Effect of Pressure on the Thermolysis of Nitroalkanes in Solution

Article abstract of DOI:10.1021/jo9705632

The effect of pressure up to 1.1 GPa on the rates of decomposition of two acidic nitroalkanes, nitromethane and 2-nitropropane, was measured. The mechanisms of thermolysis are inferred from kinetic studies and product analysis. The rate-controlling step for nitromethane decomposition in toluene at 230°C at low pressures is homolysis of the C-N bond. Beyond 20% conversion, the decomposition is autocatalytic. At high pressure, nitromethane has another reaction path which supplants homolysis. It is proposed that nitromethane forms an intermediate by cyclization of its aci-form. The high-pressure process is characterized by a first-order rate law without primary kinetic isotope effect, a low activation energy (28.5 kcal/mol), a negative activation volume (-5.5 mL/mol), and formation of products which cannot be attributed to radical intermediates. At high conversion, the reaction becomes autocatalytic as a result of accumulation of water leading to formation of products explainable by the Nef reaction. 2-Nitropropane is less stable than nitromethane. Pressure powerfully accelerates its decomposition owing to its activation volume averaging -11.2 mL/mol from 0.1 to 1.1 GPa. It is believed to cyclize via the aci-form like nitromethane. 2,2-Dinitropropane does not have a hydrogen and cannot tautomerize. In earlier work it was found to have a homolytic mechanism at high pressure. Therefore, the decomposition pathways of nitroalkanes in aprotic solvents are determined not only by the conditions but also by the availability of a hydrogen.

Full text of DOI:10.1021/jo9705632

9
048
J . Org. Chem. 1997, 62, 9048-9054
Effect of P r essu r e on th e Th er m olysis of Nitr oa lk a n es in Solu tion
,
†
J iang Wang* and Kay R. Brower
Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
Darren L. Naud
American University of Beirut (AUB), Beirut, Lebanon
Received March 27, 1997X
The effect of pressure up to 1.1 GPa on the rates of decomposition of two acidic nitroalkanes,
nitromethane and 2-nitropropane, was measured. The mechanisms of thermolysis are inferred
from kinetic studies and product analysis. The rate-controlling step for nitromethane decomposition
in toluene at 230 °C at low pressures is homolysis of the C-N bond. Beyond 20% conversion, the
decomposition is autocatalytic. At high pressure, nitromethane has another reaction path which
supplants homolysis. It is proposed that nitromethane forms an intermediate by cyclization of its
aci-form. The high-pressure process is characterized by a first-order rate law without primary
kinetic isotope effect, a low activation energy (28.5 kcal/mol), a negative activation volume (-5.5
mL/mol), and formation of products which cannot be attributed to radical intermediates. At high
conversion, the reaction becomes autocatalytic as a result of accumulation of water leading to
formation of products explainable by the Nef reaction. 2-Nitropropane is less stable than
nitromethane. Pressure powerfully accelerates its decomposition owing to its activation volume
averaging -11.2 mL/mol from 0.1 to 1.1 GPa. It is believed to cyclize via the aci-form like
nitromethane. 2,2-Dinitropropane does not have R hydrogen and cannot tautomerize. In earlier
work it was found to have a homolytic mechanism at high pressure. Therefore, the decomposition
pathways of nitroalkanes in aprotic solvents are determined not only by the conditions but also by
the availability of R hydrogen.
In tr od u ction
Nitromethane, the simplest of the aliphatic nitro
CH NO f CH ONO f products
(2)
3
2
3
They found that the activation energy for methyl
nitrite rearrangement is 13 kcal lower than that of the
compounds, is an explosive and monopropellant. Up to
the present, many studies of its thermal stability under
9
1
-6
C-N homolysis. Wodtke et al. on the other hand found
various conditions have been reported.
