Nitroanilinodinitrobenzofuroxans - Synthesis, characterisation, thermal stability and explosive properties

Article abstract of DOI:10.1039/b103537c

Three new derivatives of 4,6-dinitrobenzofuroxan: 7-(4-nitrophenylamino)-4,6-dinitrobenzofuroxan, 7-(3,5-dinitrophenylamino)-4,6-dinitrobenzofuroxan and 7-(2,4,6-trinitrophenylamino)-4,6-dinitrobenzofuroxan, have been synthesised by condensing 4-nitroaniline, 3,5-dinitroaniline and 2,4,6-trinitroaniline with 7-chloro-4,6-dinitrobenzofuroxan, respectively. The characterisation of the compounds by IR, ¹H-NMR, mass spectrometry and elemental analysis is described along with some of the evaluated preliminary explosive properties. The compounds were found to exhibit acceptable hazards properties. Furthermore, the thermal stability measurements indicated acceptable stability.

Full text of DOI:10.1039/b103537c

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Nitroanilinodinitrobenzofuroxans—synthesis, characterisation,
thermal stability and explosive properties
Mehilal, A. K. Sikder,* R. B. Salunke and N. Sikder
High Energy Materials Research Laboratory, Sutarwadi, Pune- 411 021, India.
E-mail: ak sikder@yahoo.com; Fax: þ91 5881 1316
Received (in Montpellier, France) 19th April 2001, Accepted 25th July 2001
First published as an Advance Article on the web
Three new derivatives of 4,6-dinitrobenzofuroxan: 7-(4-nitrophenylamino)-4,6-dinitrobenzofuroxan, 7-(3,5-
dinitrophenylamino)-4,6-dinitrobenzofuroxan and 7-(2,4,6-trinitrophenylamino)-4,6-dinitrobenzofuroxan,
have been synthesised by condensing 4-nitroaniline, 3,5-dinitroaniline and 2,4,6-trinitroaniline with 7-chloro-
1
4,6-dinitrobenzofuroxan, respectively. The characterisation of the compounds by IR, H-NMR, mass
spectrometry and elemental analysis is described along with some of the evaluated preliminary explosive
properties. The compounds were found to exhibit acceptable hazards properties. Furthermore, the thermal
stability measurements indicated acceptable stability.
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-
1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are well-estab-
lished and important military explosives of unusual stability
due in part to their cyclic structures.1 Explosives for military
use must be able to withstand extreme climatic conditions and
are required to give acceptable functionality responses to
impact and friction tests.
further purification. Melting points were determined in open
capillaries and are uncorrected. The IR spectra were recorded
on a Perkin–Elmer infrared spectrometer using KBr matrices.
1H-NMR spectra were recorded on a Brucker 90 MHz
instrument. Chemical shifts were recorded in ppm with refer-
ence to tetramethylsilane (TMS) as internal standard. Ele-
mental analyses were performed on a Carlo–Erba EA 1108
elemental analyzer. Mass spectra were determined by electron
ionisation at 70 eV on a Finnigan Mat 1020B GC/MS with a
solid probe.
In order to meet these requirements, a primary strategy has
been the development of polymer bonded explosives (PBXs),
low vulnerability propellants (i.e., having a low flame tem-
perature and good mechanical properties) and thermally stable
explosives. The thermal stability of a molecule can be achieved
by approaches such as the introduction of amino groups into a
nitroaromatic ring,2 condensation of picryl chloride with an
appropriate triazole moiety,3,4 or through the introduction of
conjugation.5 Further developments towards low sensitivity
and greater thermal stability have been made with benzofur-
oxans, an extremely fruitful class of compounds for use as new
explosives. The furoxan ring results in increased density com-
pared to nitro analogues whereas imino groups tend to reduce
sensitivity, which in turn increases overall stability and per-
formance, and also in many cases contributes to the heat of
formation. Of course, the performance of an explosive also
depends on several physical properties, including oxygen bal-
ance and heat of formation.6–9
Bearing these requirements in mind, we were interested in
developing new explosive molecules based on benzofuroxan,
possessing the energy of heterocyclic nitramines but having the
intrinsic stability of an aromatic cyclic configuration, as found
in well-known thermally stable explosives having high deto-
nation velocities and power.
In the following sections, we report the synthesis, char-
acterisation, thermal and explosive behaviour of three new
benzofuroxan derivatives, which may be used as strong
energetic fillers with reduced vulnerability in insensitive
munition.
