Bis(1,2,4-oxadiazole)bis(methylene) Dinitrate: A High-Energy Melt-Castable Explosive and Energetic Propellant Plasticizing Ingredient

Article abstract of DOI:10.1021/acs.oprd.8b00076

Discussed herein is the synthesis of bis(1,2,4-oxadiazole)bis(methylene) dinitrate, determination of its crystal structure by X-ray diffractometry, calculations of its explosive performance, and sensitivity measurements. Steps taken to optimize the synthesis process and to improve yields of the dinitrate are also discussed. Bis(1,2,4-oxadiazole)bis(methylene) dinitrate has a calculated detonation pressure 50% higher than that of TNT. The dinitrate compound exhibits a relatively high decomposition temperature that is rarely observed for nitrate-based compounds. The dinitrate was found to have lower sensitivities to impact and friction compared with RDX. It is believed that intramolecular hydrogen bonding observed in the crystal lattice assists in the relatively high thermal stability and relatively low sensitivity of the material.

Full text of DOI:10.1021/acs.oprd.8b00076

Communication
Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX
pubs.acs.org/OPRD
Bis(1,2,4-oxadiazole)bis(methylene) Dinitrate: A High-Energy Melt-
Castable Explosive and Energetic Propellant Plasticizing Ingredient
†
,†
,‡
§
§
Eric C. Johnson, Jesse J. Sabatini,* David E. Chavez,* Rosario C. Sausa, Edward F. C. Byrd,
†
†
Leah A. Wingard, and Pablo E. Guzman
̀
†
Energetics Technology Branch, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States
Explosive Science and Shock Physics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
Energetic Materials Science Branch, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States
‡
§
*
S Supporting Information
7
and is sensitive to reactions in an alkaline environment. Also
ABSTRACT: Discussed herein is the synthesis of bis-
1,2,4-oxadiazole)bis(methylene) dinitrate, determination
concerning is the generation of red water and pink water, two
types of wastewater that are generated from the TNT
manufacturing process, which find their ways into the waste
(
of its crystal structure by X-ray diffractometry, calculations
of its explosive performance, and sensitivity measurements.
Steps taken to optimize the synthesis process and to
improve yields of the dinitrate are also discussed. Bis(1,2,4-
oxadiazole)bis(methylene) dinitrate has a calculated
detonation pressure 50% higher than that of TNT. The
dinitrate compound exhibits a relatively high decom-
position temperature that is rarely observed for nitrate-
based compounds. The dinitrate was found to have lower
sensitivities to impact and friction compared with RDX. It
is believed that intramolecular hydrogen bonding observed
in the crystal lattice assists in the relatively high thermal
stability and relatively low sensitivity of the material.
8
stream. Red water is waste generated during the TNT
purification process (sulfitation). It is characterized by its
alkaline pH of 8 and consists of a complex mixture of
nitroaromatics and inorganic salts. Pink water is wash water
from the TNT finishing process following sulfitation, in which
the TNT is dried, flaked, and packaged. Pink water, which is
acidic (pH ∼3) is typically saturated with the amount of TNT
that will dissolve in water and is produced as a result of
equipment washing processes following demilitarization or
munitions filling operations. The U.S. Environmental Protec-
tion Agency has declared TNT a pollutant and has pushed for
9
its removal from military munitions.
The removal of TNT as an explosive ingredient has garnered
some success militarily, as it has been replaced with melt-
castable eutectic formulations based on dinitroanisole
INTRODUCTION
■
10
1
(
DNAN). Unfortunately, DNAN has a low density of 1.52
The development of high-energy-density materials (HEDMs)
−3
−1
g cm and a detonation velocity of only 5670 m s , making it
with excellent performance and reasonable sensitivity is an
overarching goal in the field of energetic materials. High-
energy-density melt-castable explosives are a particularly
challenging area in which to attain this goal. Melt-cast
explosives are sought because they allow for scalable and
efficient manufacturing processes. However, these materials
must also possess specific unique properties, which significantly
narrows the range of new target materials that can be pursued.
