ROLE OF INTERMOLECULAR REACTIONS IN THERMOLYSIS OF AROMATIC NITRO-COMPOUNDS IN SUPERCRITICAL AROMATIC SOLVENTS

Several nitroarenes were decomposed isothermally in dilute supercritical solution in benzene or toluene and in the vapor phase in the temperature range of 290-380-degrees-C in sealed glass tubes with pressures up to 100 MPa. The mechanisms of thermolysis are inferred from kinetic studies and product analysis. The initial rate-controlling step for nitrobenzene and p-nitrotoluene decomposition is probably intermolecular hydrogen abstraction to form an ArNO2H radical intermediate. The nature of the transition state is deduced from the activation volume (DELTA-V*), H/D kinetic-isotope effect, and a linear free-energy relationship between the ionization potential of the hydrogen donor and the logarithm of the decomposition rate. A concurrent pathway for o-nitrotoluene is an intramolecular reaction in which anthranil is an intermediate. The behavior of 1,3-dinitrobenzene and 1,4-dinitrobenzene resembles that of nitrobenzene, whereas 2,4-dinitrotoluene and 2,6-dinitrotoluene decompose in the same manner as o-nitrotoluene. Activation parameters are given and detailed mechanisms proposed.