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Cyclodextrin Nitrate Ester/H2SO4 as a Novel Nitrating System for Efficient Synthesis of Insensitive High Explosive 3-Nitro-1,2,4-triazol-5-one Mukesh B Deshmukh, Nilesh D Wagh, Arun K Sikder, Amulrao U. Borse, and Dipak S Dalal Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on November 26, 2014

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Industrial & Engineering Chemistry Research

Cyclodextrin Nitrate Ester/H2SO4 as a Novel Nitrating System for Efficient Synthesis of Insensitive High Explosive 3-Nitro-1,2,4-triazol-5-one Mukesh B. Deshmukh,† Nilesh D. Wagh,‡ Arun K. Sikder,§ Amulrao U. Borse,† and Dipak S. Dalal†* †

Department of Organic Chemistry, School of Chemical Sciences, North Maharashtra

University, Jalgaon - 425 001, India ‡

School of Environment and Earth Sciences, North Maharashtra University, Jalgaon -

425 001, India §

High Energy Materials Research Laboratory (HEMRL), Pune - 411 021, India

ABSTRACT: A highly efficient and novel protocol for the synthesis of 3-Nitro-1,2,4-triazol-5one (NTO) by using Cyclodextrin Nitrate Ester (CDN) as a solid and safer catalyst is described. 1. INTRODUCTION Propellants, explosives and pyrotechnics are the essential energy producing materials for space and weapon applications. Research on Insensitive high explosive is becoming prominent worldwide as these are safe to handle, less sensitive to shock, friction and percussion as compared to the conventional explosives such as RDX, HMX, PETN etc. PETN when used as a secondary explosive shows high sensitivities while RDX has been identified as toxic and probably carcinogenic.1-2 The conquer explosive is that which has good chemical and thermal stability, less sensitivity along with to achieve its high performance. The term “Insensitive Munition (IM)” is used to illustrate armaments, which are particularly harmless and are difficult

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to initiate accidentally, but at the same time have the power and reliability to fulfill the requirements to complete the mission.3 3-Nitro-1,2,4-triazol-5-one (NTO) is one of the significant insensitive high explosives (IHEs) and gaining interest in the field of high energy materials (HEMs). It has high velocity of detonation which is comparable with RDX, less sensitivity to shock, friction and percussion than RDX. In addition, NTO has more thermal stability and high energy release on decomposition. NTO possesses excellent physical properties, together with a desired combination of high energy when detonated and relatively low sensitivity to unintentional detonation, forming

it a modern candidate of choice for propellant and

explosives applications. Also NTO has many applications, such as in melt-castable, generalpurpose, insensitive high-explosive (IHE) formulations and plastic-bonded explosives. Furthermore, NTO can easily be pressed without a binder into desired shapes having a high density. It possesses acidic nature pKa 3.7 due to the labile N-H bond and forms stable salts with many metals, some of the heavy metal salts are primary explosives.4-6 NTO as a high explosive material was first published by Lee et al.7-8 According to Lee et al the intermediate 1,2,4-triazol-5-one (TO), was prepared by condensing semicabazide hydrochloride and formic acid then nitration of TO with 70% nitric acid to get 3-Nitro-1,2,4triazol-5-one (NTO) in nearly 80% yield. Chipen et al9 reported the nitration of TO with fuming nitric acid/water with yields of NTO up to 67%. Kroger et al10 reported nitration of TO by fuming nitric acid with initial cooling with yields of NTO to 70-75%. Also Katritzky et al11 reported that using conventional nitration i.e. nitric acid/Sulfuric acid, nitration of TO yields only 32% NTO. The comparison of yields in literature methods is shown in Table 1. Since afterwards, the R&D work on NTO has been continuously pursued with great attention in process development and scale up to pilot plant scale.12-14


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Several nitrate esters including propyl nitrate, butyl nitrate, octyl nitrate, decyl nitrate, 2-octyl nitrate, 1,5-pentamethylene dinitrate, 2-chloroethyl nitrate and t-amyl nitrate have been reported for their conversion into respective nitro alkanes by reaction with sodium nitrite and ethyl malonate in suitable solvents (DMSO/hexamethyphosphoramide) with optimum conversions.15 The nitration of aliphatic esters and phenyl acetate with ethyl nitrate was reported in the presence of potassium amide in liquid ammonia at -33 °C which gives α-nitro esters as well as nitroalkanes and dialkyl carbonates.16 Decyl nitrate and n-amyl nitrate were also known for the nitration of 3,4-xylenol to give 6-nitro-3,4-xylenol which was used as a reagent for the colorimetric determination of the nitrate ion.17 The guanidine nitrate, nitroguanidine as well as the simple nitrate ester, ethylene glycol dinitrate (EGDN) were accounted for the nitration of deactivated arenes.18 Also under different reaction conditions, mono nitration of phenol was investigated using isopropyl nitrate as a nitrating agent over various zeolite catalysts.19 Our curiosity to develop a novel system for the synthesis of high energy materials encouraged us to employ Cyclodextrin Nitrate Ester (CDN) along with Sulfuric acid which acts as a nitrating agent. As per literature report, no one was used CDN as a nitrating agent and we are the first to explore it’s utility as a novel nitrating agent to synthesize 3-Nitro-1,2,4-triazol-5-one (NTO). CDN has good thermal stability, solid in nature, safe to handle and good solubility in organic solvents which makes it synthetically more useful.