The deflagration temperature10 was determined by heating
0.02 g of sample in a glass tube in a Wood’s metal bath at a
heating rate of 5 ꢀC min 1; the temperature at which the
sample ignited/decomposed was recorded as the deflagration
temperature. Differential thermal analysis (DTA) was recorded
on a micro-DTA apparatus fabricated in our laboratory by
heating 10 mg of sample at a rate of 10 ꢀC min
1
in the pre-
sence of static air. The impact sensitivity was determined by the
falling hammer method using a 2.0 kg drop weight, and friction
sensitivity was determined on a Julius Peter apparatus fol-
lowing standard methods.11 The velocity of detonation12 and
detonation pressure13 were calculated by methods reported in
the literature.
1,3-Dichloro-2,4,6-trinitrobenzene14 (as prepared from
styphnic acid by treatment with pyridine followed by phos-
phorus oxychloride, mp 129–130 ꢀC), 4-nitroaniline (mp 149–
151 ꢀC), 3,5-dinitroaniline (prepared by nitration of benzoic
acid via the Schmidt reaction, mp 161–163 ꢀC) and 2,4,6-tri-
nitroaniline15 (also prepared from picric acid by treatment
with pyridine and phosphorus oxychloride followed by ami-
nation, mp 189–191 ꢀC) were prepared in our laboratory and
used as starting materials. Sodium azide was obtained com-
mercially and used as received. Methyl alcohol, acetic acid and
dichloromethane from Qualigen were used as solvents.
Synthesis
1-Azido-3-chloro-2,4,6-trinitrobenzene, 1. 1,3-Dichloro-2,4,6-
trinitrobenzene (styphnyl chloride; 5 g, 17.73 mmol) was
transferred into a three-necked round-bottom flask fitted with
a mechanical stirrer and dropping funnel. To this, 25 ml of
dimethylformamide (DMF) was added slowly and
sodium azide (1.30 g, 0.02 mol), dissolved in 10 ml of distilled
water, was added dropwise to the reaction flask under stirring
Experimental
Methods and materials
Unless otherwise stated, all common reagents and solvents
were used as supplied from commercial sources without
DOI: 10.1039/b103537c
New J. Chem., 2001, 25, 1549–1552
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This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2001

at 20–22 ꢀC over half an hour. After complete addition of
sodium azide, the stirring was continued for another 3 h
while maintaining the same temperature. The reaction mixture
was poured into crushed ice and left overnight to precipitate.
The product, which settled out, was filtered off, washed
thoroughly with distilled water and finally dried at room
View Article Online
124, 107, 91. Anal. calc. for C12H5N7O10 (MW 407): C, 35.38;
H, 1.22; N, 24.07; found : C, 35.16; H, 1.10; N, 23.85%.
7-(2,4,6-Trinitro-1-iminophenyl)-4,6-dinitrobenzofuroxan, 5.
In a three-necked round-bottom flask fitted with reflux
condenser and dropping funnel, 2 (5.21 g, 20 mmol) was
introduced with 2,4,6-trinitroaniline (9.20 g, 40.35 mmol) to
which 150 ml of methanol was added. The reaction mixture
was refluxed for 5 h, cooled down to ambient temperature
and finally poured into crushed ice. The yellow precipitate
thus obtained was filtered off, washed with water and dried
in a water-jacketed oven at 60 ꢀC. The product was further
recrystallised from dichloromethane to yield 7.5 g (83%) of
pure product; mp 179–80 ꢀC. The DTA results showed no
exotherm up to 400 ꢀC. IR (KBr) cm 1: 3322 (m, N H str),
3082 (m, Ar H str), 1636, 1590, 1495, 1274 (mw, furoxan
ring), 1528 and 1274 (s, NO2 asym and sym str), 934 (m,
temperature. Yield 4.2
g (92%). The compound was
recrystallised from methanol and was found to melt at 45–
47 ꢀC. IR (KBr) cm 1: 3089 (m, Ar H str), 2165 (s, N3)
1616 (m, C¼C str), 1545 and 1335 (s, NO2 asym and sym
str). 1H-NMR (DMSO-d6 , TMS) d: 8.5 (s, 1H, aromatic
proton). Anal. calc. for C6HN6O6Cl (MW 288): C, 25.00; H,
0.34; N, 29.17; found: C, 24.89; H, 0.46; N, 29.01%.
