The thermal decomposition of various substituted N-methyl-N-nitroanili
nes dissolved in indifferent solvents and piperidine has been investig
ated. Activation volumes and product analyses support evidence that th
e rate-determining step is the reversible homolysis of the nitramine b
ond. The activation volumes range from +18 to +36 ml mol(-1). A non-li
near Hammett relationship is attributed to an increase in secondary ca
ged reactions, namely rearrangement and oxidation. Arylnitramines with
electron-donating substituents yield greater amounts of the thermal r
earrangement products than those with electron-deactivating groups at
ambient pressures. Decomposition of arylnitramines with electron-donat
ing substituents under high pressures(ca. 1.2 GPa) favours caged react
ions over separative diffusion.