High energy tetrazole-based compounds as lead replacement explosives

Andrzej WOJEWÓDKA, Tomasz WITKOWSKI ? Faculty of Chemistry, Silesian University of Technology in Gliwice

Abstract:
This paper describes the analysis of possibilities of finding the substitute for lead azide among the derivatives of 1H-tetrazole and 5-aminotetrazole. The synthesis methods  of these compounds and their derivatives are presented. This paper presents the data on the standard enthalpy of formation, sensitivity to friction and impact as  well as the data on the standard parameters of detonation: pressure (C-J)  and velocity.

Please cite as: CHEMIK 2013, 67, 1, 19-24

According to the American Society for Testing and Materials (ASTM), energetic materials are chemical compounds or mixtures containing fuel and oxidiser, which ? due to a rapid reaction, have the ability to release heat and gas products. However, only energetic materials having the ability to detonate under specific conditions are called the explosives. Among them, the following explosives can be distinguished: resistant to incidental stimuli, secondary explosives and primary explosives with the increased sensitivity to external stimuli ? used in small amounts to initiate the reaction in secondary explosives. Primary explosives, secondary explosives, propellants, pyrotechnic (compounds and compositions) and rocket propellants are characterised by many practical applications that can be classified into two groups: military and civil. The detonation transition of explosives may be initiated by a self-driven decomposition process reaching the maximum ? detonation, values ? Deflagration to Detonation Transition, DDT) or by a shock wave.

The standard enthalpy of formation of explosives is a crucial value influencing the amount of energy that can be released during the decomposition ? it is a favourable high positive value related to a large quantity of bonds present in modern explosives: N?N, N?O, N?C. The average energy of bonds between nitrogen atoms equals to:

High energy tetrazole 01

thus, the chemical compounds containing a large number of nitrogen atoms release a large amount of energy during the decomposition as the result of the formation of molecular nitrogen, as opposed to the traditional explosives releasing energy due to oxidation of C-C, C-H bonds inside a molecule.
The modern explosives should satisfy a series of other requirements:
? C/H/N ratio should be characterised by low carbon and hydrogen content and high nitrogen content
? they should be a chemical compound (not a mixture) containing an adequate quantity of fuel and oxidiser
? they should have high density providing high detonation parameters
? a high positive value of the standard enthalpy of formation ? reflected as an amount of heat released during the decomposition of explosives
? relatively high insensitivity to the following stimuli: mechanical stimuli, electrostatic discharge, heat
? low solubility in water, material stability and compatibility
? safety for the environment.

An interest in explosives has been systematically increasing for the last several years. One of the reasons for such an interest is an attempt to obtain a chemical compound which could satisfactory replace lead azide Pb(N3)2 while constructing a detonator, a blasting cap and laser firing caps: detonating and firing cap. Lead azide is a very good initiator of secondary explosives and it is quite cheap. On the other hand, it also has many disadvantages such as being very toxic to human beings, hazardous to the environment (present on a candidate list of substances of very high concern prepared by the European Chemicals Agency ? decision no. ED/77/2011) and particularly sensitive to mechanical stimuli (simple stimuli: friction 0.1÷1 N and impact 2.4÷4.0 J) [1], which makes this compound specifically dangerous when treated manually. It is sensitive to water in a form of vapour and carbon dioxide (a problem related to material compatibility). That?s why it is so important to invent its substitute which could meet these very high requirements.

Many research centres are looking for a solution to this problem among heterocyclic compounds rich in nitrogen ? derivatives of azoles [2]. The enthalpy of azole formation depends on the ring structure, while the properties of end compounds such as heat of formation, density, melting temperature, oxygen balance, etc. can be modified by substituting hydrogen atoms. Derivatives of azoles ? mainly of tetrazoles, triazoles and imidazoles ? function as ions in the course of obtaining high energy salts. They constitute a wide base for developing new explosives. From among five-membered heterocyclic rings, 1H-tetrazoles and 5-aminotetrazoles are widely applied.

Tetrazoles are aromatic compounds containing one carbon atom and four nitrogen atoms. Unsubstituted 1,2,3,4-tetrazole is theoretically present in 3 tautomeric forms: 1H-tetrazole, 2H-tetrazole and 5H-tetrazole (Fig. 1) ? the two former forms were discovered and confirmed. Tetrazole can be found in a crystalline form only as 1H-tetrazole [3÷5], whereas it appears in the solution in two tautomeric forms: as 1H- and 2H-tetrazole. Depending on the polarity of the used solvent, they may have different proportions and the highest the polarity is, the larger the quantity of more polar 1H isomer can be found [6].

High energy tetrazole 02

1H-Tetrazole can be obtained from the process of dipolar cycloaddition [2+3] which occurs between hydrocyanic acid and hydrazoic acid (Fig. 2, method 1) [2, 8] or between sodium azide and sodium cyanide [2, 9] (Fig. 2, method 2). The most convenient method for obtaining 1H-tetrazole is the reaction between sodium azide, ammonium chloride and triethyl orthoformate in the concentrated acetic acid, which is performed at 80°C for 6 hours (Fig.2, method 3) [2, 7].

