Kinetics of the Synthesis of NTO in Nitric Acid

Vitold L. Zbarsky*, Nikolaj V. Yudin

Mendeleev University of Chemical Technology, 9 Miusskaya Square, 125047, Moscow (Russia)

Abstract

The nitration kinetics of 1,2,4-triazol-5-one in 70 – 100% nitricacid were investigated. Formation of N-nitro-1,2,4-triazol-5-oneunder these conditions was observed, and its influence on the totalprocess was studied. The activation energies of formation anddecomposition of N-nitro-1,2,4-triazol-5-one and of the nitrationof 1,2,4-triazol-5-one were determined.

Keywords: 1,2,4-Triazol-5-one, Nitration Kinetics, N-nitro-1,2,4-Triazol-5-one

1 Introduction

Among explosives of the last generation, a special place isreserved for 3-nitro-2,4-dihydro-3H-1,2,4-triazol-5-one(NTO). In spite of its significant drawbacks, such as itshigh acidity (pKa¼ 3.67) and low heat of explosion (Q¼4100 kJ/mole), it is widely used due to a low sensitivity toshock waves and ease of manufacture.NTO can be obtained easily by nitration of triazol-3-one

(TO) in dilute and concentrated nitric acid [1], or in amixture of sulfuric and nitric acid with a low percentage ofsulfuric acid [2].However, the synthesis ismore complicatedthan it appears at a first glance; in particular, there is adiscrepancy between the reaction times for completereaction in 65% and 98% HNO3. The NTO synthesisprocess in concentrated nitric acid [3] exhibits only a weakdependence on such important parameters as the ratio ofreactants (HNO3 and TO), the temperature, the degree ofdilution of the spent acid, and someother factors. In order toexplain these features of the process and to estimate theprospects of its further improvement, a detailed study on thekinetic process was performed. An analysis of the kineticsand themechanism of nitration of TO in 70 – 100%HNO3 ispresented here.

2 Experimental

The solubility of NTO in nitric acid was determined bytwo methods:

1) A suspension of NTO in nitric acid was heated slowly, atapproximately 0.5 – 1 K min�1. The temperature atwhich the last crystal disappeared was noted.

2) A solution of NTO in nitric acid was slowly cooled, atapproximately 0.5 – 1 K min�1. The temperature atwhich first crystal appeared was noted.

The experiments were performed in a heated 150 mLvessel connected to an ultra-thermostat to maintain aconstant temperature (�0.1 K). Three replicate measure-ments were taken.Studies of the kinetics of NTO nitration in 72 – 100%

HNO3 were performed in homogeneous systems with asignificant excess of HNO3, using a Specord M40 spectro-photometer. The reaction was stopped by diluting thereaction mixture to ~1.5% HNO3. After the reaction wasquenched, the spectrum from 250 to 400 nm was measured.

3 Results and Discussion

Our preliminary experiments on nitration of TO inconcentrated HNO3 showed that although the yield ofNTO did not depend on temperature in the range 0 – 30 8C,the reaction time significantly decreased with increasingtemperature. At these temperatures, the reaction time wasindependent of both the initial acid concentration in therange from 90 to 98%HNO3 and the ratio ofHNO3 to TO inthe range of 4 – 8 moles HNO3 per mole TO. Surprisingly, itwas found that the yield dropped significantly in 98%HNO3

if the HNO3 to TO ratio was greater than 10.Since the decrease in the yield could have been a result of

increased losses due to solubility of NTO in the dilute spentacid, we determined the dependence of its solubility in nitricacid on nitric acid concentration and on temperature (Fig. 1,Table 1).To determine the enthalpy of dissolution of NTO during

heating (dissolution) and cooling (crystallization) we haveused the Shreader equation [4]:

lnM¼A�DsolH/T (1)

whereM is themole fraction ofNTO in the solution, andT isthe temperature in K.The solubility of NTO in nitric acid at different temper-

atures, calculated from the obtained dependences, is pre-sented in Figure 2.* Corresponding author; e-mail: zbarsky@rctu.ru

