P.N. Gaponik

Thermal decomposition of aminotetrazoles

Abstract The thermal decomposition of 5-aminotetrazole (5-AT), 1-methyl-5-aminotetrazole (MAT), 1,5-diaminotetrazole (DAT), poly-1-vinyl-5-aminotetrazole (PVAT) and sodium salt of 5-aminotetrazole (SAT) have been studied by thermogravimetry, thermal volumetric analysis (TVA), DSC, DTA and evolved gas analysis (EGA). The kinetic parameters of the thermal decomposition of aminotetrazoles were calculated either by the Ozawa method or by the method of invariant kinetic parameters (IKP). The gaseous products, volatile condensed products and solid residues were identified by FTIR and gas chromatography–mass-spectrometry (GS/MS). The total energies and the energies of chemical bonds of various isomeric forms of 5-AT and MAT have been calculated ab initio using MP2/6-31G∗∗ theory level and MNDO approximation. Based on the content of products of thermal decomposition and the kinetic consideration, the mechanism of thermal decomposition of aminotetrazoles has been derived. Two routes of the splitting of tetrazole ring leading either to elimination nitrogen or hydrogen azide are suggested. The contribution of each route is changing upon the advancement of the process. It was assumed, that hydrogen azide splits out from the prothotropic forms of the tetrazole ring, which have hydrogen atoms by nitrogens in the ring. Experimental study as well as literature data on the amino-imino tautomerism are in agreement with the suggested mechanism of the decomposition of the tetrazole ring. It is shown that secondary reactions significantly extend variety of the products of thermal decomposition of aminotetrazoles.

Thermal decomposition of aminotetrazoles: Part 1. 5-Aminotetrazole

The thermal decomposition of 5-aminotetrazole (5-AT) has been studied by thermogravimetry, thermal volumometry, DSC, DTA and EGA. Solid products of thermal decomposition have been identified by IR spectroscopy and gaseous products by IR and mass spectrometry. Theoretical considerations of thermodynamic characteristics and energies of atomic bonding in the 5-AT tautomeric forms and intermediates have been carried out by the MO SCF method in the MNDO approximation. On the basis of IR and available literature data it is shown that dehydrated 5-AT exists mainly in the imino form in the solid state. Thermal treatment leads to increasing content of the amino form. The thermal decomposition of the imino form of 5-AT starts just after melting and results in hydrogen azide and carbodiimide. The linear polymer of carbodiimide and melamine derivatives have been identified in the solid residue. Further increase of temperature in the course of linear heating leads to another route of thermal decomposition involving the amino form of 5-AT accompanied by the evolution of nitrogen. Apparent activation energies of these routes determined from non-isothermal thermogravimetric data amount to 165 and 135 kJ mol−1, respectively.

The thermal decomposition of tetrazoles

Abstract A brief review of the literature on the thermal decomposition of chemical compounds containing a tetrazole heterocycle is given. It is shown that there are two radically different pathways of the tetrazole cycle fragmentation connected with the formation of a molecule of nitrogen or azides. The elimination of nitrogen from 2,5-disubstituted tetrazoles results in a nitrilimine. The elimination of nitrogen from 1,5-disubstituted tetrazoles leads to the formation of a nitrene. The stabilization of active intermediate products depends on the chemical properties of the substituents and the conditions under which the process is carried out, and leads to a wide spectrum of final products for the thermal decomposition of tetrazoles. Kinetic studies of the thermolysis of tetrazoles show that the mechanism of heterocycle fragmentation can vary with varying temperature. The elimination of nitrogen from tetrazoles is preceded by a high-polarity transition state.

The thermal decomposition of aminotetrazoles. Part 2. 1-methyl-5-aminotetrazole and 1,5-diaminotetrazole

Abstract The thermal decompositions of 1-methyl-5-aminotetrazole and of 1,5-diaminotetrazole have been studied using thermogravimetry, differential scanning calorimetry and thermal volumetric analysis. The solid residues, and the high boiling point and gaseous products of the decompositions have been collected and identified using IR spectroscopy and mass spectrometry. Both aminotetrazoles start to decompose just after melting: 1-methyl-5-aminotetrazole at 495 K and 1,5-diaminotetrazole at 460 K. The decomposition is accompanied by elimination of gaseous and high boiling point products, partial evaporation of the original substances and formation of thermally stable residues. Both 1-methyl-5-aminotetrazole and 1,5-diaminotetrazole, in the solid state and probably in the melt, coexist in amino and imino tautomeric forms. Therefore, two competing mechanisms of tetrazole ring-breaking, with elimination of respectively nitrogen or hydrogen azide molecule, are proposed.

Thermal decomposition of tetrazole: Part I. Programmed heating

Abstract The thermolysis of tetrazole was studied using differential scanning calonmetry, differential automatic gas volumetry and complex thermal analysis. The thermoanalytical and kinetic parameters from heat-evolution, gas-release and thermogravimetry data were calculated. The results reveal the complex nature of the tetrazole thermolysis. The mechanisms of thermal decomposition seem to be different in the gas phase and in the liquid state.

