Effect of Fluoro Substituents on Polynitroarylenes: Design, Synthesis and Theoretical Studies of Fluorinated Nitrotoluenes

Article abstract of DOI:10.1002/cplu.201800523

The replacement of traditional polynitroarylenes by their fluorinated derivatives has attracted great attention due to the improvements on detonation performance caused by the fluorine effect. A straightforward synthesis of three novel fluorinated nitroto

Full text of DOI:10.1002/cplu.201800523

Accepted Article
Title: Effect of Fluoro Substituents on Polynitroarylenes: Design,
Synthesis and Theoretical Studies of Fluorinated Nitrotoluenes
Authors: Junlin Zhang, Yingzhe Liu, Jing Zhou, Fuqiang Bi, and
Bozhou Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.201800523
Link to VoR: http://dx.doi.org/10.1002/cplu.201800523
A Journal of
www.chempluschem.org

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
Effect of Fluoro Substituents on Polynitroarylenes: Design,
Synthesis and Theoretical Studies of Fluorinated Nitrotoluenes
Junlin Zhang,[a,b] Yingzhe Liu,[a] Jing Zhou,[a] Fuqiang Bi,[a] and Bozhou Wang*[a]
Abstract: The replacement of traditional polynitroarylenes by their
fluorinated derivatives have attracted great attention due to the
improvements on detonation performances caused by fluorinated
effect.
A straightforward synthesis of three novel fluorinated
nitrotoluenes with different degree of nitration were achieved under
selected high temperatures. The existence of fluorine exerted
remarkable influence on the nitration process through electron-
withdrawing effect and possible mechanism of the transformation was
proposed according to the experimental results. Thermal
decomposition behaviors of the fluorinated nitrotoluenes were
investigated by differential scanning calorimetry and all the
compounds showed ideal low melt points for melt-cast process. X-ray
analysis of the molecules were carried out with electronic density and
electrostatic potential on molecular surface of the optimized structures
calculated by density functional theory. The good thermal and
detonation properties make the newly developed fluorinated
nitrotoluenes possible to replace trinitrotoluene (TNT) and
dinitrotoluene (DNT) in the formation of melt-cast energetic materials.
Figure 1. FLUORINE-rich groups in energetic materials
Polynitroarylenes have contributed enormously to the progress
and prosperity of mankind and are still applied as one of the most
important class of explosives[7]. Although the detonation
performances are not outstanding, some polynitroarylenes as
2,4,6-trinitrotoluene (TNT) and dinitrotoluene (DNT) exhibit ideal
melting point (80~110℃) which permits melt casting of charges
and make them highly important energetic binder for cast
compositions.[8] From synthetic point of view, TNT and DNT were
obtained by nitration treatment of toluene, meanwhile, DNT was
usually obtained as annoying mixtures of 2,6-dinitrotoluene and
2,4-dinitrotoluene. (Figure 2)
Introduction
Fluorinated energetic materials are subject of intensive research
due to the excellent detonation properties.[1] Calculation studies
demonstrate that high fluorine content along with the presence of
hydrogen leads to the formation of hydrogen fluoride (HF) upon
detonation, generating a large amount of energy, and the
existence of fluorine could also improve the densities of the
molecules.[2] During recent decades, strategies for the
preparation of fluorinated energetic materials focused on the
introduction of fluorine-rich groups as pentafluorosulfanyl (–SF5)[3],
difluoramino (–NF2)[4], fluorodinitromethyl[5] and fluoronitromethyl-
ONN-azoxy[6] group, which have been widely applied in the design
and synthesis of new energetic structures with great success.
(Figure 1) However, intrinsic relationship between the promotion
of energy-density level and introduction of fluorine is still unclear,
which severely hampers the development of new high-performing
energetic materials.
Figure 2. TNT and DNT structures
Nitration by mixed acid is the most widely used method for direct
nitration of aromatic molecules and any increase or decrease in
electron density will significantly influence the nitration effect.[9]
Compared with the nitration of electron-deficient aromatic
substrates, nitration of electron-rich aromatic substrates are much
easier.[10] In aromatic structures, the newly formed nitro groups
are generally located at the o- and p-positions of the activate
substituents, whereas at the m-positions of the deactivate
substituents.[11] (Figure 3) For the aromatic substrates with some
highly strong deactivating substituents, direct nitration will be
extremely difficult.[12] Therefore, direct nitration of fluorinated
aromatic substrates has long been regarded as a formidable task
due to the strong deactivating effect caused by the existence of
fluorine.
