Activation energies and frequency factors in the thermal decomposition of paraffin hydrocarbons

Abstract

Further evidence supports the view that the thermal decomposition of paraffin hydro­carbons occurs by simultaneous chain and non-chain (molecular) mechanisms, the latter being isolated by addition of suitable inhibitors such as nitric oxide. It is concluded that certain small surface effects are secondary disturbances of an essentially homogeneous reaction. With one group of hydrocarbons, type 1, including ethane (propane), iso -butane, iso -pentane, neo -pentane and neo -hexane, this molecular reaction is of the normal unimolecular kind, showing a single transition from the first order towards the second as the pressure drops, and an activation energy which is independent of the initial pressure. In the second group, type 2, including n -butane and higher normal paraffins, 2∙3-dimethyl butane, 2-methyl pentane and 3-methyl pentane, the molecular reaction seems to be a super­position of two unimolecular reactions with different pressure dependences and different activation energies. There is a double order transition, and a marked variation with pressure of the apparent activation energy. This apparent activation energy is influenced by the addition of certain gases (propylene, ethane, propane) in such a way as to be in any given case a unique function of the total pressure of paraffin plus added gas. The frequency factors in the equation k = A e -E/RT have been determined. With the paraffins of type 2, the ‘high- pressure’ components of the reaction give values within the range expected by the theories which relate A to a vibration frequency, and assume the localization of the critical energy in a single bond. With some of the paraffins of type 1 the frequency factors are outside the probable range (10 12 to 10 14 s -1 ), and the values suggest that the transition states may be less sharply defined than the localization condition demands. The apparent values of the frequency factors associated with the ‘low-pressure’ components of the reactions are still higher and appear to demand a special interpretation which must be the subject of further investigation.