7-Chloro-4,6-dinitrobenzofuroxan, 2. 1 (5 g, 17.33 mmol)
was carefully transferred into a two-necked flask fitted with a
reflux condenser and acetic acid (20 ml) was added. The
reaction mixture was refluxed for 2 h over an oil bath; the
mixture was then cooled to ambient temperature and
subsequently poured into crushed ice. The yellow precipitate
thus obtained was filtered off and washed with distilled water
until it was acid-free. The product was crystallised from
methanol. Yield 3.5 g (77%); mp 110–112 ꢀC; DTA: 114 ꢀC
(endotherm) and 205 ꢀC (exotherm). IR (KBr) cm 1: 3096
(m, Ar H str), 1620 and 1580 (m, C¼C and C¼N str), 1535
1
substituted benzene ring). H-NMR (CDCl3, TMS) d: 9.4 (s,
1H, benzofuroxan ring), 9.0 (s, 2H, trinitrophenyl ring), 7.30
(br, 1H, NH). EI-MS (70 eV) m=z: 264, 228 (100%), 212,
198, 166, 152, 137, 107, 91. Anal. calc. for C12H4N8O12
(MW 452): C, 31.85; H, 0.88; N, 24.77; found: C, 31.75; H,
1.06; N 24.56%.
Results and discussion
1
and 1345 (s, NO2 asym and sym str), 815 (s, C Cl str). H-
NMR (DMSO-d6, TMS) d: 8.75 (s, 1H, aromatic proton).
Anal. calc. for C6HN4O6Cl (MW 260): C, 27.69; H, 0.38; N,
21.53; found C, 27.51; H, 0.26; N, 21.30%.
Synthesis and structure
7-Chloro-4,6-dinitrobenzofuroxan, 2, was synthesised from
styphnic acid following an essentially different route from that
of Norris et al.16 The reaction steps followed for the pre-
paration of the parent compound 2, along with compounds 3,
4 and 5 are outlined in Scheme 1. The dipyridinium styphnate
was prepared from styphnic acid by treatment with pyridine; it
was then chlorinated using phosphorus oxychloride. Azidation
of the chlorinated product and subsequent cyclisation pro-
duced 2, which was subsequently condensed with the nitro-
aniline derivatives, thus resulting in the target molecules with
very good yields and excellent purity. This method of pre-
paration of the nitroanilinobenzofuroxans has been achieved
even more conveniently through a simplified method for the
preparation of the halide benzofuroxans, 2, which can be used
without further purification. Mention is made here of the
possibilities for large-scale synthesis of thermally stable,
insensitive energetic molecules and their parent compounds
from inexpensive starting materials.
Compounds 3, 4 and 5 are light yellow to dark yellow solids
and the analytical and spectroscopic data unambiguously
proved the structures of the compounds with no evidence of
isomeric structures. This is further supported by the fact that
formation of the furazan oxide involves ring opening to give an
‘‘ortho-dinitrosobenzene’’ as a transient intermediate, fol-
lowed by very rapid interconversion to the cyclised product.17
The X-ray crystallography18 and electrospectroscopy for che-
mical analysis (ESCA)19 results, together with IR, 1H-NMR
and mass spectrometry, further confirmed the structures of the
benzofuroxans.
The EI-MS fragmentation species suggested below are ten-
tative in the absence of the use of techniques such as accurate
mass measurement, fast atom bombardment and chemical
ionisation, as well as the mass spectra of deuterated analogues.
The EI-MS of these compounds were expected to show similar
patterns; however, on analysing the results we found that the
number of nitro groups and their positions in the aromatic ring
greatly influence the decomposition pathways and species.
The mass spectral data (relative intensity >5%) of com-
pounds 3, 4 and 5 are given in the experimental section. The EI
mass spectra show no molecular ions, nevertheless, certain
interesting ions that are of diagnostic value offer a means to
7-(4-Nitrophenylamino)-4,6-dinitrobenzofuroxan, 3. To a 250
ml three-necked round-bottom flask fitted with mechanical
stirrer, dropping funnel and reflux condenser, 2 (5.21 g, 20
mmol) was transferred and 4-nitroaniline (5.6 g, 40.57 mmol)
was added along with 125 ml of methanol. The reaction
mixture was refluxed for 5 h and subsequently cooled down
to ambient temperature. The reaction mixture was placed
into ice-cold water. The off-yellow precipitate thus obtained
was filtered off on a Buchner funnel and washed with water.
