Gas-phase pyrolyses of 2-nitropropane and 2-nitropropanol: Shock-tube kinetics

Article abstract of DOI:10.1021/jp993204e

The kinetics of the thermal decomposition of 2-nitropropane was studied in a shock tube at 970-1150 K under high dilution in argon at 4.5-6 atm. The decay of 2-nitropropane and the production of NO₂ were followed spectrophotometrically at 276 and 405 nm, respectively. The major products were NO, CO, CH₃CH=CH₂, CH₃CHO, C₂H₄, and (CH₃)₂CO. The decomposition proceeded primarily through C-N bond fission, while the contribution from the five center elimination of HONO accounted for < 20% of the total loss of 2-nitropropane. A reaction mechanism consisting of 81 elementary steps was proposed, accounting quantitatively for the overall pyrolysis. When the mechanism was subjected to sensitivity and principal component analysis, only 58 reactions in the mechanism were needed to reproduce the product observed distributions. The initial stage of the decomposition of 2-nitropropanol was also studied. The rate of the pyrolysis was similar to that of the 2-nitropropane over the temperature range studied, but had a much lower activation energy.

Full text of DOI:10.1021/jp993204e

J. Phys. Chem. A 2000, 104, 1207-1216
1207
Gas-Phase Pyrolyses of 2-Nitropropane and 2-Nitropropanol: Shock-Tube Kinetics
Yi-Xue Zhang and S. H. Bauer*
Department of Chemistry and Chemical Biology, Baker Chemical Laboratory, Cornell UniVersity,
Ithaca, New York 14853
ReceiVed: September 10, 1999; In Final Form: October 29, 1999
The kinetics of the thermal decomposition of 2-nitropropane was investigated in a shock tube over the
temperature range 970-1150 K under high dilution in argon at total pressures 4.5-6 atm. The decay of
2-nitropropane and the production of NO2 were followed spectrophotometrically at 276 and 405 nm,
respectively. The unimolecular rate constant deduced from the loss of 2-nitropropane at the high-pressure
limit is: k∞ ) 1014.55(0.13 exp{(-54.2 ( 3.8 kcal/mol)/RT} s-1. The products of pyrolysis were identified and
quantified with FTIR and GC. The major products are NO, CH3CHdCH2, CO, CH3CHO, C2H4, and
(CH3)2CO. The decomposition proceeds primarily through C-N bond fission, while the contribution from
the five center elimination of HONO accounts for less than 20% of the total loss of 2-nitropropane. A reaction
mechanism that consists of 81 elementary steps is proposed; it accounts quantitatively for the overall pyrolysis.
Numerical simulations with the mechanism reproduce reasonably well the NO2 vs time profiles at different
temperatures and the distributions of major reaction products over the temperature range studied. When the
mechanism was subjected to a sensitivity and principal component analysis, it was found that only 58 reactions
in the mechanism are needed to faithfully reproduce the observed product distributions. The fragmentation
and reaction sequence in the pyrolysis is graphically presented so that the fundamental chemistry of the
overall reaction can be readily visualized. The initial stage of the decomposition of 2-nitropropanol was also
investigated following the above experimental protocol. The rate of the pyrolysis appears to be similar to that
of the 2-nitropropane over the temperature range studied, but it has a much lower activation energy. The
unimolecular rate constant can be expressed as 1011.29(0.78 exp{(-37.5 ( 3.6 kcal/mol)/RT} s-1. No pressure
dependence was observed over the pressure range investigated (4-5.3 atm), and therefore it is presumed that
the rate constant represents the high-pressure limit. Both the low A factor and the low activation energy
suggest the involvement of the OH group in the first step of the reaction. NO2 production and subsequent
consumption during the decomposition was also observed. The reaction products from this species are similar
to those of 2-nitropropane.
Introduction
by increasing the pressure of the He driver until a selected Mylar
diaphragm broke. Typical pressures in the driver and driven
Current high explosives and propellants incorporate multiple
NO2 functional groups.1,2 The initial steps in the thermal
decompositions of these compounds (e.g., hexahydro-1,3,5,-
trinitro-s-triazine (RDX), 1,3,5,7,-tetranitro-1,3,5,7,-tetraaza-
cyclooctane (HMX), and 1,3,3,-trinitroazetidine (TNAZ)) typi-
cally involve the fission or transformation of the weakest C-N
bonds.3-5 But the overall decomposition mechanisms are not
clear cut. A systematic study of simple prototype nitro com-
pounds may facilitate the determination of basic bond dissocia-
tion energies of the various types of C-N bonds and may reveal
the general mechanistic features for the fragmentation sequence
of the corresponding nitro compounds. A review of previous
investigations of these materials was recently reported by Zhang
and Bauer.6 Herein, we describe an investigation of the thermal
decomposition of 2-nitropropane. A brief exploration of the
initial stages of the decomposition of 2-nitropropanol is also
included. This suggests different routes that might be followed
in the thermal decomposition of nitro compounds.
