KINETICS AND MECHANISMS OF THE PYROLYSIS OF n-BUTANE: PART I. THE UNINHIBITED DECOMPOSITION
The kinetics of the pyrolysis of n-butane have been studied at temperatures from 520° to 590 °C, and at pressures from 30 to 600 mm Hg; the rate was followed from pressure changes and by gas chromatography. The reaction was accurately of the three-halves order; the activation energy was found to be 59.9 kcal mole−1, and the frequency factor 3.24 × 1015 cc1/2 mole−1/2 sec−1. The reaction is sensitive to surface; packing the vessel and 'conditioning' it usually led to a decrease in rate and an increase in activation energy. The reaction is concluded to be largely homogeneous, and to occur almost entirely by a free-radical mechanism; the initiation reaction is considered to be the dissociation of a butane molecule into two ethyl radicals, in its first-order region, and termination is believed to be the second-order combination of ethyl radicals. The mechanism proposed is shown to account satisfactorily for the observed behavior. The surface effect is attributed to a certain amount of initiation by abstraction, by a surface atom, of a hydrogen atom from butane, and to surface catalysis of the recombination of ethyl radicals.