The product was crystallised from dichloromethane. Yield 5.6
g (77.3%); the compound did not melt but decomposed at
172 ꢀC (DTA exotherm). IR (KBr) cm 1: 3266 (m, N H str),
3116 (m, Ar H str), 1636, 1594, 1495, 1250 (mw, furoxan
ring), 1526 and 1346 (s, NO2 asym and sym), 940 (m,
1
substituted benzene ring). H-NMR (chloroform-d1, TMS) d:
8.9 (s, 1H, aromatic proton), 8.1 (s, 2H, aromatic proton), 7.7
(s, 2H, aromatic proton), 6.8 (br s, 1H, NH). EI-MS (70 eV)
m=z: 279, 264, 235, 189, 164, 138 (100%), 152, 107, 91. Anal.
calc. for C12H6N6O8 (MW 362): C, 39.77; H 1.65; N, 23.20;
found: C, 39.54; H, 1.83; N, 22.96%.
7-(3,5-Dinitrophenylamino)-4,6-dinitrobenzofuroxan, 4. 2 (25.21
g, 20 mmol) and 3,5-dinitroaniline (7.4 g, 40.43 mmol) were
carefully transferred into a 250 ml three-necked round-bottom
flask fitted with mechanical stirrer and reflux condenser,
followed by 130 ml of methanol. The reaction mixture was
refluxed for 5 h, then cooled down to ambient temperature and
poured into crushed ice. The yellow precipitate thus obtained
was filtered off and washed with distilled water and was
crystallised from dichloromethane. Yield 6.5 g (79.8%); the
product did not melt, however, the DTA was recorded and the
endotherm found at 95 ꢀC and the exotherm at 260 ꢀC. IR
1
(KBr) cm
1588, 1490,1294 (mw, furoxan ring), 1538 and 1342 (s, NO2
: 3378 (m, N H str), 3088 (m, Ar H str), 1640,
1
asym and sym str), 956 (m, substituted benzene ring). H-NMR
(CDCl3, TMS) d: 9.1 (s, 1H, benzofuroxan ring), 8.37 (s, 1H,
phenyl), 7.75 (s, 2H, phenyl ring), 7.15 (br s, 1H, NH). EI-MS
(70 eV) m=z: 285, 283, 264, 247, 183 (100%), 169, 154, 137,
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New J. Chem., 2001, 25, 1549–1552

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Scheme 1
characterise the compounds. The highest intensity ion is
attributed to fission of the dinitrobenzofuroxan moiety linked
with condensation derivatives like mono-, di- and trinitro-
aniline. Most of the other major ions can be explained by the
loss of NO2 or NO, HNO3 and HNO or, in some cases, CO
and O losses.
by the combined loss of molecular hydrogen and hydroxyl
radical, respectively. Another daughter ion at m=z 285 (not
shown on Scheme 2) could be due to the loss of (NO2 þ NO)
followed by loss of NO2 . The peak at m=z 153 may be ascribed
to loss of NO from m=z 183.
The mass spectrum of compound 4 is described here in detail
as a representative model of this class of compounds and the
fragmentation pathways are outlined in Scheme 2. The base
peak at m=z 183 constitutes a stable species corresponding to the
formation of the nitroaniline moiety through rearrangement
involving the imino function. The stepwise loss of NO2 and NO
species from the base peak is evidenced by the formation of
peaks at m=z 137 and 107. The fission of NH2 from m=z 107
resulted in the mass peak 91. The high molecular weight peaks at
m=z 283 and 264 correspond to [M (2 NO2 þ O2)]þ , followed
Thermal and explosive properties
The thermal and explosive properties of the compounds
were evaluated and the results obtained are presented in
Table 1. Compound 3 deflagrates at 170 ꢀC, compound 4 at
256 ꢀC while compound 5 does not deflagrate up to 350 ꢀC,
as rationalised by the DTA thermograms of the corre-
sponding exotherms and endotherms. Thus, it is evident
from the results that the thermal stability of the compounds
increases in the order 3 < 4 < 5. These results undoubtedly
confirm the long-term stability of this family of compounds.
The study of the explosive properties of compounds 3, 4 and
5 reveals that the compounds are insensitive to friction and
impact, when compared to well-established reference com-
pounds like RDX, HMX and Tetryl. The insensitivity of the
compounds increases with the decreasing number of nitro
groups in the molecules. The predicted performance para-
meters such as the velocity of detonation and detonation
pressure however, follow the opposite trend. Thus, com-
pound 5 is more powerful when compared to the others,
which may be attributed to a higher oxygen balance as well
as a low hydrogen content. In addition, the overall perfor-
mance of compound 5 is more or less comparable to Tetryl
(VOD 7700 m s 1), a well-known secondary explosive used
as a booster in an explosive train. Even though the present
series of new benzofuroxan derivatives exhibits moderate
performance with satisfactory thermal properties, they may
nevertheless serve as model compounds on which to base
further investigations.