sections were 70 psig and 350-520 Torr, respectively. Two
piezoelectric pressure sensors are stationed 10.0 cm apart at the
end of the driven section. Their summed signal was recorded
and digitized in a Biomation 8100 transient recorder, and then
stored in a IBM 386 computer. The accuracy of the recorded
time intervals is (1 µs; this corresponds to an ambiguity of
∼8 K in the reflected shock temperature. The effective residence
time under reflected shock conditions (∼1.3 ms) was defined
as the elapsed time from the initiation of the reflected shock
wave to the time when the pressure signal had decayed to 80%
of the initial reflected shock level. The temperature of the
reflected shock was estimated according to the computational
procedure recommended by Gardiner et al.7 for very dilute
reaction mixtures. The entire shock tube and gas manipulating
lines were kept at 130 °C to minimize adsorption of the
nitropropanes onto the shock tube walls.
High purity Ar was used as the diluent. 2-Nitropropane
(>97%, Aldrich) and 2-nitropropanol (98%, Aldrich) were used
without further purification; typical concentrations of these
compounds in the shocked mixtures were ∼0.65% (mole
fraction). The initial i-C3H7NO2/Ar and i-C3H6(OH)NO2/Ar
mixtures were prepared in a ∼6 L Pyrex vessel and allowed to
Experimental Section
The shock tube is constructed of stainless steel; its internal
diameter is 2.50 cm. The length of the driver and driven sections
are 120 and 170 cm, respectively. Shock waves were generated
10.1021/jp993204e CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/26/2000

1208 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Zhang and Bauer
valve at the end of the driven section of the shock tube. A small
portion of this sample was then injected into the GC column
through a six-port valve for analysis. An HP 18790 chromato-
graph with a thermal conductivity detector was used to separate
and determine the concentrations of products using a 30 ft ×
1/8 in. stainless steel tube packed with Hayesep DB 100/120.
The flow rate of the carrier gas (He) was typically 25 mL/min.
The column temperature was held at 25 °C for the first 5 min
and then ramped at a rate of 12 °C/min until it reached the
final temperature 240 °C. The following major reaction products
were identified: CH4, C2H4, CH2O, CH3CHdCH2, CH3CHO,
(CH3)2CO. Traces of CH3CN, C3H8, i-C4H10, i-C4H8, and C2H6
were also formed.
Results
The unimolecular rate constant for the initial decay rate of
2-nitropropane was derived from first order plots of absorption
intensity loss, recorded spectrophotometrically at 276 nm, during
the early part of the reaction. A typical experimental scan is
shown in Figure 3a, b. The initial rapid increase in absorbance
was due to the sudden jump in temperature and pressure of the
reaction mixture upon the arrival of the incident and reflected
shock waves. In most runs the logarithm of the absorbance vs
time at 276 nm was linear up to 80% completion of the reaction
and then decreased further with a lesser slope. The Arrhenius
plot of kuni obtained from early parts of the 2-nitropropane decay
curves is shown in Figure 4. That rate constant can be expressed
as
Figure 1. UV spectra of 2-nitropropane (1.936 × 10-3 M) and
2-nitropropanol (1.804 × 10-3 M). The solvent is acetonitrile and cell
length is 1 cm.
mix at 130 °C for ∼12 h before use. Research grade CO, NO,
CH4, C2H6, C2H4, CH3CHdCH2, C3H8, i-C4H8, i-C4H10, and
CH3CHO (Aldrich) and a CH2O solution were used for GC and
FTIR calibrations.