Thermal decomposition of tetrazole-containing polymers. I. Poly-5-vinyltetrazole thermolysis

Thermal decomposition of poly-5-vinyltetrazole has been investigated by the methods of chemical and complex thermal analyses under isothermal and nonisothermal conditions, infrared spectroscopy and gas chromatography. Activation parameters of the decomposition process have been evaluated by various methods. The analysis of the obtained data leads to the conclusion that, depending on the conditions of the experiment, decomposition of polymer tetrazole cycles may proceed in two parallel ways: either destruction of the tetrazole rings with elimination of hydrogen azide, or their tautomeric isomerization into the respective azidoazomethine, from which a nitrogen molecule is then ‘chipped off’.

Thermal decomposition of tetrazole Part III. Analysis of decomposition products

Abstract By means of methods of IR and 1H NMR spectroscopy, mass spectrometry, chromatography, elemental and chemical analyses it could be stated that tetrazole thermolysis, both in a melt and in the gaseous state, proceeds mainly through elimination of a nitrogen molecule, but in the melt the nitrogen is eliminated predominantly from the azide structure and in the gaseous state it eliminates from the cyclic 2H- form. When carrying out the process in a melt, ca. 5% of the tetrazole decomposes with hydrogen azide evolution. The intermediates (nitrene or C,N-nitrilimine respectively) are unstable and, depending on the experimental conditions, they are capable of either decomposing, with formation of HCN, N2 and H2, or participation in reactions leading to formation of cyanamide polymers.

Thermal decomposition of tetrazole-containing polymers: Part II. Thermolysis of poly-2-tert-butyl-5-vinyltetrazole

Abstract The thermal decomposition of poly-2- tert -butyl-5-vinyltetrazole was investigated by the gas volumetry method in an inert atmosphere and thermogravimetrically in vacuum at different heating rates. The thermolysis proceeds in two stages within the ranges 470–590 K and 610–750 K respectively. The kinetics of the first-stage decomposition under isothermal conditions between 450 and 489 K were thoroughly studied. The kinetic order of the process is close to unity, with E a = 165 ± 15 kJ mol −1 and log A = 14.4 ± 1.1 s −1 . The activation energy value calculated on the basis of non-isothermal experiments amounts to 150–165 kJ mol −1 for the first stage and ≈ 215 kJ mol −1 for the second. The compositional analysis of the volatile products and the condensed residue carried out using the methods of mass spectrometry, chromatography and IR and UV spectroscopy made it possible to propose two parallel pathways of polymer thermolysis. The first is consistent with the mechanism which was earlier substantiated for the thermolysis of poly-5-vinyl-2-alkyltetrazoles, and consists in the splitting out of a nitrogen molecule from the tetrazole ring with the formation of C , N -nitrilimine, as well as its further thermal conversion. Such a thermolysis route is implemented mainly at low heating rates. At high heating rates another decomposition mechanism is possible along with the first; it is related to removal of a tert -butyl substituent from the tetrazole ring with the formation of 2-methylprop-1-ene and 2-methylpropane.

Thermal decomposition of sodium tetrazolate monohydrate and its mixture with tetrazole

Abstract The thermal decomposition of sodium tetrazolate monohydrate and its mixture with tetrazole (64% tetrazole) was investigated by differential scanning calorimetry and complex thermal analysis at different heating rates. It was found that the exothermal single-stage decomposition of sodium tetrazolate proceeding at a high rate within the range 596–610 K is preceded by the processes of dehydration (393–443 K), polymorphic transformation (465 K) and melting (557 K). The activation energy of sodium tetrazolate decomposition was 98 ± 5 kJ mol −1 . Using the methods of IR spectroscopy, mass spectrometry and elemental analysis, it was found that the decomposition proceeds with the elimination of a nitrogen molecule and the formation of sodium acid cyanamide. Heating the mixture leads to dehydration, melting of the tetrazole and dissolution of the anhydrous salt in the tetrazole melt. During thermolysis of the solution formed, the tetrazole decomposition temperature rises and the sodium tetrazolate decomposition temperature is lowered, compared with thermolysis of the individual components. The heat of decomposition decreases significantly relative to the total heat of thermolysis of pure tetrazole and sodium tetrazolate. These features are due to the mutual influence of the components, in particular to the possible stabilization by the sodium tetrazolate molecules in the solution formed of the tetrazole 1-H form, which leads to thermolysis of the latter mainly in the melt.

Thermal decomposition of tetrazole-containing polymers: Part 3. Thermolysis of poly-1-vinyltetrazole

Abstract The thermal decomposition of poly-1-vinyltetrazole (PVT) has been studied by thermogravimetry in isothermal conditions and by DSC and thermal volumetric analysis in non-isothermal conditions. The gaseous and condensed products of thermolysis were identified. It has been shown that nitrogen is the single gaseous product of the thermal decomposition of PVT. Cyanamide and carbodiimide fragments fixed on the polymer carbochain which are able to cyclotrimerize into triazines and isotriazines respectively were identified in the condensed phase. The kinetic curves of the thermolysis in isothermal conditions have a sufficiently long induction period and fit an equation of autocatalysis of the first order. The limiting stage of PVT thermolysis is the opening of the tetrazole rings. The activation energy of this process calculated in isothermal conditions is 148 kJ moP; in non-isothermal conditions it is 140 kJ mol−1.