Abstraction of Allylic Hydrogen of Propene by Alkyl Radicals. Thermochemistry and Kinetic Study

摘要

Hydrogen abstractions reactions from the methyl carbon of propene, which is used to represent an alkene with an adjacent methyl group, are analyzed for five alkyl radicals C~·H3, C2H~·5, n-C3H~·7, iso-C3H~·7 and CH2=C~·H (vinyl). The objective is to study (review) the thermochemistry and evaluate the kinetic rate constants from the allylic radical formation from loss of a methyl-hydrogen, as compared to abstractions from alkane methyl groups. These initial studies utilize the B3LYP76-311G (2d,p) Density Functional Theory (DFT). Enthalpies of formation are further improved using CBS-QB3 and G3 composite methods and DFT structures. Statistical mechanics and Transition State Theory have been employed to investigate the reaction kinetics. Pre -exponential factors for the bimolecular formation of the transition states, both forward and reverse, show strong temperature dependence; this is interpreted using the thermodynamic (entropy) properties of the TST structures. Barriers are observed to be higher than would be estimated from generic rules for alkyl radical abstractions from alkanes where enthalpy of reaction is similar. The objective is to try and determine and understand the difference for abstraction of allylic H atoms, where a resonant radical is formed, as compared to abstraction from alkanes. Quantum tunneling effects were analyzed with both the Wigner and Eckart methods and are compared to available literature data.