kuni ) 1014.55(0.13 exp{(-54.2 ( 3.8 kcal/mol)/RT} s-1 (1)
Analytical Methods. Changes in the concentrations of
i-C3H7NO2, i-C3H6(OH)NO2, and NO2 during the reaction were
followed spectrophotometrically by recording their UV absorp-
tions at 276 and 405 nm, respectively. The UV spectra of
2-nitropropane and 2-nitropropanol (both in CH3CN) were
determined with a HP8451A diode array UV-visible spec-
trometer (Figure 1). Their extinction coefficients at 276 nm
(room temperature) are 34 and 46 M-1 cm-1, respectively. The
extinction coefficient of NO2 at elevated temperatures was
determined by Huffman and Davidson8 and can be represented
by:9 ꢀ(M-1 cm-1) ) 179.6 - 0.05475T + 6.33 × 10-6 T2, at
405 nm, over the temperature range 400-2000 K. UV light
emitted from a xenon arc lamp (Oriel) was directed normal to
the axis of the shock tube through a quartz extension attached
to the end of the shock tube. The quartz cylinder was partially
aluminized so that the incoming light is internally reflected
several times before leaving the tube. The effective path length
(13 cm) was determined using diacetyl-Ar mixtures of known
concentration at 310 nm (extinction coefficient is 15 at 298 K).
The multiple pass configuration greatly enhances the sensitivity
of the UV absorption measurements. Details of the construction
of this extension have been described previously.9
Three methods were used to identify and quantify the reaction
products. Some of the compounds were identified by GC/MS
and GC/FTIR. Samples extracted from the terminal end of the
tube after the shock heated gas was quenched were analyzed
with a Polaris FTIR spectrometer (Matheson Instruments, Inc).
Integrated peak areas were used to determine quantitatively the
concentrations of CO and NO. Beer’s law proved valid for these
species over the concentration range encountered in this work.
A typical spectrum of a product mixture is shown in Figure 2.
The major contribution to the CO2 peak is the residual CO2 in
the FTIR chamber. Immediately after each shock, a 16 mL
sample at a known pressure was collected through the sampling
Since no pressure dependence was observed, we assume that
the rate constant is at the high-pressure limit. The reported kuni
by Gla¨nzer and Troe10 is 1015.5 exp{(-54 kcal/mol)/RT} s-1
.
While their activation energy is close to that in eq 1, their A
factor is larger by a factor of 10. They proposed that C-NO2
bond fission was the predominant fragmentation step, over this
temperature range, while the contribution from elimination of
HONO is small.
NO2 is an important intermediate in the decomposition of
nitro compounds. A clear understanding of its kinetics is crucial
for elucidating the mechanism of the overall conversion. We
monitored the NO2 concentration vs time during the decomposi-
tion under a variety of experimental conditions. Typical results
are shown in Figures 5 and 6. The NO2 profiles resemble those
observed in the decomposition of nitromethane, suggesting
similarities in the mechanisms of the two systems. At the lower
temperatures, NO2 was produced and consumed relatively
slowly; whereas at higher temperatures both processes were very
fast. The final NO2 concentrations were very low, both for low
and high-temperature runs. This is consistent with the FTIR
spectra of the product mixtures, which show no NO2 peak. The
dotted lines in the figures are computer simulations described
below.
The distributions of the major products over the experimental
temperature range are presented in Figures 7-9. As re-
ported previously for the low-temperature decompositions,
CH3CHdCH2 and NO are the most abundant species. Roughly
half of the original 2-nitropropane is converted to propylene.
There appeared traces of CH3CN, C2H6, C3H8, and C4H10 in
the final mixture, but their concentrations were not quantitatively
determined. HONO and HCN were not found in the quenched
reaction mixtures. The solid lines in the figures were derived
by numerical simulations as described below.

Pyrolyses of 2-Nitropropane and 2-Nitropropanol
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1209
Figure 2. FTIR spectra of the quenched mixture of products developed during 1.3 ms residence time under reflected shock conditions, for
2-nitropropane. Conditions: T ) 1020 K, P ) 5 atm Ar; [i-C3H7NO2]o ) 4.9 × 10-7 mol/cm3. Acetone is abbreviated as AC in the figure.
The thermal decomposition of 2-nitropropanol was also
investigated; the same procedures were followed. The UV
absorption of 2-nitropropanol at 276 nm was recorded, from
which unimolecular rate constants were estimated. A typical
kinetic curve for this compound is shown in Figure 10. Note
that during the later part of the reaction, the recorded values
deviate considerably from first order, indicating either that the
reaction was slowed by some of the secondary reactions or that
some of the products also absorb strongly in this region. The
initial rates of the reaction were used to determine the Arrhenius
parameters for the decomposition. The derived unimolecular rate
constant (see Figure 11) can be expressed as
kuni ) 1011.29(0.78 exp{(-37.5 ( 3.6 kcal/mol)/RT} s-1 (2)
NO2 accumulation and its subsequent consumption were ob-
served at 405 nm for the tested temperature range. GC analysis
of postshock mixtures shows compositions quite similar to those
generated in the 2-nitropropane system. However, quantitative
determinations of product distributions were not attempted.

