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Hexamethylenetetramine Dinitrate (HDN): The Precursor for RDX Production

by Bachmann Process

Article  in  Propellants Explosives Pyrotechnics · October 2013

DOI: 10.1002/prep.201200162

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Full Paper
DOI: 10.1002/prep.201200162

Hexamethylenetetramine Dinitrate (HDN): The Precursor

for RDX Production by Bachmann Process
Hamza Turhan,[a] Taner Atalar,[a] Nilfer Erdem,[a] Canpolat zden,[a] Bahaddin Din,[b] Nebi Gl,[b]
Emel Yildiz,*[a] and Lemi Trker[c]

Abstract: Hexamethylenetetramine dinitrate (HDN) is urements. The thermal characteristics of HDN were deter-
a rather weak explosive but is used as a precursor for the mined by DSC and TG/DTA. The DSC curve of HDN shows
synthesis of RDX, one of the most important secondary an endothermic peak at 170.5 8C corresponding to the
nitramine explosives. HDN has limited application because melting point of HDN, followed by two exothermic peaks
of its hygroscopic character. This paper reports on the syn- at 174.0 8C and 200.5 8C due to the decomposition. The dif-
thesis and characterization of HDN in high yield and purity ferences in the thermal behavior of HDN samples, which
by the reaction of hexamine with nitric acid at tempera- were thermally aged at 50 8C, 100 8C, and 150 8C in a nitro-
tures below 15 8C. It was characterized by FTIR and 1H NMR gen atmosphere were examined. Additionally, some quan-
spectroscopy, Scanning Electron Microscopy (SEM) and tum chemical properties of the nitration of hexamethylene-
Liquid Chromatography/Mass Spectrometry (LC/MS) meas- tetramine were calculated.
Keywords: HDN · Nitration · RDX · Stability of nitration products · Nitramine

1 Introduction

Hexamethylenetetramine is a heterocyclic organic com- by Henning and the process, outlined as the nitrolysis of
pound with a symmetric tetrahedral cage-like structure. It hexamethylenetetramine dinitrate (HDN) with concentrated
has a wide range of industrial applications such as its use HNO3, was patented in 1899. The use of HDN as a precursor
as raw material in adhesives, coatings, and sealing com- for manufacturing of RDX was the first attainable applica-
pounds; as cross-linking agent for the hardening of phenol tion of HDN in explosive chemistry, although the properties
formaldehyde resins, corrosion inhibitor, dye fixative for of RDX as a medical compound preceded its usage as an
textiles, fuel tablets, as stabilizer for lubricating oils, and as explosive. The preparation of HDN in high yield was essen-
starting material in the production of nitramine-based ex- tially described by Hale [3].
plosives. One of the manufacturing processes of RDX is the KA
Because of the weakly basic character of hexamethylene- process, which was independently worked out by Bach-
tetramine, it reacts with acids to form salts. The phenate, mann in the USA and Knçffler in Germany [4–8]. The KA
citrate, camphorate, and perchlorate salts of hexamine are process is a combination of the K and E processes, which
used in pharmaceutical applications. The importance of the are based on hexamine, HNO3, NH4NO3, and paraformalde-
reaction of hexamine with nitric acid is due to the energet- hyde, NH4NO3, acetic anhydride, respectively.
ic characteristics of the nitramine compounds produced The nitrolysis reaction of HDN with HNO3, NH4NO3, and
from hexamine. The mechanism of the nitrolysis reaction of acetic anhydride is known as the KA process for the pro-
hexamine is quite complicated so that research efforts on
definition the effects of reaction parameters on the struc- [a] H. Turhan, T. Atalar, N. Erdem, C. zden, E. Yildiz
ture of final and side products have widely preceded. De- Tbitak MRC
pending on the nitrolysis conditions, the CH2 N bonds in Chemistry Institute
hexamine can undergo scission and nitramine-type com- Gebze Kocaeli, Turkey
*e-mail: emel.yildiz@tubitak.gov.tr
pounds with different structures are produced. Quite
a number of nitramine compounds with cyclic and linear [b] B. Din, N. Gl
MSB Ar-Ge ve Teknol
structures were obtained because of the diversity of reac-
Dairesi Başkanlığı
tion pathways during the nitrolysis of hexamine [1, 2]. The Ankara, Turkey
unquestionable importance of cyclic nitramines as military
[c] L. Trker
explosives was improved by the high performance charac- Department of Chemistry
teristics of cyclotrimethylenetrinitramine (RDX) and cyclote- Middle East Technical University
tramethylenetetranitramine (HMX). RDX was first discovered Ankara, Turkey