298 Propellants, Explosives, Pyrotechnics 30 (2005), No. 4

D 2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/prep.200500020

The results demonstrate that the solubility of NTO in 30 –70%HNO is nearly constant. This fact can explain the weakinfluence of dilution of the reaction mixture on the yield ofNTO, but does not explain why the yield in 98%HNO3 fallsat high reactant ratio.The spectrophotometric studies of nitration kinetics in

70 – 100% nitric acid were performed in homogeneoussystems with a significant excess of HNO3. The results arewell described by pseudo-first order reaction kinetics. Atconcentrations of HNO3 exceeding 77%, a product with anabsorption maximum around 276 nm is observed in thereaction mixture in the very first minutes of the reaction.This absorption peak belongs to the previously unknownN-nitro-1,2,4-triazol-5-one (N-NTO) [5]. For concentrationsof HNO3 in the 96 – 100% range, there is a sharp decrease inthe NTO yield. These observations confirm the complexcharacter of NTO nitration.In fact, at the beginning of the process, one observes the

sharp peak of N-NTO, which is stronger at higher concen-trations of HNO3. Only insignificant absorption of NTO is

detected at this time. After a while, however, the absorptionthat is characteristic of N-NTO disappears (see Fig. 3). Thebehavior of N-NTO in solution over a wide range of acidityhas been studied (see Table 2), and it has been determinedthat the de-nitration reactionwithTO formation takes placewhen H0<�1.When H0> 1, a total destruction of the triazole ring is

observed accompanied by the formation of gaseous prod-ucts containing carbon dioxide, nitrogen and nitrogen oxide.TO is virtually absent in the products of hydrolysis. In 99%trifluoroacetic acid and 94% sulfuric acid, N-NTO disap-pears immediately after mixing, and then extremely slowformation of NTO takes place. The intramolecular rear-rangement has not been observed.The results of parallel studies of TO and N-NTO trans-

formation into NTO are shown in Fig. 4.N-NTO concentrations for both processes at equal

reaction times are almost identical, and they are determinedby the concentration of HNO3 only. This fact confirms thatthe rate of equilibration between TOandN-NTO is high. In

Figure 1. NTO solubility in nitric acid in 0 – 80% HNO3

concentration range. – NTO in 33.3% nitric acid ; ^ – NTO in51.8% nitric acid; * – NTO in 61.0% nitric acid; ~ – NTO in69.1% nitric acid; * – NTO in 80.5% nitric acid; ^ – NTO in water

Table 1. Parameters of ShreaderKs equation for solubility of NTO in water and water-nitric acid mixtures

Concentration of HNO3, weight % Cooling (crystallization) Heating (dissolution)

A B DsolH, kJ/mol A B DsolH, kJ/mol

0 �3373.3 5.1314 28 �2770.2 3.6608 2333.3 �2875.0 3.6852 24 �1941.8 1.1910 1651.8 �3001.1 4.1735 25 �2002.3 1.4753 1761.2 �2792.6 3.7505 24.5 �1936.5 1.4218 1669.1 �2525.4 3.0380 21 �1841.4 1.2313 1575 �3167.9 5.3939 26.5 �1234.3 �0.2941 1080 �2089.2 2.3930 17 �1722.0 1.6875 14

Figure 2. Solubility of NTO in nitric acid at different temper-atures. ~ – 0 8C; * – 40 8C; * – 70 8C

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addition, the data on NTO yield at the end of the reactionare presented in this figure. These data confirm theconclusion about the identity of the processes being studied.One can clearly see that the rate constant of NTO formationin the concentration interval of nitric acid from 85 – 100% isvery steady and stable (Fig. 5). This explains our results andthe results obtained earlier by Becuwe [3] concerning thelack of correlation between reaction time and HNO3

concentration in preparative tests under similar conditions.It can be asserted that N-NTO is not an intermediate

product, and that NTO formation occurs as a result of N-NTOdenitration, followed by nitration of the newly formedTO. Based on these data (including a decrease in NTO yieldin 98 – 100%HNO3), we suggest the following Scheme 1 forthe TO nitration process.The equilibrium constant K2 and rate constants k3 and k4

were calculatedusing the traditionalmethod for systemswith

reactions taking place in parallel with concomitant equili-brium [6]. The equilibrium constant K1 is equal to 3.98 [7],where I1 and I2 are ratios of free and charged forms of TO inreactions I and II;A is a function of theHNO3 concentration.