1210 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Zhang and Bauer
Figure 5. Solid line: scaled recorded [NO2] vs time for 2-nitropropane.
Conditions: T ) 1033 K, P ) 5.2 atm, [i-C3H7NO2]o ) 5.0 × 10-7
mol/cm3. The dashed line is the computed [NO2] vs time, per the
mechanism in Table 1.
Figure 3. (a) Absorbance vs time (276 nm) during the thermal
decomposition i-C3H7NO2 at 1050 K. The first and second rises in
absorbance are due to the density and temperature increases developed
by the incident and reflected shock waves, respectively. Total pressure
) 5 atm. [i-C3H7NO2]o ) 5.0 × 10-7 mol/cm3. (b) The logarithm of
absorbance for of 2-nitropropane vs time at 276 nm.
Figure 6. Solid line: scaled recorded [NO2] vs time for 2-nitropropane.
Conditions: T ) 1118, P ) 5.6 atm, [i-C3H7NO2]o ) 5.0 × 10-7 mol/
cm3. The dashed line is the computed [NO2] vs time, per the mechanism
in Table 1.
The early studies were summarized by Nazin et al.11,12 Recent
investigations with shock-tube10,25 and molecular beam tech-
niques13 refined the unimolecular rate constants and provided
convincing experimental evidence for multiple pathways for the
initiation of these reactions. Three elementary reactions have
been invoked by various researchers to explain their experi-
mental results: (1) C-NO2 bond fission; (2) HONO elimination
(when a â-H is available); (3) C-NO2 to C-ONO isomeriza-
tion. Other energetically unfavorable reactions such as HNO
elimination with an H from the geminal position, e.g.,
CH3NO2 f CH2O + HNO and i-C3H7NO2 f (CH3)2CO +
HNO, and the loss of oxygen from the nitro group, e.g.,
CH3NO2 f CH3NO + O, were also postulated.
Figure 4. Arrhenius plot of the derived rate constants, for 2-nitropro-
pane. Total pressures at reflected shock conditions are 4.5-5.5 atm.
[i-C3H7NO2]o is 4.5 × 10-7 - 5.5 × 10-7 mol/cm3. Symbols are
experimental values and the solid line is the least-squares fit of the
data.
The thermal decomposition of 2-nitropropane was studied by
Frejacquens,14 Smith and Calvert,15 and Waddington and
Warriss16 in static reactors, and by Wilde17 in a flow system.
The temperature and pressure ranges in these studies were 411-
713 K and 4-40 Torr, respectively. They found that the
Discussion
The thermal decompositions of aliphatic nitro compounds
have been extensively investigated over the past few decades.

Pyrolyses of 2-Nitropropane and 2-Nitropropanol
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1211
Figure 9. The measured formaldehyde levels (of questionable preci-
sion) developed during the thermal decomposition of 2-nitropropane
(symbols). The solid line was based on numerical simulations, per the
mechanism in Table 1.
Figure 7. Observed vs computed product distributions developed
during the thermal decomposition of 2-nitropropane. The symbols are
experimental values and the solid lines are based on numerical
simulations, per the mechanism in Table 1.
Figure 8. Observed vs computed product distributions developed
during the thermal decomposition of 2-nitropropane. The symbols are
experimental values and the solid lines are based on numerical
simulations, per the mechanism in Table 1.
pyrolysis follows first order kinetics. The rate constants deduced
in these investigations were mutually consistent, with A factors
in the range 11-11.3 and activation energies in the range
37.67-39.3 kcal/mol. Spokes and Benson18 described a very
low-pressure pyrolysis experiment with 1-nitro- and 2-nitro-
propanes. They adjusted the above magnitudes to 11.3 ( 0.2
and 40 ( 0.5 kcal /mol, respectively. Although the derived
kinetic parameters from the above investigations were very
similar, the product distributions were not. Clearly, the reactor
surfaces considerably affected the rates and the mechanisms of
pyrolysis. For example, Smith and Calvert15 found that packing
the reactor with Pyrex wool increased the rate of the reaction
so that water, acetonitrile, and acetone became the most
abundant products instead of propylene and nitric oxide. This
is also an indication that free radicals are actively involved in
these conversions. The effect of the reactor surface was further
Figure 10. (a) Absorbance vs time (276 nm) during the thermal
decomposition i-CH6(OH)NO2 at 1034 K. The first and second rises
in absorbance are due to the density and temperature increases
developed by the incident and reflected shock waves, respectively. Total
pressure ) 4.8 atm. [i-CH6(OH)NO2] ) 5.1 × 10-7 mol/cm3. (b) The
logarithm of absorbance for of 2-nitropropanol vs time at 276 nm.
