Inhibition and acceleration of paraffin decomposition by nitric oxide

Abstract

Analysis of the products from the thermal decomposition of neopentane and of isopentane in presence of increasing amounts of nitric oxide confirms the validity of pressure change as a measure of extent of reaction over the first 20 % change, provided that certain initial disturbances of a secondary character are corrected for. (These may give initial rates which are apparently too high or too low.) A method of eliminating the disturbance is adopted. For all three pentanes, the rate of decomposition first falls with increase in nitric oxide concentration (‘inhibition curve’) but at high concentrations rises again. This ‘induced reaction’ has now been studied in more detail and shown to be kinetically of the first-order with respect to both nitric oxide and paraffin. A mechanism is proposed for the induced reaction, whereby NO abstracts a hydrogen atom from the parent hydrocarbon, the resulting alkyl radical reacting rapidly with NO. In this reaction the consumption of NO tends to a value of about one molecule for each molecule of paraffin decomposed. Thus regeneration of NO from the products of its reaction with the alkyl radicals seems very unlikely. In the region of the extended flat minimum of the inhibition curve, nitric oxide consumption is small. If that attribu table to the induced reaction is allowed for, then the inhibition of chains involves very little consumption of NO. It is suggested that this small consumption, in the absence of an appropriate regeneration reaction, is unlikely if the NO maintains a steady-state concentration of radicals under the conditions of the minimum rate. Thus a simpler interpretation is that there is in fact a predominantly molecular reaction in these conditions.