High energy tetrazole 03

1H-Tetrazole is characterised by a positive standard enthalpy of formation (237 kJ/mol [10]) and high detonation parameters calculated with the use of EXPLO 5 software: pressure ?21.0 GPa, velocity ?7813 m/s [7]. Moreover, it is characterised by low sensitivity to friction (>360 N), high nitrogen content ?79.98% and negative oxygen balance (-68.52%). High impact sensitivity (<4 J) belongs to its disadvantages [7]. 1H-tetrazole can be easily deprotonated under alkaline conditions, which causes the formation of, depending on the used reagent, adequate salts (Fig. 3).

High energy tetrazole 04

The obtained chemical compounds, where: R+ (X)= NH4 + (NH3), N2H5 + (N2H4×H2O), Li+ (LiOH), Na+ (NaOH), K+ (KOH), Rb+ (RbOH), Cs+ (Cs2CO3), Sr2+ (Sr(OH)2), are characterised by: low sensitivity to friction ? above 360 N and to impact ? above 100 J, and their decomposition temperature exceeds the value corresponding to 1H-tetrazole (188°C) [7, 8, 11]. 1H-tetrazolium perchlorate is obtained from the reaction of 1H-tetrazole and perchloric acid. Then, it reacts with potassium dinitramide forming a strong explosive ? 1H-tetrazolium dinitramide, having high positive standard enthalpy of formation ? 367 kJ/mol (Fig. 4) [12].

High energy tetrazole 05

The comparison of this compound to the strong and widely applied high explosive ? hexogen, indicates that, at the same density (r=1.82 g/cm3), the discussed compound is characterised by stronger detonation parameters: detonation pressure ? 36.5 GPa, pC-J (RDX)= 35.2 GPa; detonation velocity ? 9215 m/s, D(RDX)= 8977 m/s. Unfortunately, the low temperature of decomposition (Td= 130°C) and high sensitivity to impact (2 J) make it difficult to use this compound as a safe explosive [12].

5-aminotetrazole and its derivatives
5-aminotetrazole (5-AT) can be obtained, among other things, from:
? the reaction of aminoguanidine nitrate and nitrous acid ? at first, azidoformamidinium nitrate is formed, and then it is subjected to deprotonation and then ring formation takes place (Fig. 5) [13];

High energy tetrazole 06

? the reaction of dicyandiamide and hydrazoic acid ? in this reaction dicyandiamide is subjected to polymer degradation and cyanamide is obtained which reacts with hydrazoic acid producing 5-aminotetrazole (Fig. 6) [14].

High energy tetrazole 07

The standard enthalpy of 5-AT formation is 207 kJ/mol [10]. 5-AT is a chemical compound with high nitrogen content (82.3%). It is a weak acid (pKa?6). Thus, as the result of reacting with strong inorganic acids, it forms the salts, among other things, of: nitrate [15, 16], perchlorate [17], halides (bromide, chloride, iodide) [18], picrate [19]. In these salts, 5-AT is cation (Fig. 5). Halides can be used in synthesis, through a salt metathesis reaction, of other high energy salts (Fig. 7).

High energy tetrazole 08

Chloride, bromide, iodide and 5-aminotetrazole picrate are characterised by low sensitivity to friction (>360 N) and to impact (>40J), the temperature of decomposition ?175°C and the following content of nitrogen: 49.8%, 37.8%, 30.0% 35.7%, respectively [20, 21]. 5- minotetrazolium perchlorate is sensitive to the following external mechanical stimuli: friction (8 N), impact (1.5 J) which exclude its application on a larger scale [22, 23]. 5-aminotetrazole nitrate is characterised by sensitivity to friction higher than 30 N and to impact higher than 360 J [22, 23]. There are also salts in which 5-AT is an anion containing in its structure only nitrogen, carbon and hydrogen atoms as well as high energy compounds of transition metals [2, 24÷26].

Acknowledgement
This article is co-financed by the European Union within the European Social Fund within SWIFT project POKL.08.02.01-24-005/10.

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Andrzej WOJEWÓDKA ? Sc.D (Eng), the Professor at the Silesian University of Technology, graduated from the Faculty of Chemistry of the Silesian University of Technology in 1974 where he received the academic degree of doctor in 1988. He earned habilitation at the National Science and Research Institute of Work Protection in Kiev in 2004. Research interests: chemistry and technology of explosives, processing safety. He
is the author of 49 scientific papers, 52 papers and posters, 13 patents and patent applications, 205 academic studies and scientific expertise opinions.
e-mail: Andrzej.Wojewodka@polsl.pl, phone: +48 32 237 18 35

Tomasz WITKOWSKI ? M.Sc., graduated from the Faculty of Chemistry of the Silesian University of Technology in 2010. He is a Ph.D student, Faculty of Chemistry at the Silesian University of Technology. Research interests: chemistry and technology of explosives. He is the author and co-author of 2 scientific publications and 3 posters.
e-mail: Tomasz.Witkowski@polsl.pl, phone: +48 880 253 017

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