examined by Palo et al.19 using laser-powered homogeneous
pyrolysis (with a continuos-wave CO2 laser and reactants
photosensitized with SF6). They then investigated the decom-
position via surface pyrolysis in a micropulse reactor. They
found that in the former case, the reaction proceeded entirely
through C-NO2 bond fission, whereas in the latter large
amounts of additional products such as 2-nitropropene, aceto-

1212 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Zhang and Bauer
While most of the investigations discussed above focused on
the initial steps of the dissociation, we are concerned with both
the initial steps of the reaction and the subsequent secondary
reactions that lead to the final products. A clear understanding
of the decomposition mechanism of the overall decomposition
process is critical for determining the fundamental kinetics of
high explosives.
It is evident that there are two major routes for the thermal
decomposition of 2-nitropropane: C-N bond fission and HONO
five-center elimination. The latter appears to be the dominant
reaction at low temperatures, while under our conditions the
first step is primarily C-N bond fission
(CH3) 2CHNO2 f C3H7• + NO2
(7)
The C3H7• radical further dissociates to generate several sets
of products, but primarily CH3CHdCH2 and H. Subsequent
reactions of H with NO2 and the parent compound initiate a
sequence of secondary reactions. A complete mechanism for
the conversion is presented in Table 1. To fully describe the
pyrolysis over wide temperature and pressure ranges, we
included in the mechanism as many steps as are plausible. The
rate constants for most of the reactions are available in the
literature, the NIST Chemical Kinetics Database developed by
Mallard et al.24 is the main reference for these constants. For
steps for which there are no reported rate constants, we propose
estimates based on analogous reactions. The five-center HONO
elimination as the first step of pyrolysis is also included. Our
recommended rate constant for this reaction is 1011 exp{(-41
kcal/mol)/RT} s-1. The A factor is low, consistent with the
postulate that the transition structure is a moderately rigid ring.
The rate equations for the mechanism in Table 1 were
numerically integrated using the temperatures, pressures, and
concentrations calculated for the reflected shock regime from
the measured shock speeds. The computed results were then
compared with the experimental data. The NO2 profiles at
various temperatures are adequately reproduced. Overall, the
agreement between experiments and simulations is reasonably
good. Typical curves are dashed lines in Figures 5 and 6. These
profiles are similar to the ones computed for the thermal
decomposition of nitromethane, but they differ markedly from
those recorded for 1,3,3-trinitroazetidine (TNAZ). In the case
of TNAZ, there are not sufficient reactive intermediates to
completely consume the excess NO2. However, in all of these
systems, the main NO2 consuming step is the reaction between
H and NO2. Note that there are a number of secondary reactions
that consume 2-nitropropane (reactions 9, 10, 23, 80, and 81),
but their contributions are small (less than 10%); the simulated
2-nitroproane decay curve remains first order, but deviates
somewhat from the predicted curve from the first order rate
constant in eq 1.
Figure 11. Arrhenius plot for the derived rate constants, for 2-nitro-
propanol. Total pressures at reflected shock condition are 4-5.3 atm.
[i-C3H6(OH)NO2]o is 3.5 × 10-7 -5.6 × 10-7 mol/cm3. Symbols are
experimental values and the solid line is the least-squares fit of the
data.
nitrile, and acetone were generated. These indicate that unique
routes are followed in surface promoted reactions. In contrast,
Gla¨nzer and Troe10 studied this decomposition over the tem-
perature range 915-1200 K at total gas concentration (mainly
Ar) 7 × 10-6-1.5 × 10-4 mol/cc. The conversion was followed
spectrophotometrically. They deduced k ) 1015.5 exp{(-54
kcal/mol)/RT} s-1 and proposed that the reaction was entirely
initiated by C-NO2 bond fission, while the HONO elimination
pathway was negligible. The transient behavior of NO2 was
monitored but its analysis proved to be difficult because of the
lack of the knowledge of the secondary reactions. In a separate
report20 they suggested that C-NO2 bond fission remained
dominant even at low temperatures (e.g., 400 K).