The pyrolysis
evidence there is only a small difference in activation
energy between homolysis and rearrangement. They
claimed that mechanisms 1 and 2 operate in parallel. Lee,
Sanborn, and Stromberg10 have studied effects of pres-
sure on the rate of decomposition of various explosives,
and nitroalkanes appear to be anomalous. Reported
time-to-explosion measurements on nitromethane indi-
cate that pressure decreases the time required to achieve
an explosion while an opposite effect was found for 2,2-
of gaseous nitromethane at temperatures from 305-440
7
°
C was investigated by Crawforth and Waddington. The
activation energy (E ) was 55.5 kcal/mol which agrees
well with C-N bond dissociation energy, 60 kcal/mol. The
products in order of abundance are CO, CH , H O, N
CO , and small amounts of HCN and oxides of nitrogen.
a
4
2
2
,
2
It was suggested that methyl radicals are generated by
homolysis of the C-N bond and converted to methane
by hydrogen abstraction:
1
1
dinitropropane and other explosives. Engelke et al.
found that high pressure applied to nitromethane in-
creases the concentration of the nitronate ion and pro-
posed that sensitivity is directly related to this effect.
•
CH NO f CH + NO
(1)
3
2
3
2
•
•
CH + CH NO f CH + CH NO
12,13
3
3
2
4
2
2
Miller et al.
reported that pressure increased the
decomposition rate of liquid nitromethane and a reduc-
tion of the frequency of the asymmetric stretching mode
of NO with increasing pressure was observed. They
2
Recently another mechanism involving methyl nitrite
as an intermediate was proposed by Dewar et al.8
proposed a bimolecular mechanism which accounts for
the observed products, ammonium formate and water.
Naud and Brower studied the products and rates of
*
Corresponding author.
Current address: Department of Chemistry, University of South-
†
14
ern California, Los Angeles, CA 90089-0482.
X
Abstract published in Advance ACS Abstracts, November 15, 1997.
(
(
(
1) Brasch, J . W. J . Phys. Chem. 1980, 84, 2084.
2) Brower, K. R. J . Org. Chem. 1988, 53, 3776.
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(8) Dewar, M. J . S.; Ritchie, J . R.; Alster, J . J . Org. Chem. 1985,
50, 1031.
(9) Wodtke, A. M.; Hintsa, E. J .; Lee, Y. T. J . Chem. Phys. 1986,
84, 1044.
(10) Lee, E. L.; Sanborn, R. H.; Stromberg, H. D. Proc. 5th Symp.
(Intl.) Detonation, 1970, 331.
(11) Engelke, R.; Schiferl, D.; Storm, C. B.; Earl, W. L. J . Phys.
Chem. 1988, 92, 6815.
8
4, 142.
4) Trocino, J . L. Techniques for Sensitizing and Detonating Ni-
(
tromethane-Based Explosive Systems, Bulletin J LTN-2, April 1972;
Sixth Symposium (Intl.) on Detonation, edited by Edwards, D. J ., Ed.
(
Office of Naval Research, Washington D. C.), 1976, p 99.
5) Engelke, R.; Earl, W. L.; Rohlfing, C. M. Int. J . Chem. Kinet.
986, 18, 1205.
(
(12) Miller, P. J .; Block, S.; Piermarini, G. J . J . Phys. Chem. 1989,
93, 457.
1
(
(
6) Cook, M. D.; Haskins, P. J . 19th Intl. Annu. Conf. ICT 1988.
7) Crawforth, C. G.; Waddington, D. J . Trans. Faraday Soc. 1969,
(13) Miller, P. J .; Block, S.; Piermarini, G. J . J . Phys. Chem. 1989,
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6
5, 1334.
(14) Naud, D. L.; Brower, K. R. High-Pressure Res., 1992, 11, 65.
S0022-3263(97)00563-X CCC: $14.00 © 1997 American Chemical Society

Effect of Pressure on the Thermolysis of Nitroalkanes
J . Org. Chem., Vol. 62, No. 26, 1997 9049
decomposition of solutions of 2-nitropropane with an
equimolar quantity of piperdine in THF at various
hydrostatic pressures. They found that pressure acceler-
ated decomposition and the principal products were
acetone and acetone oxime in equimolar proportions. The
key step is thought to be a bimolecular reaction of
nitronate ion with the aci form of 2-nitropropane.
Nitroalkanes with R hydrogens are weak acids and
have two tautomeric forms. It is well established that
the acid tautomer (aci form) equilibrates with the nitro
tautomer (nitro form) by way of the nitronate ion. The
equilibrium for nitroalkanes is as follows:
and activation energy, the conversion was less than 25% to
avoid autocatalysis.