For example, although a melt-cast material can have a
melting point between 70 and 120 °C , a melting point below
00 °C is ideal. This allows steam heating to be used at ambient
pressure in casting operations, which can dramatically reduce
costs in manufacturing. Other meaningful properties of a melt-
castable explosive include low vapor pressure, a significant
difference between the melting temperature and the decom-
position temperature, high density, low sensitivity, and “green”
a significantly less powerful explosive than TNT (density of
3
−1 11
1
.65 g cm− and a detonation velocity of 6900 m s ). Thus,
there is an interest in developing higher-performing melt-
castable ingredients that are not only more powerful than
DNAN but also more powerful than TNT and less environ-
mentally problematic and less toxic to human health.
A plasticizer enhances fluidity or plasticity when added to a
formulation. Energetic plasticizers are used to improve physical
properties, to act as a fuel, and to improve the overall energy
yield of a formulation. In designing an optimal plasticizer,
understanding how chemical functional groups (e.g., alkyl
substituents) affect physical properties (melting, onset of
decomposition, and safety) of candidate materials becomes
2
1
12−16
imperative.
A successful energetic plasticizer must possess
a good density, reasonable sensitivity, low chemical reactivity,
and high thermal stability as one approaches oxygen-balanced
materials.
2
and affordable synthesis. Traditional state-of-the-art melt-
castable explosives have been TNT-based. However, TNT has
fallen out of favor in the energetics community because of
toxicity and environmental concerns. TNT is listed as a possible
human carcinogen, and prolonged exposure to TNT may result
3
Previously investigated materials based on N-nitroxyalkyl-
16−19
and N-azidoalkyl-substituted nitro-based heterocycles
have
been found to suffer from stability issues due to the inherent
4
in anemia and abnormal liver function.
As a stand-alone ingredient, TNT suffers from a relatively
high vapor pressure, is susceptible to photolytic degradation,
Received: March 11, 2018
5
6
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.oprd.8b00076
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Communication
reactivity of the electrophilic ring. To mitigate the afore-
mentioned safety concerns, the development of materials with
heterocyclic Lewis basic character toward electrophilic
energetic materials such as nitrocellulose (NC) and nitramines
is desired. Inclusion of alkyl nitrate pendant chains within these
materials is important to ensure high miscibility with commonly
used energetic plasticizers. These materials may possess better
wetting and plasticization properties by allowing for softer
boundaries at material interfaces. A softer boundary reduces the
tendency to crack at the propellant grain−liner interface.
acetoxyacetyl chloride to a suspension of 3 and K CO in 1,4-
2
3
dioxane at room temperature followed by stirring overnight
afforded the presumed bis(O-acyl) intermediate, which under-
went double cyclization at the refluxing temperature to yield
bis(1,2,4-oxadiazole) 5 (Scheme 2). The yield of the cyclization
Scheme 2. Second-Generation Synthesis of 2
RESULTS AND DISCUSSION
■
Recently, the bis(isoxazole) ring system was potentially found
to offer tandem melt-castability and energetic propellant
plasticization properties. In particular, the recently synthesized
20
bis(isoxazole)bis(methylene) dinitrate was found to exhibit a
melting point of 96.2 °C, thermal onset of decomposition at
1
92.7 °C, and performance slightly lower than that of TNT
(Figure 1). In an effort to further improve the energetic output
reaction was a much improved 68%. Attempted nitrolysis of the
acetate groups with 100% HNO and 100% HNO /Ac O
3
3
2
afforded no product, and only starting material was recovered.
Subjection of 5 to mixed acid conditions did afford 2, thus
demonstrating that nitrolysis could be achieved. Unfortunately,
the yield was only 33% for this transformation, and the reaction
was significantly exothermic. The high exothermicity and low
yield of the nitrolysis reaction were deemed unacceptable from
a safety and scale-up standpoint. Thus, there was still room for
optimization regarding both the yield and safety profile of the
process.
Figure 1. Molecular structures of bis(isoxazole)bis(methylene)
dinitrate (1) and bis(1,2,4-oxadiazole)bis(methylene) dinitrate (2).
and oxygen balance, attention was turned toward the bis(1,2,4-
oxadiazole) ring system (Figure 1). Overall, it was reasoned
that the presence of more nitrogen and less carbon should lead
to a higher heat of formation and a higher density, thus
resulting in a higher-performing energetic material. Thus, we
decided to pursue the synthesis of dinitrate 2.