A number of investigators21-23 also studied the photodecom-
position of 2-nitropropane. Radhakrishnan et al.21 induced
photodissociation with UV irradiation at 222, 249, and 308 nm
under collision-free conditions and detected different sets of
products. They hypothesized the following four pathways to
account for their observations: Reactions 4 and 5 may be
The mechanism in Table 1 reproduces well the distributions
of major products. The results for CH3CHdCH2, NO, CO,
CH3CHO, C2H4, and (CH3)2CO are shown in Figures 7 and 8.
The symbols in the figures are experimental values and the solid
lines are numerical simulations based on the proposed mech-
anism. The most serious discrepancy between experiment and
simulation appears for CH2O; refer to Figure 9. The computed
trend is probably correct because the measured concentrations
are subjected to large errors. The amount of CH2O is very small
and its presence was estimated from a tiny peak in our GC plot
(a shoulder of a large peak). Further refinement of our analytical
methods is needed to clarify this point. Note that the proposed
mechanism predicts traces of C4H10, C3H6, C4H8, and CH3CN
identical, since HONO quickly dissociates to HO and NO;
reaction 3 has not been proposed previously.
Wodtke et al.13 induced infrared multiple-photon dissociation
(IRMPD) of several nitroalkanes in molecular beams. Their
experimental data revealed that the 2-nitropropane decomposed
through both HONO elimination and C-NO2 bond fission. The
branching ratio (kHONO/ kC-N) under their conditions was 0.5 in
favor of C-N bond fission.

Pyrolyses of 2-Nitropropane and 2-Nitropropanol
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1213
TABLE 1: Decomposition Mechanism of 2-Nitropropanea
no.
reaction
A
n
Ea
54.2
37.23
42.12
0
0
0
0
0
8.39
0
0
0
41.0
0
ref
1
2
3
4
5
6
7
8
9
i-C3H7NO2 f i-C3H7 + NO2
i-C3H7 f CH3CHdCH2 +H
3.548E+14
2.000E+14
2.000E+13
5.636E+13
4.571E+12
3.631E+12
1.514E+14
6.700E+13
1.380E+11
2.500E+11
2.399E+13
2.000E+13
1.581E+16
2.042E+13
3.162E+13
6.310E+12
1.995E+14
5.012E+12
1.191E+21
2.000E+14
1.000E+15
1.900E+11
2.500E+12
1.000E+12
4.000E+11
1.000E+11
7.228E+12
5.984E+11
7.228E+12
1.300E+13
5.984E+11
2.188E+15
2.489E+17
2.190E+15
6.000E+09
2.130E+14
2.000E+12
4.819E+13
1.811E+13
8.000E+12
6.030E+11
1.000E+09
1.259E+13
3.981E+12
5.010E+12
9.550E+12
2.000E+15
1.790E+08
1.000E+13
4.000E+13
3.119E+11
1.200E+15
1.300E+13
1.110E+22
7.079E+12
1.470E+14
3.467E+12
2.120E+15
9.640E+13
1.000E+07
1.000E+06
3.590E+20 (ko)
2.070E+13 (k )
1.259E+17 (ko)
1.780E+16 (k )
1.9200E+10
4.5600E+10
1.310E+11
4.710E+07
8.390e+10
2.000e+12