For low-pressure thermolysis, 30-µL samples of 10 wt %
nitromethane in toluene were sealed in 5.5 cm capillary tubes
and heated at temperatures from 250 to 302 °C. The rate of
thermal decomposition was followed by GC using a thermal
conductivity detector (TCD). The pressure in the capillary
tube depends on the volatility of the solvent, the amount of
gas evolved, and the temperature. The tube was capable of
containing 100 atm during thermolysis without bursting.
Activation volumes were obtained from plots of ln k versus
P according to eq 7.
∆V* ) -RT(δ ln k/δ P)T
(7)
Their values in each case were calculated for specific
pressure ranges where the slope of ln k versus pressure is
approximately linear. All kinetic calculations were based on
a pseudo-first-order rate law.
A Hewlett-Packard GC 5890A coupled to a Hewlett-Packard
mass selective detector 5970 was employed for product analy-
sis. Identification of products was achieved by comparing the
mass spectra to those of the authentic samples or by inter-
pretation of the fragmentation pattern. Gas-phase products,
such as CH
4 2 2 2
, CO, CO , N O, NO, and NO were analyzed by
Fourier transform-infrared spectroscopy (FTIR). No rigorous
attempt was made to quantify the products of reaction.
Nitroalkanes and solvents were obtained either from Aldrich
or Fluka Chemical Co. and used without further purification.
Resu lts a n d Discu ssion
In contrast to previous work10,12,13 using pure explo-
sives, our experiments use solutions in aprotic solvents
for the following reasons. A diluting solvent can suppress
intermolecular reactions and scavenge highly reactive
2
intermediates, such as NO and NO , which could other-
wise lead to autocatalysis. Furthermore, it is convenient
to measure reaction order by changing the reactant
concentration.
From the pressure effect on the rates of decomposition
it is possible to evaluate the molar volume difference
between reactants and their transition state which is
defined as the activation volume (∆V*):
Turnbull and Maron15 measured the ionization con-
stants for aci and nitro forms and the tautomeric equi-
librium constants in aqueous solution. They found that
the preferred tautomer is the nitro form. The equilibrium
∆V* ) V* - ΣVr
(8)
T
constants, K , for nitromethane and 2-nitropropane at
-
7
-3
2
5 °C are 1.1 × 10 and 2.75 × 10 , respectively.
The activation volume can help to characterize transi-
tion states and elucidate reaction mechanism according
to a review by Asano and le Noble.17 The facts we rely
upon to interpret the results of the effects of pressure
are that (1) homolytic reactions have positive activation
volume ranging from +5 mL/mol at low temperatures to
Methyl substituents greatly increase the stability of the
aci form.
In most of the published kinetics studies the only
intensive variable was temperature, and the role of
pressure on the rates and mechanisms of decomposition
of nitroalkanes was essentially unknown. In this paper,
we report the pressure and temperature effects on the
decomposition of nitroalkanes in solution. Through a
combination of measured activation volumes and iden-
tification of products of decomposition, we postulate their
possible decomposition mechanisms.
+
40 mL/mol at high temperatures; (2) reactions which
lead to electrical polarization either by bond formation
or bond breaking have large negative activation volumes;
(3) the observed activation volume for a reaction having
a preequilibrium step is the sum of the volume change
for the equilibrium and activation volume of the rate
determining step.
Exp er im en ta l Section
1
. Th er m a l Decom p osition of Nitr om eth a n e in
High pressure experimental work was performed using a
piston-in-cylinder assembly described by Naud and Brower.16
The decomposition temperature was maintained to within 1
Tolu en e a t P r essu r e Below 100 Atm osp h er es. De-
composition of 10 wt % nitromethane in toluene followed
a pseudo first-order rate law at low degrees of reaction
°C in the range of 195 °C to 260 °C. Gas chromatography (GC)
(
e20%). This is ascribed to homolytic cleavage of the
was used to measure the conversion of nitroalkane. Dilute
solutions of nitroalkane (in toluene) were heated, and conver-
sion of nitroalkanes ranged from 5-75%. For kinetic studies
involving determination of reaction order, activation volume,
C-N bond as the rate-determining step in the thermal
decomposition. The activation energy is 53.5 kcal/mol,
and it was obtained from a straight line of Arrhenius plot.