The optimized synthesis of 2 is summarized in Scheme 3.
Because of the toxicity profile of 1,4-dioxane, it was replaced
The synthesis of 2 initially commenced with condensation of
glyoxal with 50% aqueous hydroxylamine to afford diamino-
Scheme 3. Optimized Synthesis of 2
21
glyoxime (DAG, 3). Use of an all-aqueous system when
preparing DAG as opposed to the use of a hydroxyammonium
chloride/NaOH system was found to dramatically reduce the
21
exothermic nature of the reaction profile. Treatment of DAG
with methyl glycolate in the presence of base at high
temperature, analogous to the conditions described by
2
2
Amarasinghe, afforded bis(1,2,4-oxadiazole) 4, albeit in a
very low yield of 7%. It was found in subsequent experiments
that the yield could be improved to 11% if a solution of DAG in
DMF/toluene was added over the course of 4 h. Nitration of 4
with 100% HNO yielded 2 in 85% yield. While the synthesis
3
route was only three steps, 2 was obtained in just a 7.5% overall
yield.
The first and third steps the synthesis in Scheme 1 were
deemed acceptable, so efforts were undertaken to optimize the
yield of the double cyclization reaction. Dropwise addition of
Scheme 1. First-Generation Synthesis of Bis(1,2,4-
oxadiazole)bis(methylene) Dinitrate (2)
with the more environmentally acceptable acetonitrile. Even
though the refluxing temperature of acetonitrile (82 °C) is
lower than that of 1,4-dioxane (102 °C), cyclization still
proceeded uneventfully to furnish 5 in an identical yield of 68%.
The acetate moieties were easily cleaved in the presence of
catalytic K CO in MeOH to give 4, which was nitrated
2
3
uneventfully to yield 2. Although the synthesis sequence in
Scheme 3 adds an additional step, it is inherently safer and
significantly higher-yielding, as 2 was obtained in an overall
yield of 44%, allowing 25 g scale reactions to be performed
multiple times without incident. It is important to mention that
B
DOI: 10.1021/acs.oprd.8b00076
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Organic Process Research & Development
Communication
while the nitrolysis of 5 to give 2 is highly exothermic, the
nitration of 4 yields 2 without an exothermic event, even when
determined density, and reveal its key intra- and intermolecular
interactions (Figure 2). There are no unusual bond lengths or
2
0 g of 4 is added in one portion to cold 100% HNO with
3
stirring.
The physical properties and calculated explosive performance
23
of 2 are provided in Table 1. 2 is CO oxygen-balanced and
Table 1. Calculated Physical Properties of 2 Compared with
Those of TNT and Composition B
data category
2
TNT
Composition B
a
Tm [°C]
84.5
80.4
78.0−80.0
200.0
−
b
Tdec [°C]
183.4
−33.3
295.0
−74.0
c
ΩCO2 [%]
d
ΩCO [%]
0
−24.7
1.65
20.5
6950
−
−
−
3
e
ρ [g cm
]
1.832
29.4
8180
236.0
−79.4
1.68−1.74
26.0−28.0
7800−8000
−
f
Pcj [GPa]
Vdet [m s−1]g
h
Isp [s]
Δ H° [kJ mol−1]i
−59.3
−
f
a
b
Tm = onset temperature of melting. Tdec = onset temperature of
decomposition. Ω
balance. ρ = derived density from X-ray data. P = detonation
c
d
= CO oxygen balance.
Ω
= CO oxygen
CO
2
2
CO
e
f
cj
g
h
i
pressure. V = detonation velocity. I = specific impulse. Δ H° =
det
sp
f
molar enthalpy of formation.
has a melting point of 84.5 °C and a thermal onset of
decomposition at 183.4 °C. Thus, 2 can be classified as a
potential stand-alone melt-castable explosive material. 2 out
performs TNT in many categories by a wide margin, which is
significant since the latter material is considered the benchmark
for melt-castable explosives. The experimental density and
theoretical detonation velocity of 2 are significantly higher than
those of TNT, with a theoretical detonation pressure ca. 50%
higher than that of TNT, even surpassing the performance of
Composition B, a high-performance melt-castable formulation.