[a]
[b]
Dr. Junlin Zhang, Dr. Yingzhe Liu, Jing Zhou, Dr. Fuqiang Bi, Prof.
Bozhou Wang
Xi’an Modern Chemistry Research Institute
Xi’an, 710065, China
E-mail: wbz600@163.com
Dr. Junlin Zhang
Department of Chemistry
Technische Universität München
Garching bei München, 85748, Germany
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
investigate the chemical reactivity of the direct nitration strategies,
3,5-difluorotoluene and α,α,α,3,5-pentafluorotoluene were
chosen and treated with HNO3-H2SO4 mixture, the most widely
used condition for the preparation of polynitroarylenes. To our
disappointment, no desired products were detected. Obviously,
the electron-deficient character of the substrates hampered the
transformations and more strenuous conditions or reagents were
required. After screening various nitration conditions and
reagents, we finally arrived at conditions for the successful direct
+
nitration transformation by using Oleum-NaNO3 mixture as NO2
source under selected high temperatures. (Scheme 1)
TNT and DNT were synthesized through direct nitration of
toluene, but the products were usually obtained as mixtures with
different degree of nitration. To our delight, single product was
successfully achieved under our newly developed nitration
+
process. By using Oleum-NaNO3 mixture as the source of NO2 ,
3,5-difluoro-2,4,6-trinitrotoluene (DFTNT) was obtained at the
heating temperature of 160℃ while 3,5-difluoro-2,4-dinitrotoluene
(DFDNT) was achieved as the only product at the heating
temperature of 70℃. Obviously, temperature should be one of
most important factors for the nitration process. When α,α,α,3,5-
pentafluorotoluene was chosen as the substrate, α,α,α,3,5-
pentafluoro-2,4-dinitrotoluene (PFDNT) was also achieved as the
only product. No all-fluorinated TNT was observed during the
heating process and the compound began to decompose when
the heating temperature was further raised. From the synthetic
point of view, it seemed that the newly formed nitro groups in
PFDNT prevented the introduction of the third nitro group, leading
to the failure in the preparation of all-fluorinated TNT (PFTNT),
however, it is also possible caused by the decomposition of all-
fluorinated TNT under high temperature.
Figure 3. Direct nitration of aromatic substrates
Here, we reported a straightforward synthesis of three novel
fluorinated
nitrotoluene
molecules,
3,5-difluoro-2,4,6-
trinitrotoluene (DFTNT), 3,5-difluoro-2,4-dinitrotoluene (DFDNT)
and α,α,α,3,5-pentafluoro-2,4-dinitrotoluene (PFDNT). All the
molecules were synthesized through nitration reactions under
selected temperatures from electron deficient aromatics with high
fluorine content. Thermal decomposition behaviors were
investigated by differential scanning calorimetry and further
compared with TNT and DNT, whereas densities, heats of
formation, detonation velocities and pressures were calculated
through computational methods. In addition, the optimized
structure, electronic density and electrostatic potential on surface
of the molecules were calculated with density functional theory for
better understanding the fluorinated effect on energetic
structures[13]. The deep research on the fluorinated nitrotoluenes
will promote the studies of fluorinated energetic materials as well
as the intrinsic mechanism of nitration process under high
temperatures.
Results and Discussion
Synthetic studies and structure analysis
Replacement of traditional polynitroarylenes by their fluorinated
derivatives have attracted great attention for the improvements on
detonation performances caused by fluorinated effect, however,
synthesis of fluorinated polynitroarylenes is highly difficult due to
the limited synthetic approaches. All-fluorinated TNT, for instance,
is supposed to be a super energetic molecule, but its preparation
has never been achieved. Generally, there are two strategies to
introduce fluorine into energetic structures. The most commonly
used approach is modification of the existing molecules through
fluorination reactions. In contrast, introduction of fluorine through
direct nitration of fluorinated structures are rarely applied. During
recent application research on TNT and DNT, it occurred to us
that direct nitration of fluorinated aromatic hydrocarbons might
provide a shortcut access to new fluorinated nitrotoluenes. To
Scheme 1. Synthetic studies towards fluorinated nitrotoluenes.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
Suitable crystals of DFTNT, DFDNT and PFDNT for X-ray
analysis were obtained by dissolving the compounds in a
minimum amount of ethyl acetate held at room temperature,
followed by filtration of the crystals after the volume of ethyl
acetate had been reduced. The crystal structures of DFTNT,
DFDNT and PFDNT were illustrated in Figure 4. Interestingly,
strong hydrogen-bonding interaction was found between the
fluoride in the aromatic ring and the hydrogen in the methyl group
while no similar hydrogen-bonding interaction observed in the
crystal structures of DFTNT or PFDNT, indicating that the
hydrogen-bonding interaction of the fluorinated nitrotoluenes
mainly existed between aromatic hydrogen and fluorine atom.