Propellants Explos. Pyrotech. 2013, 38, 651 – 657  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 651
Full Paper H. Turhan, T. Atalar, N. Erdem, C. zden, B. Din, N. Gl, E. Yildiz, L. Trker

duction of RDX. In these reactions hexamine is used in the of 10 K min 1. The DSC scans were carried out over the
form of the dinitrate salt, which is formed as a crystalline temperature range of 0–300 8C in a nitrogen purge. Differ-
salt on addition of 70 % HNO3 to saturated aqueous solu- ential Thermal Analysis data were obtained in the range of
tion of hexamine below 15 8C. The use of the dinitrate salt 10–300 8C, in a nitrogen atmosphere with a heating rate of
of hexamine in the KA process reduces the amount of 10 K min 1 with a SII TG/DTA 7300 EXSTAR instrument. X-
HNO3 required for the nitrolysis reaction. The reagents are ray diffraction spectroscopic analysis was performed with
employed in stoichiometric amounts with the exception of a Shimadzu XRD 6000 unit with a ceramic Cu-X-ray tube,
HNO3, which was used in slight excess. Since the nitrolysis graphite monochromator, computer controlled theta-com-
reaction is carried out in the presence of acetic anhydride, pensating slit. Scans were collected from 4 to 80 degrees 2-
which generates anhydrous conditions, concentrated HNO3 theta, 2 second counts at 0.020 degrees steps. The particle
(98–100 %) is used. But the stoichiometric amount of HNO3 morphology of the samples was examined by scanning
is less than 98 % during the nitrolysis reaction of HDN in electron microscopy with a JEOL JSM 6335F SEM instru-
the KA process. According to the stoictiometric results, per ment at an accelerating voltage of 20 kV (Oxford Instru-
mole of hexamine or HDN two moles of RDX are produced. ments 7260 EDS, INCA/ISIS software). HDN particles were
Additionally, HDN is often used in low temperature ni- mounted on metal stubs and coated with a thin layer of
trolysis reactions in order to avoid the initial exotherm ob- Au-Pd alloy (10 nm) in an argon atmosphere with
served on the addition of hexamine to nitric acid [9]. HDN a Quorum Polaron Coater.
shows weak explosive characteristics. Heating or any igni-
tion source causes violent deflagration. Its use in explosive
2.3 Synthesis of HDN
formulations is limited by its hygroscopic nature and be-
cause it easily decomposes in the presence of water. Its HDN was synthesized as described by Hale and Bachmann
main decomposition products are HNO3, hexamine, and [1, 5]. Hexamethylenetetramine (20 g) was transferred into
formaldehyde. HDN transforms into methylhexamethylene- a jacketed glass reactor connected to a water circulator,
mononitrate when the aqueous solution was boiled [8]. equipped with a dropping funnel, a mechanical stirrer,
Some properties of HDN are given in Ref. [9]. a pH electrode, and a thermometer. Distilled water (35 mL)
HDN is easily soluble in water, but insoluble in alcohol, was added to the reactor and the solution was stirred until
ether, chloroform, and acetone. hexamethylenetetramine completely dissolved at 2.0 8C.
In this paper, the additional applications of HDN in view The calculated amount of 65 % HNO3 was added dropwise
of its deflagration behavior and as an explosive are investi- to the solution and the temperature of the reaction mixture
gated. This includes semi-empirical quantum chemical cal- was not allowed to rise above 10.0 8C (exothermic reaction).
culations (semi-empirical) on hexamine and its nitrated de- The pH changes of mixture during the addition of HNO3
rivatives, the formation of HDN, the effect of excess were monitored by a pH meter. After the HNO3 addition
amount of HNO3 to the product yield and chemical charac- was finished, the reaction mixture was stirred for additional
terization of HDN and the thermal characterization and 15 min at 5.0 8C. The white precipitate was filtered, washed
aging conditions of HDN. several times by cold, water-free acetone, and dried in
a vacuum oven at 30.0 8C. The yield of the samples was in
the range of 86–98 % (Table 1). Because of the hygroscopic
2 Experimental Section nature of HDN, the product was stored in a desiccator. The
structural formula of HDN is shown in Scheme 1.
2.1 Materials
Hexamethylenetetramine was purchased from Sigma-Al-
Table 1. Effect of an excess amount of HNO3 on the reaction yield.
drich and dried before use. 65 % HNO3 was purchased from
Merck AG. Sample Stoichiometrya)/mol Yield/wt-% HNO3 content/wt-%
H1 1.0/2.0 85.67 47.10
H2 1.0/2.2 90.15 47.31
2.2 Characterization Methods H3 1.0/2.4 95.77 47.62
1
H NMR spectra were measured in DMSO-d6 with a Bruker H4 1.0/2.6 96.93 47.50
H5 1.0/2.8 97.80 47.52
Avance 500 MHz. FTIR spectra of the samples (KBr discs) H6 1.0/3.0 97.46 47.50
were measured with a Perkin-Elmer Frontier FTIR Spectro-
photometer. DSC analyses were performed with a Perkin- a) Mol hexamine/mol HNO3.
Elmer Jade DSC instrument. The heat flow and temperature
calibrations of DSC were performed using indium standard.
The experiments were carried out at a constant heating
rate and almost constant sample mass of 5  0.1 mg was
used. The samples for DSC analyses were accurately weigh-
ed in unsealed alumina pans and heated at a heating rate Scheme 1. Structural formula of HDN.

