EFFECT OF PRESSURE ON THE THERMOLYSIS OF NITROALKANES IN SOLUTION

The effect of pressure up to 1.1 GPa on the rates of decomposition of two acidic nitroalkanes, nitromethane and 2-nitropropane, was measured . The mechanisms of thermolysis are inferred from kinetic studies and product analysis. The rate-controlling step for nitromethane decomposi tion in toluene at 230 degrees C at low pressures is homolysis of the C-N bond. Beyond 20% conversion, the decomposition is autocatalytic. A t high pressure, nitromethane has another reaction path which supplant s homolysis. It is proposed that nitromethane forms an intermediate by cyclization of its aci-form. The high-pressure process is characteriz ed by a first-order rate law without primary kinetic isotope effect, a low activation energy (28.5 kcal/mol), a negative activation volume ( -5.5 mL/mol), and formation of products which cannot be attributed to radical intermediates. At high conversion, the reaction becomes autoca talytic as a result of accumulation of water leading to formation of p roducts explainable by the Nef reaction. 2-Nitropropane is less stable than nitromethane. Pressure powerfully accelerates its decomposition owing to its activation volume averaging -11.2 mL/mol from 0.1 to 1.1 GPa. It is believed to cyclize via the aci-form like nitromethane. 2,2 -Dinitropropane does not have alpha hydrogen and cannot tautomerize. I n earlier work it was found to have a homolytic mechanism at high pres sure. Therefore, the decomposition pathways of nitroalkanes in aprotic solvents are determined not only by the conditions but also by the av ailability of alpha hydrogen.