2.000e+12
2.100e+11
1.000e+11
0
0
0
b
b
i-C3H7 f CH3 + C2H4
26
27
27
27
28
b
29
b
27
b
30
31
32
31
33
33
34
35
35
36
b
37
38
b
36
b
39
40
40
41
42
43
34
34
37
34
34
b
i-C3H7 + CH3 f i-C4H10
-0.68
0.68
0
0
0
0
0
0
0
0
0
0
0
0
0
-3.8
0
0
2.0
0
0
0
0
0
2.5
0
0
i-C3H7 + CH3 f CH3CHdCH2 + CH4
i-C3H7 + H f CH3CHdCH2 + H2
i-C3H7 + H f C3H8
H + NO2 f HO + NO
i-C3H7NO2 + CH3 f i-C3H6NO2 + CH4
i-C3H7NO2 + OH f i-C3H6NO2 + H2O
i-C3H7 + OH f CH3CHdCH2 + H2O
i-C3H7 + NO2 f i-C3H7ONO
i-C3H7ONO f i-C3H7O + NO
i-C3H7O + NO f i-C3H7ONO
i-C3H7O + NO f i-C3H7NO2
i-C3H7O + NO f CH3)2CO + HNO
i-C3H7ONO f (CH3)2CO + HNO
i-C3H7ONO f CH3CHdCH2 + HONO
HONO + M fHO + NO + M
i-C3H7O f (CH3)2CO + H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
0
0
38.90
37.90
48.37
21.50
17.50
2.78
0
0
0
0
1.30
5.82
2.90
1.56
5.82
0
0
0
0
43.0
0
0.99
0.993
0
1.987
0
33.40
36.50
31.40
46.11
79.10
2.464
0
4.207
27.09
12.52
0
22.592
11.127
0
2.6407
35.01
15.10
0
i-C3H7O f CH3CHO + CH3
CH3CHdCH2 + OH f CH2CHdCH + H2O
i-C3H7NO2 + H f i-C3H6NO2 + H2
i-C3H7 + HNO fC3H8 + NO
CH3O + NO2 f CH2O + HONO
CH3CHdCH2 + OH f CH2dCCH3 + H2O
CH3CHdCH2 + H f C2H4 + CH3
CH3CHdCH2 + H f CH2CHdCH + H2
CH3CHdCH2 + H f n-C3H7
CH3CHdCH2 + H f i-C3H7
CH3CHdCH2 + H f CH2dCCH3 + H2
H + H + M f H2 + M
2.5
-1.0
H + OH + M f H2O + M
-2.0
CH3 + CH3 f C2H6
-1
HNO + HNO f H2O + N2O
HNO f H + NO
0
0
0
0
0
0
0
0
0
0
0
0
0
5.64
0
0
0
0
CH3 + HNO f CH4 + NO
OH + HNO f H2O + NO
H + HNO f H2 + NO
HNO + NO2 f HNO2 + NO
HNO + HCO f NO + CH2O
i-C3H6NO2 f CH3CHdCH2 + NO2
CH3CHdCH f CH3 + C2H2
CH3CHdCH f CH3CtCH + H
CH3CHO + M f CH3 + HCO + M
CH2dCCH3 f CH3CtCH + H
CH3CHO f CH3 + HCO
CH3CHO + CH3 f CH4 + CH3CO
CH3CHO + OH f CH3CO + H2O
CH3CHO + H f CH3CO + H2
CH3CHO + NO2 f CH3CO + HONO
CH3CO + M f CH3 + CO + M
CH3 + NO2 f CH3O + NO
CH3O + M f CH2O + H + M
CH3 + C2H4 f CH3CHdCH2 + H
H + NO f HNO
CH2O + H f HCO + H2
CH2O + M f CO + H2 + M
H + N2O f OH + N2
34
b
26
26
42
44
45
41
45
45
46
45
47
54
48
34
b
0
-7.5
0
-0.41
1.27
0
0
0
0
49
34
b
i-C3H6NO2 f CH2O + CH3CNOH
CH3CNOH f CH3CN + OH
CH3 + NO2 + (M) f CH3NO2 + (M)
0
0
0
b
-6
-0.6
0
47
47
b
63
CH3NO2 + (M) fCH3 + NO2 + (M)
42.0
58.5
9.14
8.67
17.61
16.24
18.99
0
0
b
64
65
66
67
68
69
70
71
72
n-C3H7 + H2 f C3H8 + H
2.48
3.28
1.82
2.00
2.38
0
0
0
0
27
27
42
36
36
b
b
b
b
i-C3H7 + H2 f C3H8 + H
CH3CO + H2 f CH3CHO + H
CH3CHdCH2 + CH3CO f CH3CHO + CH2CHdCH2
CH2CHdCH2 + H2 f CH3CHdCH2 + H
CH2CHdCH2 + CH3 f n-C4H8
CH2CHdCH2 + CH3 f i-C4H8
0
0
0
CH3CHdCH2 + OH f CH3CHO + CH3
CH3CHdCH2 + OH f CH2O + C2H5

1214 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Zhang and Bauer
TABLE 1 (Continued)
no.