652 www.pep.wiley-vch.de  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Propellants Explos. Pyrotech. 2013, 38, 651 – 657
Hexamethylenetetramine Dinitrate (HDN): The Precursor for RDX Production by Bachmann Process

3 Results and Discussion of the nitrogen atoms seems to be difficult, actually the
proton(s) in those structures (HTN, HTetN) belong(s) to the
It is well-known that the molecular stabilities of com- nitro unit rather than forming quaternary trialkyl ammoni-
pounds can be evaluated and compared with atomic um (NHR3). These computational results are supported by
charges of corresponding molecules (Mulliken atomic the experimental findings. Based on the PM3 computed
charges, Natural Bond Orbital (NBO) analysis etc.). As an ex- Mulliken charges (seen in Figure 1), protonation of hexam-
ample the correlation between the nitro charge and molec- ine makes the ring more electropositive and enable a nucle-
ular stability is particularly useful for exploring the relation- ophilic attact to the ring so it can be easily opened (more
ship between Qnitro and the heat of detonation of nitro reactive). It is known from the literature that HDN is a reac-
compounds. However, the atomic charge is a defined quan- tive intermediate in the conversion of hexamine to RDX.
tity and not a physical observable. In our case, hexamine Additionally, Mulliken atomic charges reveal that mono-
(HX) and its mono- (HMN), di- (HDN), tri- (HTN) and tetrani- and di-protonation of hexamine by nitric acid are feasible
trate salts (HTetN) were subjected to semi-empirical quan- instead of tri- and tetra-protonation.
tum chemical calculations (PM3 method). The computed The dinitrate salt of hexamine was synthesized by reac-
geometry optimized structures of the considered molecules tion of hexamethylenetetramine with 65 % HNO3, at a tem-
are shown in Figure 1. perature not above 15.0 8C [3, 5]. Because of the exothermic
As can be seen from the PM3 computed structures, the reaction, the careful observation of the reaction tempera-
tri- and tetra-nitrated forms are different from mono- and ture is crucial for a high product yield. HDN is soluble in
di-nitrated hexamine. The third and fourth protonation (H + )

Figure 1. PM3 geometry optimized structures and Mulliken atomic charges of the considered species.

