Reactions of allylic radicals that impact molecular weight growth kinetics

Abstract

The reactions of allylic radicals have the potential to play a critical role in molecular weight growth (MWG) kinetics during hydrocarbon oxidation and/or pyrolysis. Due to their stability (when compared to alkyl radicals), they can accumulate to relatively high concentrations. Thus, even though the rate coefficients for their various reactions are small, the rates of these reactions may be significant. In this work, we use electronic structure calculations to examine the recombination, addition, and abstraction reactions of allylic radicals. For the recombination reaction of allyl radicals, we assign a high pressure rate rule that is based on experimental data. Once formed, the recombination product can potentially undergo an H-atom abstraction reaction followed by unimolecular cyclization and β-scission reactions. Depending upon the conditions (e.g., higher pressures) these pathways can lead to the formation of stable MWG species. The addition of allylic radicals to olefins can also lead to MWG species formation. Once again, cyclization of the adduct followed by β-scission is an important energy accessible route. Since the recombination and addition reactions produce chemically-activated adducts, we have explored the pressure- and temperature-dependence of the overall rate constants as well as that for the multiple product channels. We describe a strategy for estimating these pressure-dependencies for systems where detailed electronic structure information is not available. We also derive generic rate rules for hydrogen abstraction reactions from olefins and diolefins by methyl and allyl radicals.

A Shock Tube Study of H Atom Addition to Cyclopentene

“…A similar result could be obtained by lowering the preexponential factor of , but a larger barrier is in better accord with theoretical values . At 1000 K, our rate constant for ring closure ( k −3 ) is 3× to 7× larger than values estimated by Wang et al (Table ). In our model, the use of a smaller value of k −3 would require a corresponding reduction in k 4 (beta scission of allyl) to fit the Tsang and Walker data.…”

Investigation of n-pentane pyrolysis at elevated temperatures and pressures in a variable pressure flow reactor

“…As the concentrations of the higher olefins build up, so do the concentrations of the resonantly stabilized allylic radicals. Recently Wang et al 35 have shown that these allylic radicals have important pathways leading to molecular weight growth. Additionally the stability of the allylic radicals allows for the buildup of their concentrations, this leads to a decrease in the decomposition rate of pentane.…”

The Impact of Resonance Stabilization on the Intramolecular Hydrogen‐Atom Shift Reactions of Hydrocarbon Radicals

“…The ring‐strain energies are also provided; the energy barriers for the corresponding bimolecular H‐atom abstraction reactions are from previous studies 23. 24…”

“…This is primarily due to the bimolecular component of the activation energy, which is about 16 kcal mol −1 for the H‐atom abstraction by a primary allylic radical from a secondary allylic site 23. 24 The analogous bimolecular abstraction reaction of an alkyl radical has an activation energy of around 12 kcal mol −1 35. With the exceptions of reactions 4.1 and 5.1, the ring‐strain energies are similar to those for the corresponding reactions in Scheme or .…”

“…The impact of resonance structure on the bimolecular component of the activation energy is well documented 23. 24 Results show that the activation energy for H‐atom abstraction at an allylic site in an olefin is lower than that for abstraction at an alkyl site, whereas the barrier for H‐atom abstraction by an allyl radical is higher than that for abstraction by an alkyl radical. Depending upon the location of the conjugated bonds, the ring‐strain component might be expected to increase.…”