Figure 4. ORTEP diagram (50% probability) of DFTNT, DFDNT and
PFDNT.
Eelectrostatic potentials (ESPs) on 0.001 au (electrons/Bohr3)
isosurfaces of electron density of the molecular structures DFTNT,
DFDNT and PFDNT were studied by density functional theory and
the general gradient approximation method in Dmol3 module of
Material Studio 8.0 software with the Perdew, Burke, and
Ernzerhof (PBE) exchange-correlation functional and the DNP
basis set to further clarify the experimental results and
mechanism of the transformations[13]. According to theoretical
investigation of the relationship between stabilities and the
charges of the nitro group in organic nitro compounds, nitro group
Mulliken charges can be regarded as a structural parameter to
estimate the stabilities of the nitro compounds. The compound
with more negative Mulliken Charges of the nitro group will be
more insensitive and usually gives a larger value of impact
sensitivity H50[14]. Calculations of DFTNT, DFDNT and PFDNT
including optimizations, Mulliken population and frequency
analyses were performed and the results were demonstrated in
Figure 5. Compared with DFDNT, the Mulliken charges of nitro
group in PFDNT is much more positive, which indicated that
PFDNT is much less stable; therefore, the failure in the
preparation of all-fluorinated TNT was more likely caused by the
decomposition of PFDNT under high temperature.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
influenced the packing form of crystals remarkably. After the
introduction of fluorine into DNT and TNT, the densities of
molecules increased noticeably.
Table 1. Hirshfeldsurfaces[a] of crystal stacking[b] of TNT, DFTNT, DNT,
DFDNT and PFDNT
O…H
46.0%
O…H
27.0%
O…H
48.8%
O…H
32.0%
F…O
O…C
18.5%
F…O
24.5%
H…H
19.4%
F…O
17.1%
F…F
O…O
17.9%
O…C
16.5%
O…O
10.4%
F…H
TNT
O…O
DFTNT
DNT
16.2%
O…O
10.8%
O…H
10.8%
DFDNT
PFDNT
16.3%
O…C
13.7%
O…O
25.5%
25.2%
10.7%
[a] Only attributes of the Hirshfeld surface larger than 10% were displayed.
[b] Hirshfeld surface analyses were achieved through CrystalExplorer 3.0
program.
Figure 5. ESPs on isosurfaces of electron density of TNT, DNT, DFTNT,
DFDNT and PFDNT’s crystal structures.
Hirshfeld surfaces are widely used to identify and quantify the
interaction nature and proportion in crystals.[15] Hirshfeld surface
analyses of DFTNT, DFDNT and PFDNT were achieved through
CrystalExplorer 3.0 program[16] and further compared with DNT
and TNT to study their intermolecular interactions. (Table 1 and
Figure 6) After the fluorine was introduced into DNT and TNT, the
interactions between fluorine and other atoms appeared.
Interestingly, extra interactions of F…O and F…H increased
significantly in DFDNT while extra interactions of F…O and F…F
increased sharply in PFDNT. In contrast, compared with TNT,
only extra interaction between fluorine and oxygen emerged
apparently in DFTNT. Different intermolecular interactions
Figure 6. Hirshfeldsurfaces of crystal stacking of nitrotoluenes and their
derivatives.
Studies on energetic properties and thermal decomposition
behaviors of the fluorinated nitrotoluenes
TNT and DNT were widely applied as ideal organic solvents in the
preparation of melt-cast explosives for their capability in the
dissolution of numerous energetic materials resulting in
acceptable processing viscosities. After the introduction of
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
fluorine into these nitrotoluenes, most of the performances of the
energetic molecules were improved obviously except for the
sensitivities, which were higher than TNT and DNT due to the
weaker hydrogen-bonding interactions. (Table 2)
Table 2. Physical and energetic properties of newly developed fluorinated
nitrotoluenes compared with TNT and DNT
Compd
DFTNT
263
TNT
227
DFDNT
218
PFDNT
272
DNT
182
M
[g·mol−1
Ω[a]
]
-51.7
1.82
-73.9
1.65
18.4
102
-80.7
1.73
-47.0
1.82
-114.3
1.54
12.9
125
[%]
ρ[b]
[g·cm−3
ΔfH[c]
[kJ·mol−1
IS[d] (H50
[cm]
]
-273.8
53
-326.6
62
-917.3
29
]
)
P[e]
[Gpa]
27.2
19.5
6909
17.9
17.1
16.4
6263
[f]
VD
7435
6642
6077
[m·s−1
]
[a] Oxygen balance assuming the formation of CO2. [b] Gas pycnometer
(25°C). [c] Calculated molar enthalpy of formation. [d] Impact sensitivity. [e]
Detonation pressure. [f] Detonation velocity.