Propellants Explos. Pyrotech. 2013, 38, 651 – 657  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.pep.wiley-vch.de 653
Full Paper H. Turhan, T. Atalar, N. Erdem, C. zden, B. Din, N. Gl, E. Yildiz, L. Trker

Figure 2. Change in pH of the reaction mixture during the addi-

tion of HNO3.
Figure 3. Comparison of the FTIR spectra of crude HDN and hex-
amine.
water but the solution decomposes on standing and form-
aldehyde is formed.
During the first additions of 65 % HNO3 to the concen- stretching vibration), and 1312–1384 cm 1 (NO2 stretching)
trated hexamine/water solution, the reaction mixture was have differentiated the HDN formation from hexamethyle-
clear, the product easily dissolved and no precipitation oc- netetramine.
curred. The product started to precipitate when the pH The 1H NMR spectrum of crude HDN is shown in
value of the reaction mixture sharply decreased and Figure 4. The results also confirmed the purity of HDN (d =
reached 1.83. The pH changes of reaction mixture during 4.8 ppm (s, 12 H, CH2), d = 8.7 ppm (br. s, protonated
the addition of HNO3 were monitored by a pH meter. The amines).
pH vs. HNO3 addition interval graph is shown in Figure 2. For LC/MS analysis, the HPLC system was coupled with
The effect of an excess amount of HNO3 on the reaction an Agilent Technologies 6410 Triple Quad LC/MS mass
yield was investigated. The amounts of ingredients and the spectrometer. HDN was analyzed on a Zorbax SB-C18 4.6 
reaction yields are given in Table 1. The excess amount of 250 mm 5 mm column with a mobile phase consisting of
HNO3 was increased from 10 to 40 wt-% and the concentra- 43 % of a 1 mM acetic acid solution and 57 % of methanol
tion of hexamethylenetetramine in water was kept constant at a flow rate of 0.6 mL min 1. The column temperature
at 45 wt-%, near to saturation at 5.0 8C. As shown in was not controlled and the injection volume was 20 mL.
Table 1, the reaction yield increases with increasing the The MS system was equipped with an electrospray ioniza-
excess amount of HNO3 and the highest yield was obtained tion source.
in the case of 40 wt-% excess. Further increasing the HNO3 Nitrogen was used both as drying gas at 320 8C (at the
amount (more than 40 wt-%) had no positive effect on the flow of 10 L min 1) and nebulizer gas at 2.41  105 Pa. The
HDN yield. capillary voltage was 4000 V and the fragmentor voltage
HDN precipitated in the form of a white powder that was 135 V. Data was acquired at negative ion mode.
was separated from the reaction mixture, washed several The LC-MS chromatogram of HDN eluted at 4.75 min is
times with water-free acetone (previously cooled to 0 8C) shown in Figure 5. HDN (C6H14N6O6) has a molecular weight
and dried in a vacuum oven at 30.0 8C.
The nitric acid contents of the crude samples were deter-
mined by titrating water solutions (0.1 g per 100 mL) with
0.1 n NaOH standard solutions to predict the degree of
protonation. The acidity of the solutions was calculated on
a dry basis to percent nitric acid. The results are given in
Table 1. According to the titration results, all crude samples
were found to contain 47.1–47.6 % of HNO3. The HNO3 con-
tent of the HDN samples is closely related to the amount
of HNO3 required for the compound, which was synthe-
sized by using 1 mol of hexamine and 2 mol of HNO3
(47.34 % HNO3 as stoichiometric) [3].
The chemical composition of HDN was characterized by
FTIR spectroscopy. Figure 3 shows a comparison of the FTIR
spectra of crude HDN and hexamethylenetetramine. The
main characteristic transmittance bands at 642 cm 1 (NO2
deformation), 860 cm 1 (NO stretching), 1274 cm 1 (O NO2 Figure 4. 1H NMR spectrum of HDN.