O

O HN

NH N

CDN/H2SO4/H2O

HN

60-65°C, 3h

NH N

O2N

H TO

NTO

Scheme 1. Synthesis of 3-Nitro-1,2,4-triazol-5-one (NTO)


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Table 1. Comparison of previous methods and present work Entry

Reagents

Yield (%)

1

Nitric acid/Sulfuric acid (conventional)11

32

2

Fuming nitric acid/water9

67

3

Fuming nitric acid (with initial cooling)10

70-75

4

Nitric acid (70%)6-8

80

5

Present work

88

2. MATERIALS AND METHODS 2.1 Materials All the reagents and chemicals used in the present study were of AR grade and used as such. TLC experiments were performed on pre-coated silica gel plates using Hexane:Ethyl acetate (1:9) as the eluent. The uncorrected melting points were reported using open capillary method. The IR spectra were recorded on Shimadzu FTIR Affinity-1 spectrophotometer in KBr matrix. 1H NMR spectra were recorded on 400 MHz Bruker Avance II Spectrometer instrument with TMS as an internal standard. 13C NMR spectra were recorded on 100 MHz Bruker Avance II Spectrometer instrument. Mass spectra were recorded on Waters Q-TOF Micromass instrument. TGA and DSC studies were undertaken on a PerkinElmer 4000 Thermogravimetric Analyzer instrument and PerkinElmer 4000 Differential Scanning Calorimeter instrument respectively, operating at 10 °C/min heating rate in nitrogen atmosphere with 5 mg of sample. UV spectra were recorded on Shimadzu UV 2450 spectrophotometer instrument using acetonitrile as a solvent.


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2.2 Synthesis of CDN20 4.5 g of β-CD was added slowly to 29 g of fuming nitric acid over a period of 15-20 min at 10-20 °C and then continued to stir for 1 h. Finally, cool it to 0 °C and poured into 200 mL of ice-water mixture. The solid product was filtered out, washed with water until the pH of washed water was nearly 7, dried and then weighed (6.8 g). The Yield was 88%.

2.3 Synthesis of NTO Synthesis of 3-Nitro-1,2,4-triazol-5-one (NTO) was carried out in two steps as 1) Condensation of semicarbazide hydrochloride with formic acid to get 1,2,4-triazol-5-one (TO)7 and 2) Reaction of 1,2,4-triazol-5-one (TO) with CDN and Sulfuric acid in presence of water to obtain 3-Nitro-1,2,4-triazol-5-one (NTO) (Scheme 1).

2.3.1 Synthesis of 1,2,4-triazol-5-one (TO) Semicarbazide hydrochloride (15 g, 0.134 mol) and formic acid (15 mL, 88%) were charged in a 50 mL round bottom flask. The mixture was heated with stirring up to 92-95 °C for 3 h. Here semicarbazide hydrochloride gets gradually dissolved and the precipitate of product was observed in the flask. The excess of formic acid was removed by distillation under reduced pressure until crystallization occurred. 34 mL water was then added and distillation continued to dryness. The contents were filtered and recrystallized from ethanol to get 8.062 g of pure 1,2,4triazol-5-one (TO). The yield was 70 %, mp 234 °C (lit.6,23 mp 232 °C, lit.24 mp 234 °C). 1

H NMR (400 MHz, DMSO-d6): δ = 7.69 (s, 1 H), 11.34 (s, 1 H, NH), 11.46 (s, 1 H, NH) ppm.

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C NMR (100 MHz, DMSO-d6 + CDCl3): δ = 135.91, 155.78 (Carbonyl) ppm.


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IR (ν, cm-1): 1690 (C=O), 3065 (NH), 2833 (CH). UV/VIS: λmax : no remarkable wavelength (λ) maximum. MS (70 eV, 150 °C): m/z = 86 (M+1).

2.3.2 Synthesis of 3-Nitro-1,2,4-triazol-5-one (NTO) CDN (1.945 g, 1 mmol) was charged in a 25 mL round bottom flask and concentrated Sulfuric acid (3 mL, 96%) was added under cooling. The mixture was stirred at room temperature for 1h. Then 1 mL of water was added slowly dropwise under cooling followed by the addition of 0.170 g (2 mmol) of TO and stirred the reaction mixture at 60-65 °C for 2 h. The reaction was slight exothermic and brown fumes evolved. Cool it to room temperature and poured over 15-20 mL of crushed ice-water mixture. Extracted with 3 x 25 mL of ethyl acetate, the combined ethyl acetate layer was dried over sodium sulphate and then distilled out under reduced pressure to get solid 3-Nitro-1,2,4-triazol-5-one (NTO). The obtained NTO was purified with column chromatography using Hexane:Ethyl acetate (8:2) as eluent. The yield was 88 %. 1

H NMR (400 MHz, DMSO-d6): δ = 12.80 (s, 1 H, NH), 13.16 (s, 1 H, NH).

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C NMR (100

MHz, DMSO-d6 + CDCl3): δ = 147.59, 154.11 (Carbonyl) ppm. IR (ν, cm-1): 1709 (C=O), 3204 (NH), 1543 (asym N-O), 1354 (sym N-O). UV/VIS: λmax : 325 nm. MS (70 eV, 150°C): m/z = 129 (M-1).

3. RESULTS AND DISCUSSION Herein, we are reporting an efficient method for the conversion of 1,2,4-triazol-5-one (TO) to 3-Nitro-1,2,4-triazol-5-one (NTO) by employing Cyclodextrin Nitrate Ester (CDN) along with Sulfuric acid as a novel nitrating system. CDN is solid and safer in handling than nitric acid for the synthesis of NTO. The benefit being a solid is its manual carrying and handling


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otherwise in case of fuming nitric acid, the handling is hazardous. Thus CDN offers a safer strategy than using fuming and concentrated nitric acid which is to be combined with Sulfuric acid during conventional nitration reaction. We optimized the parameters by examining the reaction which involves TO, Sulfuric acid and water to afford the appropriate product NTO (Scheme 1). In our experiments, the nitronium ion species get generated by using CDN and Sulfuric acid. For this purpose, we did experiments with 1 mL, 2 mL and 3 mL of Sulfuric acid to get complete dissolution of CDN and maximum amount of nitronium ion species. We observed that the 3 mL of Sulfuric acid was the optimum amount for dissolution of 1 mmol of CDN for synthesis of NTO. To gain optimization of reaction conditions, we screened the amounts of CDN for their ability to catalyze the reactions (Table 2). For this purpose, we performed the reactions to find out the best amount of catalyst by varying CDN amount in constant reaction conditions. Table 2. Synthesis of NTO catalyzed by different amounts of CDNa

a

Entry

mmol of TO

mmol of CDN

Yieldb (%)

1

2

1

88

2

2

0.8

70

3

2

0.6

68

4

2

0.4

66

5

2

0.2

53

Reaction conditions: TO (2 mmol), Sulfuric acid (3 mL), water (1 mL), temp. (60-65 °C), reaction time (3 h). bIsolated yield. We have checked the role of solvents by using the same ratio of reactants. We noted that

there was no reaction in EtOH, EtOAc, Acetone, CH2Cl2, CHCl3, DMF and DMSO evenafter 5 h


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while in Acetic acid, mixture of products were obtained. Thus we performed the reaction in absence of organic solvent using Sulfuric acid and obtained higher yield of 88% with shorter reaction time of 3h. The effect of temperature on reaction which reveals that the optimum temperature for the reaction was 60-65 °C. At lower temperature, the reaction was not proceeded while at higher temperature, the reaction mass becomes sluggish as listed in Table 3.

Table 3. Effect of temperature for synthesis of NTO Entry

Temperature (°C)

Time (h)

Yield (%)

1

0-5

5

N. R.a

2

10-15

5

N. R.a

3

25-30

5

N. R.a

4

40-45

5

50

5

60-65

3

88

6

75-80

5

Sluggish

a

No Reaction

Our experimental investigation demonstrate that using 0.5 equivalent of CDN (1 mmol), 3 mL of sulfuric acid and 1 mL of water is the best ratio for giving the maximum yield. Also as shown in Table 1, the yields reported in literature were less whereas in present work the yield is optimum. Here more advantageous fact is that for conversion of TO to NTO, less molar ratio i.e. 0.5 equivalent (1 mmol) of CDN was required than reported in literature6-8.