K1 ¼TOHþ½ �

TO½ � � Hþ½ � ; I1 ¼TOHþ½ �TO½ � ð2Þ

K2 ¼N�NTO½ � � Hþ½ �TO½ � � NOþ

2

� � ; I2 ¼N�NTO½ �

TO½ � ð3Þ

log I2 ¼ pK2 þ lgNOþ

2

� �Hþ½ � ¼ pK1 þA; ð4Þ

To calculate the value of I2weuse the formula formaterialbalance at the initial moment of reaction.

Table 2. Dependence of the rate constant for N-NTO decomposi-tion on acidity.

Acid H0a pH k1, min�1

58.1% H2SO4 �4.02 0.008244.9% H2SO4 �2.86 0.0007027.7% H2SO4 �1.59 0.000500.1 N HCl 1 0.00330.1 N F3CCOOH 1 0.00411.5% HNO3 1.13 0.00670.01 N HCl 2 0.01050.01 N CH3COOH 3.37 0.1

a the Hammet acidity function [4]

Figure 3. The dependence of optical density of solutions of TOnitration reaction mass on the reaction time and HNO3 concen-tration. – – – in 87.4% nitric acid; 1 min, – · – · in 87.4% nitric acid;180 min, –— in 99.0% nitric acid; 1 min, — in 99.0% nitric acid,180 min.

Figure 4. The formation of NTO from TO and N-NTO in nitricacid at 25 8C. & – fraction of N-NTO in the mixture with TO atdissolution N-NTO in nitric acid in 1 minute; * – fraction of N-NTO in the mixture with TO at dissolution TO in nitric acid in 1minute; * – yield of NTO from TO; & – yield of NTO from N-NTO

Scheme 1.

300 V. L. Zbarsky and N. V. Yuding

Propellants, Explosives, Pyrotechnics 30 (2005), No. 4 D 2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

I2 ¼1þ I1TO½ �0

N�HTO½ � � 1ð5Þ

With the correctly chosen functionA, the angular factor ofthe dependence of log (I2) on A should be close to 1 (seeFig. 6). The best results are obtained using the functionA asfollows [8]:

A¼ log[NOþ2 ]þHR

When two parallel processes take place the kineticequations of the first order have the following form:

NTO½ � ¼ TO½ �0�k03

k03þk0

41� e k0

3þk0

4ð Þ�t� �

N�NTO½ �

¼ TO½ �0�k03

k03þk0

4� e k03þk04ð Þ�t

TO½ �0�k03

k03þk0

4¼ NTO½ �end

NTO½ �endTO½ �0

¼ w

wherew is the yield of NTO and k03 and k0

3 are apparent rateconstants of reactions III and IV.

logðk3Þ ¼ logðk03Þ þ logð1þ I1 þ I2Þ ð6Þ

logðk4Þ ¼ logðk04Þ þ logðð1þ I1 þ I2Þ=I2Þ ð7Þ

One can see from the plot in Fig. 7 that, at HNO3

concentrations of 98 – 100%, the decomposition rate of N-NTO is higher than the rate of NTO formation. It wasmentioned above that the termination of growth of the rateconstant of TO nitration when the nitric acid concentrationexceeds 85% is a result of a fast decrease in the unproto-nated TO concentration (TO is in the free base form). Toexplain this result, the competing equilibria involving TOprotonation and N-nitration were taken into account, andthe real rate constants k3 and k4 were calculated. In thisscenario for TO nitration, the first-order rate constantincreases monotonically with increasing nitric acid concen-tration (see Fig. 8). These observations confirm the correct-ness of the chosen kinetic model. The termination of growthof the reaction rate at 85 – 100% of nitric acid is caused by adecrease in concentration of TO due to equilibria I and II.

Figure 6. Dependence of I2 on concentration (HRþ log (aH2O))

Figure 7. Dependence of observed constants of the formation of(*) NTO and (*) N-NTO on the concentration of HNO3.