654 www.pep.wiley-vch.de  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Propellants Explos. Pyrotech. 2013, 38, 651 – 657
Hexamethylenetetramine Dinitrate (HDN): The Precursor for RDX Production by Bachmann Process

Figure 5. LC-MS chromatogram of HDN.

Figure 7. DSC curves of unaged and aged samples.

Figure 6. LC-MS spectrum of HDN.

of 266 g mol 1. In the LC-MS spectrum (Figure 6), the peaks

[M + 75] , [M + 121] , and [M + 151] are attributed to
[HDN + NO2 + CH3O 2H] , [HDN + 2NO2 + CH3O 2H] ,
[2HDN + 2NO2 + 2CH3O 3H] , respectively.
DSC and TG/DTA measurements were used to examine
the thermal properties of HDN. Three sets of measurements Figure 8. TG/DTA thermograms of HDN and hexamine.
were performed to investigate the thermal aging behavior
of HDN.
Aging measurements were carried out by heating the No peak representing hexamine could be observed in
sample in a hermetic aluminum pan starting from 10 to 50 the DSC curves of the aged and unaged samples. It has to
to 100 to 150 8C at 10 K min 1; the experiment was held be noted that hexamine does not melt. It sublimes in
isothermally at 50–100–150 8C for 1 h, afterwards cooled vacuo at 230–270 8C and decomposition begins at tempera-
down to 10 8C at 100 K min 1, held at 10 8C for 5 min, and tures above 270 8C. The overlaid TG/DTA thermograms of
reheated to 300 8C at 10 K min 1 by DSC. The overlay DSC unaged HDN and hexamine are shown in Figure 8. The TG
curve of the unaged and aged samples at 50–100–150 8C is curve of hexamine exhibits a single step mass loss, but in
shown in Figure 7. In the DSC curve of the unaged sample, the TG curve of HDN an additional weight loss step at
one endothermic peak is observed at 170 8C, which corre- 165 8C, according to the thermal decomposition tempera-
sponds to the melting temperature of HDN. The experi- ture of HDN could be observed. The broad endotherm in
mentally determined melting point of the sample (165 8C) the DTA curve of hexamine (dotted green line) stands for
is in good agreement. At further increasing the tempera- the sublimation of hexamine.
ture two exothermic peaks were observed, one at 174 8C The color difference of the unaged and the samples
and a sharp peak at 200 8C, which represents the thermal aged at 170 8C is shown in Figure 9. The unaged HDN sam-
decomposition of HDN. In the DSC curves of the samples ples in sealed vials were heated at 50, 100, 150, and 170 8C
aged at 50 8C and 100 8C for 1 h, no detectable differences for 1 h and the aged samples were characterized by FTIR
in thermal transitions at 170 8C (endotherm) and 174 8C spectroscopy. The main characteristic peaks of HDN in the
(small exotherm) were observed, but the peak temperature FTIR spectrum of the sample aged at 50 8C confirmed that
of the exothermic peak shifted upwards and a new broad no structural degradation occurred at 50 8C. But when the
exothermic peak at 215 8C was observed. In the DSC curve samples were heated to 100 and 150 8C, significant spectral
of the sample aged at 150 8C, one broad endothermic peak differences were observed. The overlaid FTIR spectra of
with a shoulder at 205 8C and a sharp transition at 214 8C unaged HDN and the samples aged at 170 8C are shown in
was observed. Figure 10.

Propellants Explos. Pyrotech. 2013, 38, 651 – 657  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.pep.wiley-vch.de 655
Full Paper H. Turhan, T. Atalar, N. Erdem, C. zden, B. Din, N. Gl, E. Yildiz, L. Trker

Figure 9. Color difference of unaged and aged HDN.