This activation energy is close to the dissociation energy
(
(
15) Turnbull, D.; Maron, S. H. J . Am. Chem. Soc. 1943, 65, 212.
16) Naud, D. L.; Brower, K. R. J . Org. Chem. 1992, 57, 3303.
(17) Asano, T.; le Noble, W. J . Chem. Rev. 1978, 78, 409.

9
050 J . Org. Chem., Vol. 62, No. 26, 1997
Wang et al.
Sch em e 1
F igu r e 1. Rate constant and conversion of 10 wt % ni-
tromethane decomposed in toluene under low pressure at 250
°
C.
Sch em e 2
of the C-N bond, 60 kcal/mol. The activation entropy
change (∆S*) calculated from the Eyring equation is
a
+3.1R (J /K mol). The high E and large preexponential
factor (positive ∆S*) suggest that decomposition is a
unimolecular radical process and C-N bond homolytic
scission is the rate-determining step. The gaseous
products detected by FTIR for 30% decomposition, in
order of abundance are CO
2
, N
2
O, CH
4
, and CO. It is
high reactivity in redox and condensation reactions
leading to tarry products.
2. Th er m a l Decom p osition of Nitr om eth a n e in
Tolu en e u n d er High P r essu r e. At 0.7 GPa, the
decomposition rate of 10 wt % nitromethane was mea-
sured at various temperatures, and the Arrhenius plot
probable that methane is formed from methyl radicals
by hydrogen abstraction and that methyl radicals are
generated by homolysis of the C-N bond. The liquid
products analyzed by GC/MS in order of abundance are
benzonitrile, benzyl alcohol, benzyltoluene, water, and
benzamide. The yield of these products reaches a plateau
when the conversion of nitromethane is 20%. The
involvement of toluene in the radical decomposition is
shown in Scheme 1.
is a perfect straight line. The E
and ∆S* were 28.5 kcal/
a
mol and -14.6R (J /K mol), respectively. These low
activation parameters clearly demonstrate that the de-
composition mechanism at higher pressure is different
from that at low pressure. It is not a radical process.
When nitromethane decomposition exceeds 20%, an
increase in decomposition rate is observed in Figure 1,
FTIR analysis did not give any evidence of CH
4
formation
and the percentage yield of radical products such as C
6
H
5
-
but CO
2
, N
2
O, and traces of water were found. A higher
CN decreases. An insoluble tarry material was also
formed. The radical mechanism is therefore overtaken
by another mechanism at higher conversion. A trace
amount of water added to a nitromethane solution was
found to increase the rate of decomposition 30-fold. This
suggests that water accumulating at high conversion
catalyzes the decomposition of nitromethane and leads
to autocatalysis through the Nef reaction shown in
Scheme 2.
ratio of N
2
2
O to CO was found at higher conversion. The
yield of black polymeric substance was greater at higher
conversions. It varied in consistency from tar to powdery
solid depending on conditions. The products in solution
were analyzed by GC/MS after removal of the black tar.
Water was formed, but no product derived from toluene
could be detected. It is evident that toluene plays no
direct chemical role at high pressure and that the radical
decomposition mechanism can be ruled out. Acceleration
of the decomposition rate by pressure was observed at
pressures up to 1.1 GPa at 230 °C. The activation volume
At higher conversion the ratio of CH
4 2
to N O decreased.