Scheme 2
New J. Chem., 2001, 25, 1549–1552
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Table 1 Some characteristic properties of nitroanilinodinitrobenzofuroxan
Properties
3
4
5
Deflagration temperature/ꢀC
DTA/ꢀC
170
172
256
95 (endotherm)
260 (exotherm)
>350
180
(endotherm,
no explosion
up to 400 ꢀC)
1.83
7.07
92
32.4
(endotherm-
cum-exotherm)
3
Density/g cm
Oxygen balance (CO)/(%)
1.89
30.93
127
> 36
6538
201.97
1.86
17.6
115
36
7157
238.16
Impact sensitivitya (50% explosion ht)/cm
Friction sensitivityb (insensitive up to)/kg
Velocity of detonationc /m s
7639
266.97
1
Detonation pressurec /kbar
a
b
c
Sensitivity to impact: RDX, 35 cm; HMX, 32 cm; Tetryl, 85 cm. Sensitivity to friction: RDX, 12 kg; HMX, 12 kg; Tetryl, 36 kg. Calculated
values (ref. 12, 13)
5
6
K. G. Shipp and L. A. Kaplan, J. Org. Chem., 1966, 31, 857.
A. B. Shermetev and T. S. Pinina, in 27th International Annual
Conference of Fraunhofer-Institut fur Chemische Technologie (ICT),
ed. T. Keicher, Pfinztal, Karlsruhe, Federal Republic of Germany,
1996, pp. 30.1–30.14.
W. Naixing, C. Boren and O. Yuxiang, Propellants, Explos., Pyr-
otech., 1994, 19, 145.
W. P. Norris, Naval Weapon Center Technical Publication
(NWCTP) 6522, Naval Weapons Center, China Lake, CA, USA,
June 1984.
Conclusions
Three new dinitrobenzofuroxan derivatives, showing moder-
ately powerful explosive properties, have been synthesised and
fully characterised. Of the three molecules, compound 5
appears promising and may find applications where thermal
stability and high performance are required. Furthermore, the
ease with which the starting materials can be prepared from
commercially available compounds suggests that the products
3, 4 and 5 would be economically viable to synthesise on a
large scale for applications as explosives.
7
8
9
W. P. Norris and A. P. Chafin, Naval Weapon Center Technical
Publication (NWCTP) 6598, Naval Weapons Center, China Lake,
CA, USA, June 1989.
10 Encylopedia of Explosives and Related Items, ed. B. T. Fedoroff and
O. E. Sheffield, Picatinny Arsenal, Dover, NJ, USA, 1st edn.,
1960, vol. 1, pp. XVI–XVII.
Acknowledgement
11 L. Avrami and R. Hutchinson, in Energetic Materials: Technology
of Inorganic Azides, ed. H. D. Fair and R. F. Walker, Plenum Press,
New York, USA, 1st edn., 1977, vol. 2, ch. 4, pp. 111–159.
12 L. P. Rothestein and R. Peterson, Propellants Explos., 1979, 4, 56.
13 Explosives, Propellants and Pyrotechnics. Land Warfare: Brasseys
New Battlefield Weapon and Technology Series, ed. A. Bailey and
S. G. Murray, Brassey’s, Pergamon Press, Oxford, UK, 1st
edn., 1989, vol. 2, p. 33.
The authors thank Dr H. Singh, Director of the High
Energy Materials Research Laboratory, Pune, for the facil-
ities and encouragement provided during the course of this
work.
14 M. Warman and V. I. Siele, J. Org. Chem., 1961, 26, 2997.
15 R. Boyer, E. Y. Spencer and G. F. Wright, Can. J. Res., 1946, 24, 200.
16 W. P. Norris, A. Chafin, R. J. Spear and R. W. Read, Hetero-
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17 F. B. Malloxy and A. Cammrate, J. Am. Chem. Soc., 1966, 88, 64.
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W. E. Noland, Chem. Ind. (London), 1962, 562.
19 B. Chen and Z. Liao, in 21st International Annual Conference of
Fraunhofer-Institut fur Chemische Technologie (ICT), ed. H. Schu-
bert and K. Menke, Pfinztal, Karlsruhe, Federal Republic of
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