reaction
A
n
Ea
ref
73
74
75
76
77
78
79
80
81
C2H5 f H + C2H4
4.310e+12
2.110e+12
1.000e+11
8.430e+10
2.050e+13
3.120e+12
1.000e+15
8.000E+11
8.000E+11
1.19
0
0
0
37.20
0.813
41
0.262
0.131
0.131
55.0
0
42
26
b
36
36
36
b
CH2CHdCH2 + NO f CH2CHCH2NO
i-C3H7NO2 f HONO + CH3CHdCH2
CH2CHdCH2 + CH2CHdCH2 f CH2CCH2 + CH3CHdCH2
CH2CHdCH2 + n-C3H7 f C6H12
0
CH2CHdCH2 + i-C3H7 f CH3CHdCH2 + CH3CHdCH2
CH2CHCH2NO f CH2CHdCH2 + NO
i-C3H7NO2 + H f (CH3)2CHNO + OH
i-C3H7NO2 + H f i-C3H7 + HONO
-0.35
0
0
0
b
b
0
a Rate constants are in the form k ) A(T/298)nexp{-Ea(kcal/mol)a/RT} with the unit of cm3 mol-1 s-1 for bimolecular reactions. The Troe
center-broadening parameter for the reaction 64 and 65 is Fcent ) 0.3732 exp(-T/250) +0.6268 exp(-T/901). b Rate constant assigned in this
report.
in the final product mixture. This is in agreement with our
experiments. The mechanism also predicts a significant amount
of CH2dCHCH2NO in the final products, but its existence has
not been established by the available analytical techniques.
The low A factor and activation energy for the thermal
decomposition of 2-nitropropanol indicates that the first step
of the reaction involves a tight transition structure. The most
plausible step is a near concerted elimination of NO2 and OH
via a six member ring, an H-bonded transition structure that
converts to OH‚‚‚NO2 and CH3CHdCH2 (propylene). Then the
transient species OH‚‚‚NO2 quickly dissociates. The existence
of a six-member ring that incorporates OH and NO2 in
compounds such as 2-nitropropanol has been extensively
considered with respect to the possible presence of an intramo-
lecular hydrogen bond.52
Secondary conversions may be induced by reactions between
i-C3H6(OH)NO2, NO2, OH, and CH3CHdCH2. Note that the
Arrhenius parameters for 2-nitropropanol are similar in mag-
nitudes to those for the HONO elimination in the thermal
decompositions of 2-nitropropane and nitroethane.53

Pyrolyses of 2-Nitropropane and 2-Nitropropanol
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1215
Figure 12. (A) Fragmentation and reaction sequence in the thermal decomposition of 2-nitropropane. Reactions initiated by the intermediates
i-C3H7ONO, CH3, and CH2CHdCH2 are shown in Figure 12B. (B) Reactions initiated by i-C3H7ONO, CH3, and CH2CHdCH2 in the decomposition
of 2-nitropropane.
Sensitivity Analysis
following criteria were used to determine the importance of any
particular step in the mechanism: a step was considered
important if it belongs to a reaction group with eigenvalues >
1.0 × 10-5, and unimportant if it belongs to a reaction group
The mechanism in Table 1 was subjected to a sensitivity/
principal component analysis as discussed previously.6,25 The

1216 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Zhang and Bauer
with an eigenvalue < 1.0 × 10-5. In any event, if the magnitude
of the eigenvector of a reaction is less than 0.1, it is considered
unimportant. The mechanism listed in Table 1 was first analyzed
assuming that all the species in the mechanism were “observed”,
i.e., requiring that the reduced reaction mechanism from the
analysis faithfully reproduces concentration profiles of all the
inserted species. It was found that 16 steps (6, 14, 15, 16, 18,
24, 25, 32, 39, 43, 44, 47, 50, 51, 55, and 67) were unimportant
during the entire decomposition process. The remaining mech-
anism was then further analyzed by considering only those
species for which the concentrations reach at least 10-4 of
[i-C3H7NO2]o during part of the decomposition. These are
i-C3H7NO2, NO2, CH3CHdCH2, C2H4, CH4, H2, C3H8, NO,
H2O, (CH3)2CO, HONO, CH3CHO, i-C4H8, C3H5NO,
CH2CCH2, CO, CH3CN, CH3NO2, n-C4H8, and C6H12. It was
found that seven more steps could also be omitted without
altering the time evolution of the listed species: 11, 33, 34, 35,
59, 60, 78. The differences between the concentrations computed
via the reduced and the full mechanism are less than 5%. A
simplified mechanism tree is graphically presented in Figure
12. Note that the following atoms and free radicals are the most
reactive species in this system: i-C3H7, H, CH3, OH, and HNO.
HONO elimination accounts for less than 20% of the total loss
of i-C3H7NO2 over the temperature range covered in this
investigation.
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Acknowledgment. The authors gratefully acknowledge the
financial support for this investigation from Army Research
Office under Grant No. DAAH04-95-1-0130.
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