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.