As potential ideal energetic materials for melt-cast explosives,
researches on thermal behaviors of DFTNT, DFDNT and PFDNT
are crucial for their further applications. Therefore, the research
on thermal behaviors of DFTNT, DFDNT and PFDNT were carried
out through DSC method. (Figure 7) Under heating, continuous
endothermic curves could be observed from the DSC experiment,
indicating the sustained volatilizations of these molecules. DFTNT
and PFDNT melted at the temperature of 78.0℃ and 80.1℃,
which are close to the melt point of TNT and DNT. However,
DFDNT melted at a much higher temperature (101.8℃). The
higher melt temperature is very likely caused by the strong
hydrogen-bonding interaction in DFDNT. (Figure 4) With the
further rise of heating temperature, a spontaneous endothermic
volatilization process of DFTNT, DFDNT and PFDNT were also
observed clearly, which means large amount of vapor was
generated after the melt process. When the heating temperature
was up to 220℃, the decomposition process of DFDNT and
PFDNT started. DFTNT showed even better thermal stability and
the decomposition of DFTNT started only when the heating
temperature was higher than 290℃. DSC studies showed that the
thermal decomposition processes of DFTNT, DFDNT and PFDNT
were complex, which was caused by secondary decomposition
reactions of the pyrolysis products. The low melting points and
good thermal stabilities of the molecules rendered them excellent
application prospect in the preparations of melt-cast energetic
materials.
Figure 7. DSC curve of DFDNT, PFDNT and DFTNT.
Conclusions
Three novel fluorinated nitrotoluenes were obtained via nitration
processes under selected high temperatures and their structure
studies were achieved through quantum calculation and crystals
analysis. The amount of the nitro groups induced to the highly
electron deficient aromatics was dependent on the reaction
conditions and single dinitration products could be successfully
obtained without their isomers under the newly developed
nitration method. The Mulliken Charges of nitro group in PFDNT
is much more positive than DFDNT, indicating molecule structure
can easily decompose in the preparation of all-fluorinated TNT
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
X. Fan, X. Gang, W. Qing, S. Gao, ChemPlusChem 2016, 81, 1156-
1159. d) M. Xu, Z. Ge, X Lu, H. Mo, Y. Ji, M. Hu, RSC Adv. 2017, 7,
47271-47278.
under high temperature. Hirshfeld surface analyses showed that
different intermolecular interactions influenced the packing form
of crystals remarkably and the introduction of fluorine into DNT
and TNT led to obvious increase of molecule densities.
Furthermore, the good energetic properties, low melting points
and good thermal stabilities of the molecules rendered them
excellent application prospect in the formation of melt-cast
energetic materials.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
H. Martinez, Z. Zheng, W. Dolbier, J. Fluorine. Chem. 2012, 143, 112-
122.
C. Ye, G. Gard, R. Winter, R. Syvret, B. Twamley, J. Shreeve, Org. Lett.
2007, 9, 3841-3844.
R. Butcher, R. Gilardi, K. Baum, N. Trivedi, Thermochim. Acta. 2002, 384,
219-227.
M. Kettner, K. Karaghiosoff, T. Klapötke, M. Sućeska S. Wunder, Chem.
Eur. J. 2014, 20, 7622-7631.
O. Lukýanov, G. Pokhvisneva, T. Ternikova, N. Shlykova, M. Shagaeva,
Russ.Chem. Bull. 2011, 60, 1703-1711.
Experimental Section
D. Badgujar, M. Talawar, S. Asthana, P. Mahulikar, J. Hazar. Mater. 2008,
151, 289-305.
Reagents and sample preparation
TNT (99.0%), DNT (99.0%) and Oleum (50%) were supplied by Xi’an
Modern Chemistry Research Institute. 3,5-Difluorotolune, 3,5-
difluorobenzotrifluoride and sodium nitrate were purchased from Accela
Chem Bio Co., Ltd (Shanghai, China) without further purification.
