Mechanism of decomposition of nitroaromatics. Laser-powered homogeneous pyrolysis of substituted nitrobenzenes

Laser-powered homogeneous pyrolysis (LPHP) has been used to study the gas-phase thermal decomposition of nitrobenzene (NB), m-dinitrobenzene (m-DNB), p-nitrotoluene (p-NT), o-nitrotoluene (o-NT), and 2,4-dinitrotoluene (2,4-DNT) under conditions where surface-catalyzed reactions are precluded. The Arrhenius parameters have been determined by comparative rate measurements relative to cyclohexene decomposition and the reaction mechanisms have been established. In all cases, the rate-limiting step is the homolysis of the Ar-NO2 bond. The measured Arrhenius parameters for homolysis range from log A = 14.5 to 16.4 and E/sub a/ from 67.0 to 70.0 kcal/mol. The c-NO2 bond dissociation energies (kcal/mol) and values of log k (s ) for NB, m-DNB, p-NT, o-NT, and 2,4-DNT have been derived from these results and are as follows: 71.4 +/- 2.0, (15.5 +/- 0.5) - (68.2 +/- 1.7)/2.3RT; 73.2 +/- 2.4, (14.5 +/- 0.5) - (70.0 +/- 2.1)/2.3RT; 71.4 +/- 2.3, (14.9 +/- 0.5) - (68.2 +/- 2.0)/2.3RT; 70.2 +/- 2.5, (16.4 +/- 0.6) - (67.0 +/- 2.2)/2.3RT; 70.6 +/- 2.0, (15.3 +/- 0.4) - (67.4 +/- 1.7)/2.3RT. 15 references, 6 figures, 4 tables.