The 1,2,4-oxadiazole moieties possess weak Lewis basic
character as well as maximum pendant alkyl nitrate
functionality. Hence, this material may also serve as an
energetic plasticizer in nitrate-based formulations, potentially
reducing volatility and migration during thermal and
mechanical shock events.
Figure 2. (A) Molecular conformation of compound 2. The non-
hydrogen atoms are shown as 50% probability displacement ellipsoids.
The nonlabeled atoms result from inverting the asymmetric unit (−x,
−y + 1, −z + 2). (B) Crystal packing viewed along the b axis. Dashed
blue lines represent intramolecular interactions, whereas dashed red
lines represent intermolecular contacts, including H3A···N2 and O3···
O3.
angles. The 1,2,4-oxadiazole rings are planar (root-mean-square
deviation = 0.0021(2) Å) and the alkyl nitrate groups adopt a
trans configuration with respect to the rings. The C3 atom is
nearly coplanar with the ring (atom-to-mean plane distance =
0
.03 Å), whereas the C3−O2 bond is twisted slightly out of the
plane, as evidenced by the torsion angles O1−C2−C3−O2 =
21.8(3)° and N2−C2−C3−O2 = 162.8(2)°. The dihedral
angle between the oxadiazole ring and the nitrate group is
3.5(1)°. Bifurcated intramolecular contacts between the O3
−
The sensitivities of 2 toward impact, friction, and electro-
static discharge (ESD) were determined and compared with
those of the commonly handled and processed explosive RDX
8
atoms and the H3B and C2 atoms [O3···H3B = 2.360(2) Å;
O3···C2 = 2.856(3) Å] contribute to its stability, whereas van
der Waals contacts between the H3A and N2 atoms on
(Table 2). 2 was found to exhibit lower sensitivities to impact
i
Table 2. Sensitivities of 2 Compared with Those of RDX
adjacent molecules [H3A···N2 = 2.562(1) Å; symmetry code
a
b
c
(i): −x + 1, −y, z −2] and between the O3 and O3 atoms on
compound
IS
FS
ESDS
ii
adjacent molecules [O3···O3 = 3.025(1) Å; symmetry code
RDX
6.2
8.7
156
282
0.125
(
ii): −x + 1, −y + 1, −z + 1] dominate its intermolecular
2
0.125
interactions. In the crystal lattice, the oxadiazoles are arranged
face-up in rows [centroid-to-centroid distance = 6.028(1) Å,
plane-to-plane shift = 6.025(2) Å], forming planes parallel to
the b axis. The rings are stacked along the a axis with some in
close proximity [centroid-to-centroid distance = 3.444(2) Å;
plane-to-plane shift = 1.5542(3) Å] (see Figure 2B).
a
b
c
IS = impact sensitivity. FS = friction sensitivity. ESDS =
electrostatic discharge sensitivity.
and friction and an identical ESD sensitivity compared to RDX
and is therefore deemed to be a relatively safe material to
handle. The relatively low sensitivities of 2 to impact and
friction are noteworthy since there is a common belief among
many in the energetic materials community that nitrate-based
materials possess high sensitivities to impact and friction.
Single-crystal X-ray diffractometry was used to unequivocally
identify 2, confirm its structure, provide an experimentally
The diffraction patterns of 2 are consistent with its being a
̅
member the triclinic crystal system (P1) with one molecule in
its unit cell. On the basis of its molecular mass and lattice
constants [a = 4.8405(4) Å, b = 6.0293(4) Å, c = 9.4356(7) Å,
α = 80.399(6)°, β = 77.125(7)°, and γ = 78.831(6)°], we
obtain a density of 1.832 g cm− at 295.9 (1) K.
