of 2,6-DNT sample was terminated at only 459 K.
Fig. 3 MALDI TOF mass spectra of DNT isomers after
The increase of sample purity with 0.05% was observed
after measurement performed this way. Measurement that
leads to higher temperatures caused measuring pans
unsealing and examined substance pouring out.
Process of DNT isomers decomposition was analyzed
also in isothermal conditions. Cycle of measurements of
melting and isothermal conditioning at temperatures below
500 K was performed for each DNT isomer excluding
2,6DNT (because of problems mentioned above). Exemplary
series of 2,5-DNT melting is shown in Fig. 4.
As the result of consecutive processes of isothermal
conditioning, melting peaks become wider, lower, and shift
in the direction of lower temperatures, what is related to
impurities production. Samples after conditioning were
submitted for MALDI TOF analysis. Table 3 contains the
comparison of MALDI TOF results for all of examined
substances for the most intense peaks.
Similar decomposition products (the same molecular
masses) were obtained for all DNT isomers samples
conditioned isothermally, alike as earlier for non-isothermal
conditioning. It indicates again that regardless substituents
position, the initial stage of decomposition follows the
same way. From the other hand, the participation of
particular oligomers in the spectrum is dependent on
substituent position in the molecule. Peak with the highest
intensity was obtained for M/z equaled 688 (2,3-DNT) and
1137 (the rest of isomers).
Problems with partial decomposition measurements
performance in case of 2,6-DNT isomer could be related to
high pressure of compound vapor below the decomposition
temperature. To investigate this issue, total decomposition
process in samples of mass above 10 mg was studied with
thermogravimetry. The first series of measurements was
conducted in pans covered (but not closed) by the lid with
the hole from the safety reasons. Obtained TG and DSC
curves are presented in Fig. 5.
The mass loss of DNT isomers occurred in the
temperature range of 400–550 K. It was related to the sample
evaporation for all isomers. The mass loss process for
2,6DNT sample started and ended at the lowest temperature.
Rapid exothermic process of decomposition, which could
destroy measuring nest, was not observed. That is why
consecutive measuring series was conducted with the pan
close tight with the lid with the hole. Results are presented
in Fig. 6.
Closing of pans caused the increase of mass loss
beginning of about 70 K. Process of 2,6-DNT mass loss
began and ended at the lowest temperature again. It was
related to the sample evaporation (Tonset = 563 K,
Tmax = 567 K). Also for 2,5-DNT and 3,5-DNT, the mass
loss was connected to sample evaporation
(correspondingly: Tonset = 571 K, Tmax = 579 K; Tonset = 586 K,
Tmax = 592 K). Insignificant exothermic process related to
decomposition is visible at the end of evaporation process.
Below 1% of solid residue remained after the
decomposition of 2,5 2,6 and 3,5-DNT at the temperature of 650 K.
Process of 2,3-DNT mass loss was exothermic with
overlaying strong endothermic effect (Tonset = 587 K,
Tmax = 591 K). Its course is irregular, what is related to
blocking of the lid hole with decomposition products. Solid
residue after 2,3-DNT decomposition (at 650 K) was 4%.
Process of 2,4 and 3,4-DNT mass loss was exothermic
(correspondingly: Tonset = 578 K, Tmax = 582 K;
Tonset = 587 K, Tmax = 592 K). Solid residue value after
decomposition at 650 K was the highest and equaled
correspondingly 12 and 13%.
Fig. 4 Series of DSC curves of 2,5-DNT melting process preceded
by isothermal conditioning
Obtained results confirmed that problems with the
analysis of 2,6-DNT decomposition initial stage are
regarded to high vapor pressure below the decomposition
temperature. 2,4-DNT was the compound decomposing at
the lowest temperature. These results are compatible with
literature data [
DNT isomers are polar molecules, and their dipole moment
may be responsible for their tendency to aggregation. High
polarity leads to the molecules approach and
intermolecular reactions. Increased temperature, up till 480 K, does
not cause advanced decomposition, but contrary,
aggregation is the dominant reaction at these temperatures. MALDI
TOF analysis confirms this thesis, as well as the decrease of
melting enthalpy higher than cryometric estimated increase
of impurities content. Besides the aggregation causes the
Fig. 6 TG (a) and DSC (b) curves of mass loss and related thermal
effects of DNT isomers samples heated with the temperature increase
rate of 5 K min-1 in pans closed tight with the lid with 75 lm hole
decrease of molar impurities fraction, it may be also
decreased with the lack of solubility of impurities in the
liquid phase of DNT. It also testifies for the aggregation
because macromolecules hardly dissolve in liquid organic
substances. Aggregation causes the increase of content of
compounds which are thermally more stable than initial
substance, and improvement of the sample stability. The
analysis of the initial stage of DNT isomers decomposition
revealed that both at the ambient and increased
temperature, similar macromolecular aggregation products are
formed and it does not depend on substituents location.
Mass spectra of thermally conditioned DNT samples
contain strong signals m/z with values of (among others) 688,
1063, 1137 and 1211.
Thermogravimetric studies confirmed that problems
with the examination of the initial stage of 2,6-DNT
decomposition are related to high vapor pressure below the
decomposition temperature. 2,4-DNT is the isomer
decomposing at the lowest temperature.
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