KINETICS AND MECHANISMS OF THE PYROLYSIS OF n-BUTANE: PART II. THE REACTION INHIBITED BY NITRIC OXIDE

Abstract

The kinetics of the pyrolysis of n-butane, when maximally inhibited by nitric oxide, were studied at temperatures from 540° to 610 °C, and at pressures from 30 to 550 mm Hg. The reaction has a short induction period and is accurately of the three-halves order; the activation energy was 65.9 kcal mole⁻¹ and the frequency factor 5.3 × 10¹⁶ cc^1/2 mole^−1/2 sec⁻¹. The reaction was somewhat less inhibited by surface than was the uninhibited reaction. Excess of carbon dioxide had no effect on the rate. The results are explained in terms of a free-radical mechanism for the maximally inhibited decomposition. It is proposed that the initiation step in the inhibited decomposition is mainly C₄H₁₀ + NO → C₄H₉ + HNO. This is followed by the ordinary chain-propagating reactions, and by processes such as C₂H₅ + NO → C₂H₅NO. The main chain-terminating step, of the type β + βNO, is concluded to be C₂H₅ + C₂H₅NO → C₄H₁₀ + NO or C₂H₆ + C₂H₄ + NO. This scheme leads to three-halves-order kinetics, and provides a satisfactory quantitative interpretation of the experimental behavior.