3
C
DOI: 10.1021/acs.oprd.8b00076
Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development
Communication
CONCLUSIONS
equiv). The reaction mixture was stirred at ambient temper-
ature for 48 h and then concentrated in vacuo to give a crude
■
In summary, an efficient, scalable route to bis(1,2,4-oxadiazole)-
bis(methylene) dinitrate (2) has been developed, its crystal
structure has been determined, and its explosive performance
and sensitivity have been evaluated. The four-step synthesis
affords 2 in an overall yield of 44%. 2 was found to have a low
sensitivity to impact, friction, and ESD, and it was estimated to
have energy in excess of that of Composition B and a
detonation pressure 50% higher than that of TNT. The
characteristics and performance properties of 2 suggest that this
material has high potential to serve as a powerful stand-alone
melt-castable explosive material as well as an energetic
plasticizing ingredient with nitrocellulose-based propellant
formulations in an effort to reduce the volatility/migration
issues that arise during cookoff.
solid, which was purified by trituration with 200 mL of H O.
2
The solid was collected by Bu
̈
chner filtration and oven-dried at
6
0 °C overnight to afford 16.0 g of diol 4 (95% yield) as a white
powder. T = 197.8 °C; Tdec = 231.5 °C (onset), 278.2 °C
melt
1
(
peak); H NMR (90 MHz, DMSO-d ) δ 6.16 (s, 2H), 4.87 (s,
6
13
4
H); C NMR (90 MHz, DMSO-d ) δ 178.50, 156.94, 52.74;
6
−1
IR (neat) cm 3339.57 (w), 1575.77 (m), 1431.04 (m),
202.56 (m), 1081.27 (s).
1
[
3,3′-Bis(1,2,4-oxadiazole)]-5,5′-diylbis(methylene)
Dinitrate (2). To a 250 mL round-bottom flask immersed in
an ice bath was added 100 mL of 100% HNO . After the nitric
3
acid was chilled to 0 °C, diol 4 (20.0 g, 101.0 mmol, 1.00 equiv)
was added in four equal portions over 1 h. After the addition
was complete, the reaction mixture was stirred for 4 h, during
which time the ice bath was allowed to melt, and the reaction
mixture was allowed to warm to ambient temperature. The
reaction mixture was poured onto crushed ice with stirring.
EXPERIMENTAL SECTION
■
General Methods. Chemicals and solvents were used as
received from Sigma-Aldrich. Diaminoglyoxime (3) was
synthesized according to the literature.
spectra were recorded using an Anasazi 90 MHz instrument.
The chemical shifts quoted in parts per million in the text refer
to typical standard tetramethylsilane in CDCl as the solvent.
Infrared spectra were measured with a Bruker Alpha-P FTIR
instrument. Melting and decomposition temperatures were
measured at a heating rate of 5 °C/min using a TA Instruments
Q10 DSC instrument. Single-crystal X-ray diffraction studies
were performed with a SuperNova Dualflex diffractometer
containing an EosS2 charge-coupled device detector and a Mo
Kα radiation source (λ = 0.71073 Å).
Caution! Although we did not experience any problems handling
the compounds described in this paper, when handling energetic
materials such as 3 and 2, proper laboratory precautions should be
taken. Laboratories and personnel should be properly grounded,
and safety equipment such as heavy Kevlar/steel gloves, reinforced
Kevlar coat, ballistic face shield, ear plugs, and blast shields are
necessary.
After 1 h, the solid was collected by Bu
dried in a well-ventilated fume hood to afford 24.7 g of dinitrate
(85% yield) as a white powder. T
̈
chner filtration and air-
21
1
13
H and C NMR
2
= 84.5 °C; Tdec = 183.4
melt
1
°
C (onset), 214.6 °C (peak); H NMR (90 MHz, DMSO-d ) δ
6
13
6
6
1
.16 (s, 4H); C NMR (90 MHz, DMSO-d ) δ 175.48, 159.43,
6
3
4.26; IR (neat) cm− 2950.51 (w), 1653.16 (s), 1574.20 (m),
1
415.61 (m), 1350.42 (m), 1282.23 (s), 1212.83 (s), 1059.18
(
m).
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.oprd.8b00076.