Figure 10. Overlaid spectra of aged and unaged samples.

The FTIR spectrum of the HDN sample aged at 170 8C Figure 11. SEM images of crude HDN particles.
displayed the characteristic NO3 (asym.) stretching vibra-
tion band at 1361 cm 1, the NO3 out of plane deformation
at 825 cm 1, the NO3 in-plane deformation of nitrate salts
at 1763 cm 1. After the aging period, the sharp absorption use it for the production of HMX as well were made. The
band in the spectrum of unaged HDN at 3034 cm 1 disap- heat of explosion and detonation velocity values are not in
peared and a new sharp absorption band at 1674 cm 1 agreement with the values of nitramine based explosives
emerged. but they are comparable with those of ammonium nitrate
The particle surface of the crude HDN was observed by (heat of explosion: 1592 kJ kg 1, as calculated [10]), the
scanning electron microscopy. main component of ANFO.
Figure 11 shows HDN crystals with different magnifica- HDN was synthesized by protonation of hexamine with
tions. As shown in the SEM images, the HDN particles have 65 % HNO3, at 3 8C. The simultaneous detection of
regular shapes with smooth edges and flat crystal faces. changes in pH during the protonation and the optimum
The crude product has different particle sizes, one of them amount of HNO3 for highest product yield was investigat-
was measured as 45  40 mm. ed. According to theoretical calculations, only mono- and
di-protonation of hexamine by HNO3 are feasible. Charac-
terization by 1H NMR and FTIR spectroscopy confirmed the
structure and purity of HDN.
4 Conclusion
The thermal characteristics of HDN are different than
HDN, the dinitrate salt of hexamine, is mainly a precursor those of hexamine. The DSC thermograms of the samples
for the production of RDX, although several attempts to aged at 50, 100, and 150 8C revealed that the storage con-

656 www.pep.wiley-vch.de  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Propellants Explos. Pyrotech. 2013, 38, 651 – 657
Hexamethylenetetramine Dinitrate (HDN): The Precursor for RDX Production by Bachmann Process

ditions and application temperatures of HDN limit its use in [5] W. E. Bachmann, J. C. Sheehan, A New Method of Preparing
explosive formulations. the High Explosive RDX, J. Am. Chem. Soc. 1949, 71, 1842–
1845.
[6] W. E. Bachmann, W. J. Horton, E. L. Jenner, N. W. MacNaughton,
Acknowledgments L. B. Scott, Cyclic and Linear Nitramines Formed by Nitrolysis
of Hexamine, J. Am. Chem. Soc. 1951, 73, 2769 – 2773.
[7] V. Gilpin, C. A. Winkler, Studies of RDX and Related Com-
The Authors thank Zekayi Korlu, Mustafa Candemir, Nevin
pounds VIII. Thermochemistry of RDX Reactions, Can. J. Chem.
Bekir for their technical assistance. This work was per- 1952, 30, 743–748.
formed with the financial support of the Scientific and [8] K. W. Dunning, W. J. Dunning, Methylenenitramines. Part I. The
Technological Research Council of Turkey. Reaction of Hexamine Dinitrate with Nitric Acid at Low Tem-
peratures, J. Chem. Soc. 1950, 2920–2924.
[9] R. Meyer, J. Kçhler, A. Homburg, Explosives, Wiley VCH, Wein-
heim, Germany, 5th. ed. 2002, p. 169.
References [10] D. Buczkowski, B. Zygmunt, Detonation Properties of Mixtures
[1] J. P. Agrawal, R. D. Hodgson, Organic Chemistry of Explosives, of Ammonium Nitrate Based Fertilizers and Fuels, Cent. Eur. J.
John Wiley & Sons, New York, NY, USA, 2007, pp. 243 – 244. Energ. Mater. 2011, 8, 99 – 106.
[2] T. Urbanski, Chemistry and Technology of Explosives, Vol. 3, Per-
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