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3.1 UV Analysis The UV spectrum of TO and NTO recorded on Shimadzu UV 2450 spectrophotometer instrument using acetonitrile as a solvent demonstrates that TO has no remarkable wavelength (λ) maximum while NTO shows wavelength (λ) maximum at 325 nm. The comparison between the graphs of TO and NTO is shown in Figure 1 which are the representative and characteristics of TO and NTO.11 These graphs supports the conversion of 1,2,4-triazol-5-one (TO) to 3-Nitro1,2,4-triazol-5-one (NTO).

Figure 1: UV spectrum of TO and NTO a: NTO, b: TO 3.2 Thermogravimetric Analysis The TGA of TO undertaken on a Perkin Elmer 4000 Thermo Gravimetric Analysis instrument operating at 10 °C/min heating rate in nitrogen atmosphere with 5 mg of sample


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shows the 98% weight loss in the temperature range from 200 °C to 318 °C. The TGA of NTO demonstrate the 75% weight loss in the temperature range from 157 °C to 314 °C is in good agreement with the data reported in literature.14,21 The comparison between the TGA of TO and NTO is shown in Figure 2.

Figure 2: TGA of TO and NTO a: NTO, b: TO

3.3 Differential Scanning Calorimetry The DSC study of TO undertaken on Perkin Elmer 4000 Differential Scanning Calorimeter instrument, operating at 10 °C/min heating rate in nitrogen atmosphere with 5 mg of sample shows exothermic peak which starts at 230 °C and sharp peak maximum at 239 °C. The DSC of NTO demonstrates one distinct transition with an onset temperature of 245 °C and a peak melting temperature of 272 °C. They are in good agreement with literature values.21-23 The comparison between DSC of TO and NTO is depicted in Figure 3.


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Figure 3: DSC of TO and NTO

3.4 Elemental Analysis Elemental Analysis performed on Flash EA 1112 Series instrument shows C, 18.38; H, 1.51 and N, 43.21 whereas theoretical values are C, 18.46; H, 1.54 and N, 43.08. It is observed that experimental values are in good agreement with stoichiometric percent composition. The Elemental analysis report is shown in Figure 4.


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Figure 4: Elemental Analysis of NTO


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3.5 Plausible Mechanism β-Cyclodextrin (β-CD) is a cyclic oligomer of D-glucose having heptamar glucose unit25 possessing toroidal cyclic structure with secondary hydroxyl groups at C-2 and C-3 on their more open face and the primary C-6 hydroxyl group on the opposite secondary face.26 β-CD has the ability to bind with organic molecule in hydrophobic central cavity by supramolecular interactions like van der Waals' interaction, hydrophobic interactions, hydrogen bonding and electrostatic interactions between charged part of guest molecule and beta cyclodextrin.27 A series of organic reactions catalyzed by β-CD has been reported that activates the substrate by supramolecular interactions.28

H

H2SO4

CDN

O

NO2 O2N

O H

H2O

O2N O H

O NO2 H

TO

H

O

NO2 O2N H N

O

Work up in Water HN

NH N

O2N

NTO

N N

Partially Nitrated CD O2N O H

O H

O

H

O NO2 H

Scheme 2. Plausible mechanistic pathway for synthesis of NTO


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The plausible mechanistic pathway of NTO synthesis by using Cyclodextrin Nitrate Ester and Sulfuric acid is shown in Scheme 2. First the protonation of CDN takes place in presence of Sulfuric acid and forms nitronium ion species in situ which are being attacked by the olefinic carbon of the TO to produce NTO. Here TO may get activated with microenvironment of CDN cavity by the formation of hydrogen bonding with carbonyl group and free N-H group followed by electrophilic attack of nitronium ion on TO to give NTO. It contributes to the high yield of NTO in which CDN not only acts as a novel nitronium ion source but also the catalyst for the synthesis of NTO.

4. CONCLUSIONS We have developed a novel protocol for the synthesis of 3-Nitro-1,2,4-triazol-5-one (NTO) by using Cyclodextrin Nitrate Ester (CDN). CDN has good thermal stability, solid in nature, safe to handle, and good solubility in organic solvents which makes it synthetically more useful. Also CDN has far better nitration ability than other nitrating agents. From all these properties coupled, CDN is a good candidate for the synthesis of NTO from TO and would also be useful for the synthesis of other explosives in future. We found that this method was very efficient and the NTO was prepared in better yields than other methods.

ASSOCIATED CONTENT Supporting Information The IR, 1H NMR, 13C NMR and Mass spectra of TO and NTO. Also Characterization of CD and CDN used in this study. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-2572257432; Fax: +91-2572258406. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Author thanks the Ministry of Defence, ARMREB, DRDO, New Delhi for the financial support (ARMREB/HEM/2011/136) and Sophisticated Analytical Instrumentation Facility, Panjab University, Chandigarh for providing the spectral characterization.

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