Experimental procedures and analytical data for all new
1
13
compounds ( H, C NMR, and FT-IR spectra and DSC
traces for compounds 2, 4, and 5) and X-ray crystallo-
graphic data for compound 2 (PDF)
[
3,3′-Bis(1,2,4-oxadiazole)]-5,5′-diylbis(methylene)
AUTHOR INFORMATION
■
*
*
Diacetate (5). To a 2 L round-bottom flask equipped with a
stir bar were sequentially added 1 L of acetonitrile, 3 (15.0 g,
.127 mol, 1.00 equiv), and K CO (35.1 g, 0.254 mol, 2.00
equiv). The flask was fitted with a pressure-equalizing liquid
addition funnel, and a solution of acetoxyacetyl chloride (35.4
Corresponding Authors
E-mail: jesse.j.sabatini.civ@mail.mil. Phone: 410-278-0235.
E-mail: dechavez@lanl.gov. Phone: 505-665-2742.
0
2 3
ORCID
g, 27.9 mL, 0.254 mol, 2.00 equiv) in 200 mL of CH CN was
Jesse J. Sabatini: 0000-0001-7903-8973
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
3
added dropwise over 2 h. The reaction mixture was stirred
overnight at ambient temperature, and the flask was then fitted
with a reflux condenser. The reaction mixture was heated to
reflux, stirred for 4 h, and then cooled to room temperature.
The solid was collected by Buchner filtration and discarded.
̈
The mother liquor was transferred to a 2 L round-bottom flask
and concentrated in vacuo to afford a crude solid. The crude
Funding
The authors thank the U.S. Army and the Joint Munitions
Technology Development Program for financial support in
carrying out this work. Los Alamos National Laboratory is
operated for the U.S. Department of Energy by Los Alamos
National Security (LANS, LLC) under Contract DE-AC52-
06NA25396.
solid was purified by trituration with 500 mL of H O. The solid
2
̈
was collected by Buchner filtration and oven-dried at 60 °C
overnight to afford 24.4 g of bis(1,2,4-oxadiazole) 5 (68%
1
yield) as a white powder. Tmelt = 107.5 °C; H NMR (90 MHz,
1
3
DMSO-d ) δ 5.53 (s, 4H), 2.18 (s, 6H); C NMR (90 MHz,
6
DMSO-d ) δ 176.91, 169.84, 159.37, 56.45, 20.09; IR (neat)
Notes
6
−1
cm 1745.23 (s), 1577.13 (m), 1208.08 (s).
The authors declare no competing financial interest.
The CIF file for compound 2 can be obtained from the
Cambridge Crystallographic Data Centre (deposit@ccdccam.
ac.uk) free of charge, quoting the CCDC reference number
1819160.
[
3,3′-Bis(1,2,4-oxadiazole)]-5,5′-diyldimethanol (4).
To a 250 mL round-bottom flask equipped with a stir bar
were sequentially added 50 mL of MeOH, 5 (24.0 g, 85.1
mmol, 1.00 equiv), and K CO (1.18 g, 8.51 mmol, 0.100
2
3
D
DOI: 10.1021/acs.oprd.8b00076
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Organic Process Research & Development
Communication
(
22) Amarasinghe, K. K. D.; Maier, M. B.; Srivastava, A.; Gray, J. L.
ABBREVIATIONS
■
One-pot synthesis of 1,2,4-oxadiazoles from carboxylic acid esters and
amidoximes using potassium carbonate. Tetrahedron Lett. 2006, 47,
TNT = trinitrotoluene; DNAN = dinitroanisole; RDX = royal
demolition explosive
3
(
629−3631.
23) (a) Byrd, E. F. C.; Rice, B. M. Improved Heats of Formation of
Energetic Materials Using Quantum Mechanical Calculations. J. Phys.
Chem. A 2006, 110, 1005−1013. (b) Byrd, E. F. C.; Rice, B. M. A
Comparison of Methods to Predict Solid Phase Heats of Formation of
Molecular Energetic Salts. J. Phys. Chem. A 2009, 113, 345−352.
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DOI: 10.1021/acs.oprd.8b00076
Org. Process Res. Dev. XXXX, XXX, XXX−XXX