Formaldehyde was not found. This may be due to its

Effect of Pressure on the Thermolysis of Nitroalkanes
J . Org. Chem., Vol. 62, No. 26, 1997 9051
Ta ble 1. P seu d o F ir st-Or d er Ra te Con sta n ts for
Th er m olysis of Nitr om eth a n e a t 230 °C,
0
0
.7 GP a , a n d 2-Nitr op r op a n e a t 195 °C,
.9 GP a
k (s- ) × 10
1
6
sample
no.
nitroalkane
solvent
run 1
run 2
1
2
3
4
5
6
7
10% CH3NO2
10% CH3NO2
5% CH3NO2
20% CH3NO2
10% CD3NO2
5% 2-nitropropane
10% 2-nitroporpane
benzene
toluene
toluene
toluene
toluene
toluene
toluene
1.00
4.95
5.23
6.00
5.25
5.68
5.58
5.65
11.80
13.50
Sch em e 3
F igu r e 2. Activation volume profile of 10 wt % nitromethane
decomposed in toluene at 230 °C.
autocatalytic reaction. Table 1 gives the pseudo-first-
order rate constants at 230 °C under 0.7 GPa. Sample 2
has a 2-fold higher concentration than sample 3, but both
have the same rate constant. The same result is found
for samples 2 and 4. The rate law thus appears to be
first order in nitromethane. The results in Table 1 show
that k
H D
/k is about 0.9-1.15. Clearly there is no primary
kinetic isotope effect, and C-H bond cleavage is a fast
preequilibrium step. An isotope exchange reaction was
performed to test the rate of nitronate formation.
A
solution made up of 1:1 by volume of nitromethane and
d
5
3
-nitromethane was heated at 230 °C under 0.7 GPa for
0 min. It was found that exchange goes to completion
, h , h and d -nitromethane in
F igu r e 3. Rate constant and conversion of 10 wt % ni-
with a mixture of h
3
2
d
1
1
d
2
3
tromethane decomposed in toluene at 230 °C, 0.7 GPa.
the statistical ratio of 1:3:3:1. The formation of nitronate
ion is therefore not rate limiting because only 2%
decomposition occurred during the isotope exchange.
Pressure greatly accelerates the formation of nitronate
ion, but this process does not control the rate of decom-
position. It may, however, be an essential preequilibrium
step. The mechanism shown in Scheme 3 is proposed.
The energy barrier for cyclization in Scheme 3 should
be lower than that of isomerization of nitromethane to
methyl nitrite, because nucleophilic addition of oxygen
anion to a double bond with partial positive charge is
normally facile as in saponification. This is probably the
reason such a low activation energy was found. For the
isomerization of nitromethane to methyl nitrite, a three-
plot given in Figure 2 is a straight line, and the activation
volume is -5.5 mL/mol. This also suggests that rate-
controlling step at high pressure is not homolysis because
homolysis is characterized by positive activation volume.
Figure 3 is a relation between the pseudo-first order rate
constant and reaction time. It is obvious that there is a
gradual change in mode of decomposition. The first
phase is from zero to about 8 h during which the rate of
decomposition remains constant and follows first-order
kinetics. The second is in the range of 8 h to 13 h where
decomposition accelerates and follows an autocatalytic
mechanism. This suggests that kinetic studies should
be limited to the early stage of decomposition to avoid

9
052 J . Org. Chem., Vol. 62, No. 26, 1997
Wang et al.
Ta ble 2. Effect of Ad d itives on Th er m a l Sta bility of
Nitr om eth a n e a t 230 °C, 0.7 GP a
k (s- ) × 106
1
sample
run 1
run 2
1
0% CH3NO2 in toluene
4.95
3.10
343.3
28.3
5.68
toluene dried by sodium
addition of 3% water
addition of 3% methanal
addition of 3% pyridine
100.8
membered cyclic transition state was proposed in which
8
the carbon atom has to be five-coordinated and the
energy barrier is at least as high as 47.0 kcal/mol. A
sequence involving a cyclization like that of Scheme 3
has been used in synthesis.18 Compound I in Scheme 3
should readily decompose to form the major products,
formaldehyde and N
derived from eq 9.
2
O. The first-order rate law can be
F igu r e 4. Activation volume profile of 10 wt % 2-nitropropane
decomposed in toluene at 230 °C.
from kinetic data at 1.1 GPa which is an indication of
the complex reaction. The rate law was determined to
be the first-order. The products are acetone, water, N
2
O,
and tar. Acetone oxime was not found although it is
stable under the reaction conditions. This indicates that
the following bimolecular reaction for decomposition of
When benzene was used as solvent instead of toluene,
the apparent rate constant, kobs, decreased by a factor of
2
-nitropropane catalyzed by amine does not occur in the
5
K
. This result may be attributed to a solvent effect on
. At relatively high nitromethane decomposition, the
reaction was autocatalytic as under low pressure. Table
gives the effect of additives on the thermal stability of
absence of amine.
T
2
nitromethane. It is evident that substances capable of
accelerating proton transfer can catalyze the decomposi-
tion.
3
. Th er m a l Decom p osition of 2-Nitr op r op a n e in
Tolu en e a t High P r essu r e. The pressure effect on the
decomposition rate of 10 wt % 2-nitropropane in toluene
was studied in the pressure regime from 0.10 GPa to
Figure 5 is a plot of rate constant k vs decomposition
time. When the decomposition time is less than 100 min,
the decomposition obeys first-order kinetics, but beyond
1
.1GPa at 230 °C. The activation volume calculated from
2
h, the rate increases substantially. Therefore, 2-nitro-
Figure 4 is -11.2 mL/mol. The extraordinary negative
activation volume indicates that decomposition is not a
radical process. It is unusual for an activation volume
to maintain a large magnitude at high pressure. Activa-
tion volumes for some Diels-Alder reactions have been
measured in excess of -50 mL/mol for the first 0.1 GPa,
but they rapidly decrease with reduction of free volume
at high pressure. This large activation volume of 2-ni-
tropropane may be indicative of an ionic preequilibrium
such as autoprotolysis. The pressure effect on 2-nitro-
propane is greater than that on nitromethane (-5.5 mL/
mol) and can arise from at least two reasons: (1)
propane like nitromethane has different decomposition
mechanisms at low and high conversion. The products
in the gas phase are N
2 2 3 2
O, CO , (CH ) CO, and a small
amount of CH . As in the case of nitromethane, it has
4
been found that water dramatically catalyzed the de-
composition of 2-nitropropane. Because of the similari-
ties between nitromethane and 2-nitropropane, a parallel
decomposition mechanism can be invoked.
At 230 °C and 1.1 GPa, the decomposition rate of
2
-nitropropane is 16 times greater than that of ni-
tromethane. The previously mentioned difference in
tautomeric equilibrium constants may be a factor. The
observed activation volume for the proposed mechanism
is the sum of the volume changes of the equilibrium and
subsequent rate-determining step. At high conversion
of 2-nitropropane, the autocatalytic mechanism (addition
of water to the aci-form yielding acetone and nitroxyl)
dominates the decomposition.
In contrast to nitromethane and 2-nitropropane, 2,2-
dinitropropane is an explosive compound without R
hydrogen and cannot form the nitronate ion. In earlier
work,14 the activation volume was found to be +3.6 mL/
2
-Nitropropane has a longer chain structure and, conse-
quently, its volume in a three-member transition state
would be compact (i.e. ball-like) and (2) the K
of
-nitropropane was found to be lager in aqueous solution
T
2
than for nitromethane; this means that 2-nitropropane
exhibits greater charge separation and solvation effect
than nitromethane in the preequilibrium process. The
very low activation energy, 15.4 kcal/mol, was derived
(18) Young, A.; Levand, O.; Luke, W. K. H.; Larson, H. O. Chem.
Commun. 1966, 230,

Effect of Pressure on the Thermolysis of Nitroalkanes
J . Org. Chem., Vol. 62, No. 26, 1997 9053
Sch em e 5
coefficient for ordinary liquids at ambient conditions is
-
9
2
-1
approximately 10
reduced to ca. 10
m s , which probably would be
-
11
2
-1
m s under shock initiation condi-
F igu r e 5. Rate constant and conversion of 10 wt % 2-nitro-
propane decomposed in toluene at 195 °C, 1.1 GPa.
2
0
tions such as 10 GPa and 1000 K.