General Procedure for the synthesis of DFTNT and PFDNT:
Oleum (50%, 75 mL) was placed in a three-neck flask, to which NaNO3
(4.25 g, 50 mmol) was added in several potions and the temperature was
kept below 50℃ during the addition. After the addition, fluorinated toluene
(10 mmol) was added carefully. The mixture was heated to 80℃ and
allowed to stir for 10 min at this temperature. The reaction was then further
heated to 160℃ and stirred for another 3 h before cooled to room
temperature. The mixture was poured into ice (200 g) and stirred for 1 h.
The formed solid was filtered and washed by H2O (20 mL) for three times
to give a white solid product.
R. Pasupala, D. Badgujar, G. Gore, S. Tewari, A. Sikder, Propell. Explos.
Pryot. 2011, 36, 393-403.
P. Pagoria, A. Mitchell, R. Schmidt, C. Coon, E. Jessop, Acs Symposium
1995, 623, 151-164.
[10] X. Qi, G. Cheng, C. Lu, D. Qian, Cent. Eur. J. Energ. Mat. 2007, 4, 105-
113.
[11] R. Dunbar, J. Shen, G. Olah, J. Am. Chem. Soc. 2002, 94, 6862-6864.
[12] K. Smith, T. Gibbins, R. Millar, R. Claridge, J. Chem. Soc. Perkin. Trans.
2000, 16, 2753-2758.
[13] a) Material Studio 8.0; Acceryls Inc.: San Diego, 2014. b) J. Perdew, K.
Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865-3868; c) C. Adamo,
V. Barone, J. Chem. Phys. 1999, 110, 6158-6170.
[14] C. Zhang, Y. Shu, Y. Huang, X. Zhao, H. Dong, J. Phys. Chem. B. 2005,
109, 8978-8982.
[15] a) W. Zhang, J. Zhang, M. Deng, M. Deng, X. Qi, F. Nie, Q. Zhang, Nat.
Commun. 2017, 8, 1-7. b) F. Afkhami, A. Khandar, G. Mahmoudi, W.
Maniukiewicz, J. Lipkowski, J. White, R. Waterman, S. Granda, E.
Zangrando, A. Bauzá, A. Frontera, CrystEngComm 2018, 20, 746-754.
[16] M. Spackman, D. Jayatilaka, CrystEngComm 2009, 11,19-32.
Apparatus and measurements
The thermal analysis experiments were performed with a model TG-DSC
STA 449C instrument (NETZSCH, Germany). Operation conditions:
sample mass, 0.6 mg; dynamic nitrogen; stainless steel seal cell; heating
rate, 10℃·min-1; temperature range, 20℃~500℃. Single crystal X-ray
experiment was carried out on a Bruker Apex II CCD diffractometer
equipped with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å)
using ω and φ scan mode. Structures were solved by the direct method
using SHELXTL and refined by means of full-matrix least-squares
procedures on F2 with the programs SHELXL-97. All non-hydrogen atoms
were refined with anisotropic displacement parameters.
CCDC 1873174 (DFTNT), 1873178 (PFDNT) and 1873177 (DFDNT)
contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data
Centre.
Acknowledgements
We are grateful to the financial support from National Natural
Science Foundation of China (No.21805226 and No.21805223)
and the China Postdoctoral Science Foundation (No.
2018M633552).
Keywords: Energetic materials • Fluorinated nitrotoluenes •
Melt-cast • Nitration • Thermal behaviors
[1]
a) S. Garg, J. Shreeve, J. Mater. Chem. 2011, 21, 4787-4795; b) M.
Kettner, K. Karaghiosoff, T. Klapötke, M. Sućeska, S. Wunder, Chem.
Eur. J. 2014, 20, 7622-7631; c) L. Zhai, X. Qu, B. Wang, F. Bi, S. Ping,
This article is protected by copyright. All rights reserved.

10.1002/cplu.201800523
ChemPlusChem
FULL PAPER
FULL PAPER
A straightforward synthesis of three
novel fluorinated nitrotoluenes with
different degree of nitration were
Junlin Zhang, Yingzhe Liu, Jing Zhou,
Fuqiang Bi, and Bozhou Wang*
Page No. – Page No.
achieved
under
selected
high
temperatures. Single nitration product
could be obtained through the new
nitration conditions and the good
energetic properties, low melting points
as well as good thermal stabilities of
these energetic molecules rendered
them excellent application prospect in
the formation of melt-cast energetic
materials.
Effect of Fluoro Substituents on
Polynitroarylenes: Design,
Synthesis and Theoretical Studies of
Fluorinated Nitrotoluenes
This article is protected by copyright. All rights reserved.