Thermal Decomposition Pathways of 1,3,3-Trinitroazetidine (TNAZ), Related 3,3-Dinitroazetidium Salts, and 15N, 13C, and 2H Isotopomers

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Abstract

The thermal decomposition of 1,3,3-trinitroazetidine (TNAZ) and related 3,3-dinitroazetidium (DNAZ+) salts was examined neat and in solution. TNAZ kinetics were found (160−250 °C) to be first-order and nearly identical neat and in benzene, with an activation energy of 46.6 kcal/mol (195 kJ/mol). The DNAZ+ salts were less thermally stable than TNAZ, and neat did not decompose in a first-order fashion. However, in aqueous solution the DNAZ+ salts did decompose following first-order kinetics; their rates were similar with minor differences apparently related to the strength of the anion as a conjugate base. Like simple nitramines such as dimethylnitramine, TNAZ tended to form N2O rather than N2, but unlike other nitramines it formed about as much NO as N2O. TNAZ isotopomers labeled with 13C and with 15N were prepared and used to identify the origin of the decomposition gases and the identity of the condensed-phase products. Early in the decomposition of TNAZ, most of the NO came from the nitro group attached to the azetidium ring nitrogen. Most of the N2O was the result of the nitro groups interacting with each other, while the majority of the N2 contained one nitrogen from the ring. Many condensed products have been identified, but five stand out because they are formed in the thermolysis of TNAZ and the three DNAZ+ salts [NO3-, Cl-, N(NO2)2-]. These are 3,5-dinitropyridine (M, always a minor product), 1-formyl-3,3-dinitroazetidine (L), 1,3-dinitroazetidine (K), 1-nitroso-3,3-dinitroazetidine (E), and 1-nitroso-3-nitroazetidine (G); the identity of the first four has been confirmed by use of authentic samples. Of these five, the last four have been shown to interconvert with TNAZ and each other under the conditions of these experiments. This study confirms the presence of two competitive TNAZ decomposition pathways. Under the conditions of this study, N−NO2 homolysis is slightly favored, but products, such as K, resulting from C−NO2 scission, are also well represented.

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