Under these
-
7
conditions, the diffusion length for 10 s is 1 to 2 nm,
which is approximately one to two molecular diameters
of nitromethane. This is a length that could feasibly
allow a number of rapid proton-transfer reactions to occur
Sch em e 4
-
7
in the 10 s time span. Since nitromethane is usually
detonated without extension by solvent and it is the
principal reactant in both preequilibrium reactions that
leads to the formation of the aci-form (first and third
reactions of Scheme 3), the generation of the aci-form
through the bimolecular reactions could be facile. The
heat generated from the ensuing cyclization of the aci-
form followed by secondary reactions could sustain the
shock front or promote a low-order detonation process to
full detonation. However, it must be noted that proton-
transfer reactions involving carbon acids may be too slow
even under the detonation conditions of pressure and
temperature. By our mechanism, two successive proton-
transfer reactions are needed before nitromethane reaches
the aci-form intermediate. Therefore it is questionable
whether the proposed cyclization pathway for the static
thermal decomposition of nitroalkanes could be applied
to the detonation process.
mol. The principal products are acetone, NO, and NO
2
with a small quantity of 2-nitropropane. It is interesting
to note that the behavior of 2,2-dinitropropane in aprotic
solution is very different from that of 2-nitropropane. The
1
4,19
proposed mechanism is shown in Scheme 5.
This
activation volume is appropriate for free-radical dissocia-
tion in the rate-determining step. Comparison of the
activation parameters and the characteristics of mono-
and dinitropropane makes it highly probable that the
tautomeric equilibrium of the former plays a vital role
in the mechanism of decomposition. If a link is to be
made with the aforementioned reactions and the reac-
tions of detonation of nitromethane (or any other acidic
nitroalkane), one must consider the limits imposed by
heat and mass transport on the rate of molecular colli-
sions in the detonating material. Unlike other explosives
Con clu sion
The observed pressure effects on the decomposition of
nitroalkanes in aprotic solution have indicated that the
dominant reaction channel can change with pressure.
At low pressure, nitromethane reacts by homolysis of
the C-N bond while at high pressure, it adopts a
mechanism with a negative volume, negative activation
entropy, and low activation energy. A cyclic intermediate
is proposed. The behavior of 2-nitropropane at high
pressure is similar to that of nitromethane and very
different from that of 2,2-dinitropropane.
At high conversion, both nitromethane and 2-nitropro-
pane change mechanism to a Nef-like reaction with water
formed in the first stage.
The solvent has four effects on the decomposition of
nitroalkanes: (1) a polar solvent can facilitate conversion
(i.e. nitrate ester explosives) where part of the shock
energy is consumed immediately by bond rupture, ni-
tromethane might need one or more preequilibrium steps
until a highly reactive intermediate is produced. An
estimate of the number of molecular collisions during
passage of the shock wave can be obtained from the
diffusion length of nitromethane. The time for propaga-
tion of the pressure pulse and reaction heat in a detona-
-
7
tion shock front is approximately 10 s. The diffusion
(
20) Campbell, A. W.; Davis, W. C.; Travis, J . R. Phys. Fluids, 1961,
(19) Flournoy, J . M. J . Chem. Phys. 1962, 36, 1107.
4, 498.

9
054 J . Org. Chem., Vol. 62, No. 26, 1997
Wang et al.
of tautomers; (2) dilution decreases intermolecular in-
teraction and gives reliable information about the initial
step; (3) the solvent can stabilize highly reactive inter-
Pressure appears to be a principal factor in the
chemistry of nitromethane decomposition and detonation.
The decomposition mechanisms for nitroalkanes are
very complicated. It is evident from our study that the
dominant reaction channel is determined not only by the
experimental conditions such as temperature, pressure,
and solvent properties, but also by the structural features
of the nitroalkane.
2
mediates such as NO and retard autocatalytic decom-
position; (4) if the solvent is protic it will accelerate the
decomposition by Nef reaction. The large effect of
pressure on the rate of thermal decomposition of ni-
tromethane and 2-nitropropane points to the importance
of the proton-transfer reactions leading to the formation
of the aci-form. At least three other experiments suggest
Ack n ow led gm en t. This work was supported by the
Research Center for Energetic Materials, an NSF/
Industry cooperative venture. D. L. Naud is grateful for
the financial support provided by the University Re-
search Board of AUB.
the same: (1) amine-catalyzed decomposition of nitroal-
kanes is promoted by pressure,14 (2) time-to-explosion is
decreased with increasing pressure,10 and (3) compression
of neat nitromethane in a diamond anvil cell accelerates
decomposition.1
2,13
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