Kinetics of Intramolecular Phenyl Migration and Fused Ring Formation in Hexylbenzene Radicals

Abstract

A combined ab initio/TST study was conducted to study the phenyl migration and fused ring formation of a series of 1-phenyl-hex-x-yl radicals. Phenyl shift proceeds through a two-step mechanism with a ring closure by ipso-cycloaddition, followed by a ring-opening by β-scission leading to x-phenyl-hex-1-yl radical isomers. Both steps involve a spirocyclic transition state connected by a spirocyclic intermediate. Barrier heights at the CBS-QB3 level range from 9.9 to 17.7 kcal mol–1 depending on the size of the newly formed ring. Formation of fused rings by peri-cycloaddition reactions occur through barrier heights that range from 8.8 to 28.8 kcal mol–1. The addition sites are the substituted and ortho carbons of the phenyl ring for the ipso- and peri-cycloadditions, respectively. For both types of reaction, the Arrhenius A factor decreases as the ring size increases. This is related to the number of internal rotors lost across the reaction. In addition, length of the side chain also impacts on the kinetics as longer side chains decrease the barrier height required to form a second cycle. Electrons in the cyclohexadienyl group of the spiro and fused rings are much more localized than expected, with the presence of two distinct π bonds. Rate coefficients for each reaction are provided. The unimolecular reactions are fast at most conditions, implying that ring formation and/or ring-opening processes will equilibrate 1-phenyl-hex-x-yl and x-phenyl-hex-1-yl radicals and the spiro and fused isomers.

Thermochemistry and Group Additivity Values for Fused Two-Ring Species and Radicals

“…This work is primarily focused on the thermochemistry of the aforementioned reaction pathways and derivation of GAVs so that other similar pathways could be accurately estimated. A detailed study of the transition states of the same reactions was recently reported by Khanniche et al …”

“…This work is primarily focused on the thermochemistry of the aforementioned reaction pathways and derivation of GAVs so that other similar pathways could be accurately estimated. A detailed study of the transition states of the same reactions was recently reported by Khanniche et al 26 Thermochemistry of Fused Two-Ring Aromatic Species. As found in the previous work, 1 accurate thermochemistry is needed to make correct predictions for product yield.…”

Formation of Two-Ring Aromatics in Hexylbenzene Pyrolysis

“…We are unable to characterize the exact hexylbenzene isomer because of the lack of data of hexylbenzene isomers in the NIST Mass Spectral Library . It is plausible to conclude that the formation of these compounds is due to the phenyl migration pathways studied by Khanniche et al…”

“…Figure shows the model predictions for hexylbenzene isomers formed through the phenyl migration pathway referenced in Lai et al and Khanniche et al The model prediction for hexylbenzene isomers monotonically increases, whereas in experiments, hexylbenzene isomers peak at 40% conversion. This discrepancy is due to the lack of consumption pathways for hexylbenzene isomers in the model.…”

“…Previous studies by this group − , highlighted the sparseness of thermochemical and kinetic database for aromatic intramolecular and intermolecular reactions. As a result, the thermochemistry of 132 species and kinetics of 41 reactions directly relevant to this model compound study were calculated at the CBS-QB3 level of theory.…”