Thermal decomposition of aminotetrazoles
Introduction
Aminotetrazoles possess a unique combination of properties. In spite of large positive enthalpies of formation [1], they exhibit surprisingly high thermal stability [2], [3], [4], [5]. Aminotetrazoles have the highest content of nitrogen among all organic substances (e.g. 82.3 wt.% for 5-aminotetrazole (5-AT) and 84.0 wt.% for 1,5-diaminotetrazole (DAT) ). Therefore, aminotetrazoles are prospective materials for generation of gases, as blowing agents, solid propellants and other combustible and thermally decomposing systems [6], [7]. The kinetics and mechanism of thermal decomposition of aminotetrazoles is of high interest. Since aminotetrazoles show the tendency to prototropic, ring-chain and mesoionic isomerization [2], [3], [8], the investigation seemed to be important for revealing and evaluation the role of structural factors in the thermal transformations.
Recently, we have studied the thermal decomposition behavior of tetrazole [9], [10], [11] and some aminotetrazoles [2], [3], [4]. This paper presents an overview of the comprehensive study on kinetics and mechanism of thermal decomposition of a series of aminotetrazoles of the following structures.which include 5-AT, (I, R=H), 1-methyl-5-aminotetrazole (MAT, I, R=CH3), DAT, (I, R=NH2), poly-1-vinyl-aminotetrazole (PVAT, polymer of 1-vinyl-5-aminotetrazole I, R=CHCH2), and 5-aminotetrazole sodium salt (SAT, II), where the tetrazole ring exists in the anionic form.
Section snippets
Experimental
Synthesis and purification of 5-AT, MAT, DAT and PVAT were described elsewhere [2], [3], [4]. The purified 5-AT was reacted with sodium ethylate at equivalent ratio in ethanol to prepare SAT. After completion of the reaction, solvent was distilled out under vacuum. The results of chemical analysis for SAT are as following.CH2N5Na (107,05) Calc. C 11.21 H 0.019 N 65.39 Na 21.49 Found C 11.26 H 0.021 N 66.02 Na 21.18
The thermal decomposition of the aminotetrazoles was studied either in the sealed steel
Thermal analysis
As thermogravimetry shows (Fig. 1), 5-AT, PVAT and SAT have two step of weight loss within the temperature interval of 460–900 K, whereas, MAT and DAT decompose in one main step at 470–620 K followed by a slow weight loss till 870 K (Fig. 1). The temperature intervals, heats of decomposition, specific volumes of gas evolved and weight losses for the first step of thermal decomposition of aminotetrazoles are presented in Table 1. The decomposition of 5-AT, MAT and DAT starts right after the
Conclusions
Two mechanisms of splitting of the tetrazole ring are effective in aminotetrazoles: (1) evolution of hydrogen azide from the tautomeric forms with hydrogen atom by nitrogens in the ring and (2) evolution of nitrogen molecule from the amino-tautomeric form. This assumption is based on the composition of volatile products of the thermal decomposition as well as data of the study of amino-imino tautomerism. The ab initio calculations of the total energies and the energies of chemical bonds in 5-AT
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