Thermal Decomposition of di-tert-Butyl Peroxide in Binary Mixtures near the Critical Point

The thermal decomposition of di-tert-butyl peroxide has been studied in the presence of high pressures of cyclopropane; the rate constant for the decomposition was determined from the sum of the rates of formation of tert-butanol and acetone. In the temperature range 125–160 °C, i.e. above the critical point of cyclopropane, concentrations of cyclopropane of between 2.6 and 3.6 mol/1 (60–80 amagats) were used; in the temperature range 100–124 °C, i.e. below the critical point of cyclopropane, an equivalent mean (liquid + gas) cyclopropane concentration was used. An Arrhenius plot of the rate constants for the peroxide decomposition is continuous through the critical point of the cyclopropane, and is indistinguishable from the best Arrhenius line representing all previous work on the reaction.However, an Arrhenius plot of the ratio of the rates of formation of tert-butanol and acetone shows a marked discontinuity at the critical point, but further investigation shows that this results from the unequal distribution of the reactant peroxide between the two phases which are present below the critical point of the cyclopropane.Less extensive series of experiments using propane, isobutane, cyanogen, and carbon dioxide as pressurizing gases confirm the pressure-independence of the rate of decomposition of di-tert-butyl peroxide: the experiments with propane and isobutane confirm the phase-independence of the rate; the propane experiments also exhibit the same apparent kinetic discontinuity at the critical point. Finally, rate-constant data are obtained for the reactions of the tert-butoxy radical with cyclopropane and isobutane, and for the addition of methyl radicals to cyanogen.