Kinetics and mechanism of thermal decomposition of nitropyrazoles

The decomposition of mono-, di-, and trinitropyrazole derivatives in the condensed state was studied by the manometric method. The reaction rate depends on the number and position of nitro groups in the pyrazole cycle and on the polarity of the medium and aggregate state of the substance. The activation energy of the initial non-catalytic stage of decomposition Е1 decreases on going from monoto trinitropyrazoles from 142 to 132 kJ mol−1, and the preexponential factor is 109±0.5 s−1. In a diphenyl solution the decomposition rate is lower than that in the melt, and this difference decreases with increasing in the number of nitro groups in the molecule. For the decomposition of trinitropyrazole in the solid state, Е1 decreases by 10 kJ mol−1. All these facts are explained in terms of the mechanism, according to which the reaction occurs as the oxidation of the adjacent carbon atom by the nitro group and proceeds via a strongly polar cyclic transition state.