Arrhenius parameters for the reaction of propyl radicals with propane

Abstract

The reaction of 11-propyl radicals from the photolysis of azo-n-propane with propane has been studied and Arrhenius parameters for hydrogen atom abstraction relative to the recombination of 11-propyl radicals determined. From the Arrhenius parameters obtained and the available thermodynamic data, the Arrhenius parameters of the reverse reaction, the hydrogen atom abstraction from propane by iso-propyl radicals, have been calculated. Some complicating factors in the photolysis of azopropane are discussed.Canadian Journal of Chemistry, 47, 3305 (1969) The hydrogen abstraction reactions of methyl radicals from alkanes have been extensively studied, but information 011 the analogous reactions of higher alkyl radicals is scant (1). It has generally been assumed that the reactions of the higher alkyl radicals are very similar to those of methyl except that the activation energy of abstraction would progressively increase as the C-H bond energy of the abstracting radical decreases. This expectation, though trivial, still lacks proof of general validity. The most abundant data for comparison of methyl with higher alkyl radicals are for abstraction of hydrogen from aldehydes. Here, however, the data, if trustworthy, appear to favor an opposite trend. Abstraction of hydrogen for example by Me and t-Bu from t-BuCHO has activation energies of 10.2 and 4.3 kcal, respectively. For abstraction from s-BuCHO by Me and s-Bu, the values are 10.4 and 4.9, and from i-BuCHO by Me and i-Bu 8.4 and 6.5 kcal, respectively (1). The abstraction from 12-BuCHO and EtCHO by Me, n-Bu, and Me and Et exhibits the same trend. On the other hand, abstraction of hydrogen from paraffins (for example from n-and iso-butane and cjjclohexane) by methyl and ethyl shows the expected trend in that abstraction by ethyl has activation energies about 0.8 kcal larger than by methyl (1).Hydrogen abstractions by propyl radicals from hydrocarbons have not been reported in the literature. In conjunction with another study, we have determined the Arrhenius parameters for the 12-propyl-propane reaction.12-Propyl radicals were generated by the 3660 A photolysis of azo-11-propane. It is generally accepted that photolysis of azo-alkanes in their first absorption band leads to an n -t n': excitation. The excited molecules may either undergo decomposition into nitrogen and two alkyl radicals or suffer collisional deactivation to the ground state. The decomposition evidently has a small activation energy, and the radicals produced are thermally equilibrated (2-14).The photolysis of azo-n-propane has been carried out in the presence of propane and propane-1 ,1,1,3,3,3-GI,, and rate constants for the reaction of n-propyl radicals with propane relative to the recombination of n-propyl have been measured. In the kinetic treatment allowance has been made for the reactions of propyl radicals with the parent azo compound. ExperimentalSmall portions of azo-11-propane (Merck) were purified by gas chronlatography before each run, using the method described below for separation o...