Figure 5. Rate constant for NTO formation in nitric acid at25 8C. *– from N-NTO; * – from TO

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The influence of temperature on TO nitration has beenstudied. From the kinetic data obtained, we have calculatedthe rate constants for the processes of TO nitration and N-NTO decomposition, k ’

3 and k ’4 respectively, and found the

parameters of the Arrhenius equation. The results arepresented in Table 3.N-NTO formed in the nitration reaction was found to be

an extremely unstable substance. Its ignition point wasmeasured to be as low as 84 8C. Preliminary tests haverevealed the following N-NTO properties: it decomposesviolently when mixed with water; its decomposition in aglass Bourdon-type manometer at 80 8C proceeds so fastthat it destroys the glass membrane; a few milligrams of thesubstance rubbed in an unglazed porcelain mortar givesharp clicks. N-NTO combustion properties are similar to

those of fast-burning explosives: burning velocities at0.1 MPa and 10 MPa were measured to be 10.8 mm/s and100 mm/s, respectively.

4 Conclusion

The activation energy of the process of N-NTO decom-position in 100%HNO3 is higher than the activation energyof TO nitration, which causes the NTO yield to decreasewith increasing reaction temperature. Therefore, fast gen-eration of N-NTO takes place during nitration of TO inconcentrated nitric acid. This significantly influences theprocess of NTO formation and causes a significant decreasein the yieldwhenHNO3with concentrations exceeding 98%is used. Because of low stability of N-NTO, specialprecautions should be taken to exclude its formation duringNTO production.

5 References

[1] V. F. Zhilin, V. L. Zbarsky, Production Methods and Propertiesof Explosives. 2. Nitrotriazol-5-one, Chimicheskaya technolo-giya, 2001, 5, 6 (in Russian).

[2] H. S. Kim, E. M. Goh, and B. S. Park, U.S. Patent 6583293, 2003,Agency for Defense Development of Korean Republic.

[3] A. Becuwe, A. Delclos, Low-Sensitivity Explosive CompoundsforLowVulnerabilityWarheads,Propellants, Explos., Pyrotech.,1993, 18, 1.

[4] K. J. Kim, M. J. Kim, J. M. Lee, S. H. Kim, H. S. Kim, and B. S.Park, Solubility, Density, and Metastable Zone Width of the 3-Nitro-1,2,4-triazol-5-oneþWater System, J. Chem. Eng. Data1998, 43, 65.

[5] V. L. Zbarsky, V. V. KuzKmin, and N. V. Yudin, Synthesis andCharacteristics 1-Nitro-1,4-Dihydro-1H-1,2,4-Triazol-5-one,Zhurnal Organicheskoj Khimii, 2004, 40, 1110 (in Russian).

[6] N. M. Emmanuel, D. G. Knorre, Course of Chemical Kinetics,Vysshaja Shkola, Moscow, 1974, pp. 259, 281.

[7] A. R. Katritzky, C. Ogretir, The Kinetic Nitration and Basicityof 1,2,4-Triazol-5-ones, Chimica Acta Turcica, 1982, 10, 137.

[8] D. J. Belson, A. N. Strachan, Aromatic Nitration in AqueousNitric Acid, J. Chem. Soc. Perkin Trans. II, 1989, 15.

Acknowledgements

The authors are grateful to the late Professor B. N. Kondrikovand Dr. V. Yu. Egorshev for their interest to this work.

(Received February 13, 2005; Ms 2005/105)

Table 3. The parameters of the Arrhenius equation for TOnitration and NTO decomposition.

Reaction HNO3 A E% s�1 kJ/mol

Nitration 77.9 1.06*1013 86.0Nitration 89.27 1.13*1012 76.5Nitration 100 2.02*1010 66.8Destruction 100 6.89*1012 78.5

Figure 8. Dependence of real constants (*) k3 and (*) k4 on theconcentration of HNO3.

302 V. L. Zbarsky and N. V. Yuding

Propellants, Explosives, Pyrotechnics 30 (2005), No. 4 D 2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim