Abstract:The pyrolysis of ethylene-propylene and ethylene-isobutene mixtures has been studied in a static system over the temperature range of 682"-754'K and for initial pressures of each olefin of 33-300 torr. The following molecular ene reactions were observed and the rate constants measured:
25f0.3--"8,000f100000/2.3R?Using thermodynamic data, rate constants for the corresponding retro-ene decomposition reactions were calculated and compared to kinetic data reported for similar compounds. Other products were formed … Show more
“…The activation energy is calculated to be 33.3 kcal/mol. This is similar to the experimental activation energy of 37 kcal/mol 1 The ene reactions studied theoretically. 2 Two views of the Becke3LYP/6-31G* transition structure for the ene reaction of propene with ethylene.…”
The transition structures for the ene reactions of cyclopropene with ethylene, propene, and cyclopropene have been located with ab initio molecular orbital calculations and the 6-31G* basis set and by DFT calculations with the Becke3LYP functional and the 6-31G* basis set. Several of the transition structures have also been located with CASSCF calculations. Energies of all stationary points were also evaluated with second-order Møller-Plesset theory using the RHF/6-31G* optimized geometry. The geometries of each transition structure and the energetics of each reaction are discussed and compared to the ene reaction of propene with ethylene. Calculations show that the cyclopropene ene reactions have much lower activation barriers than the propene-ethylene ene reaction, in agreement with experimental results. The transition structures have varying degrees of asynchronicity. The stabilities of the possible radical intermediates for each reaction are reflected in the geometries of the transition structures. The relief of strain in a cyclopropene, when acting as the enophile, accounts for the energetic differences in these reactions. The endo transition structure for the dimerization is lower in energy than the exo transition structure by 2.7 kcal/mol at the Becke3LYP/6-31G* + ZPE level of theory. Secondary orbital overlap of a CH bond of the enophile with the π-system at the central carbon of the ene is proposed to account for the preference for the endo transition structure. Barely stable diradical intermediates have been found for both endo and exo cyclopropene dimerization reactions, but it is likely that they are artifacts of the current level of theory.
“…The activation energy is calculated to be 33.3 kcal/mol. This is similar to the experimental activation energy of 37 kcal/mol 1 The ene reactions studied theoretically. 2 Two views of the Becke3LYP/6-31G* transition structure for the ene reaction of propene with ethylene.…”
The transition structures for the ene reactions of cyclopropene with ethylene, propene, and cyclopropene have been located with ab initio molecular orbital calculations and the 6-31G* basis set and by DFT calculations with the Becke3LYP functional and the 6-31G* basis set. Several of the transition structures have also been located with CASSCF calculations. Energies of all stationary points were also evaluated with second-order Møller-Plesset theory using the RHF/6-31G* optimized geometry. The geometries of each transition structure and the energetics of each reaction are discussed and compared to the ene reaction of propene with ethylene. Calculations show that the cyclopropene ene reactions have much lower activation barriers than the propene-ethylene ene reaction, in agreement with experimental results. The transition structures have varying degrees of asynchronicity. The stabilities of the possible radical intermediates for each reaction are reflected in the geometries of the transition structures. The relief of strain in a cyclopropene, when acting as the enophile, accounts for the energetic differences in these reactions. The endo transition structure for the dimerization is lower in energy than the exo transition structure by 2.7 kcal/mol at the Becke3LYP/6-31G* + ZPE level of theory. Secondary orbital overlap of a CH bond of the enophile with the π-system at the central carbon of the ene is proposed to account for the preference for the endo transition structure. Barely stable diradical intermediates have been found for both endo and exo cyclopropene dimerization reactions, but it is likely that they are artifacts of the current level of theory.
“…198 The experimental activation energy for this reaction was reported as 37.0 kcal/mol. 35 DFT using the B3LYP functional and 6-31G* basis sets predicts the activation energy for the reaction of propene with ethene to be 33.3 kcal/mol, 198 with a similar value of 31.5 kcal/mol yielded by the MP2 calculations using 6-31G* basis sets. 198 The feasible pathway of the ene reaction (EN-0 in Fig.…”
Section: S413 Ene Reactionmentioning
confidence: 66%
“…Computational results have shown that the reaction mechanism of the ene reaction is concerted, however, the formation of the C-C bond precedes that of the C-H bond. [101][102][103]196 The formation of 1-pentene by thermal pyrolysis of mixtures of propene and ethene is exothermic with the reaction enthalpy of −21.8 kcal/mol at 700 K. 35,197 The computed reaction energy of the ene reaction between propene and ethene is −21.0 kcal/mol with DFT using the B3LYP exchange-correlation functional and 6-31G* basis sets. 198 The MP2 reaction energy using 6-31G* basis sets is −24.5 kcal/mol.…”
Studying organic reaction mechanisms using quantum chemical methods requires from the researcher an extensive knowledge of both organic chemistry and first-principles computation. The need for empirical knowledge arises because any reasonably complete exploration of the potential energy surfaces (PES) of organic reactions is computationally prohibitive. We have previously introduced the Heuristically-Aided Quantum Chemistry (HAQC) approach to modeling complex chemical reactions, which abstracts the empirical knowledge in terms of chemical heuristics—simple rules guiding the PES exploration—and combines them with structure optimizations using quantum chemical methods. The HAQC approach makes use of heuristic kinetic criteria for selecting reaction paths that are not only plausible, that is, consistent with the empirical rules of organic reactivity, but also feasible under the reaction conditions. In this work, we develop heuristic kinetic feasilibity criteria, which correctly predict feasible reaction pathways for a wide range of simple polar (substitutions, additions, and eliminations) and pericyclic organic reactions (cyclizations, sigmatropic shifts, and cycloadditions). In contrast to knowledge-based reaction mechanism prediction methods, the same kinetic heuristics are successful in classifying reaction pathways as feasible or infeasible across this diverse set of reaction mechanisms. We discuss the energy profiles of HAQC and their potential applications in machine learning of chemical reactivity.<br>
“…Computational results have shown that the reaction mechanism of the ene reaction is concerted, however, the formation of the C-C bond precedes that of the C-H bond. 182,216,221 The formation of 1-pentene by thermal pyrolysis of mixtures of propene and ethene is exothermic with the reaction enthalpy of 21.8 kcal/mol at 700 K. 232,233 The computed reaction energy of the ene reaction between propene and ethene is 21.0 kcal/mol with DFT using the B3LYP exchange-correlation functional and 6-31G* basis sets. 234 The MP2 reaction energy using 6-31G* basis sets is 24.5 kcal/mol.…”
Studying organic reaction mechanisms using quantum chemical methods requires from the researcher an extensive knowledge of both organic chemistry and first-principles computation. The need for empirical knowledge arises because any reasonably complete exploration of the potential energy surfaces (PES) of organic reactions is computationally prohibitive. We have previously introduced the Heuristically-Aided Quantum Chemistry (HAQC) approach to modeling complex chemical reactions, which abstracts the empirical knowledge in terms of chemical heuristics-simple rules guiding the PES exploration-and combines them with structure optimizations using quantum chemical methods. The HAQC approach makes use of heuristic kinetic criteria for selecting reaction paths that are not only plausible, that is, consistent with the empirical rules of organic reactivity, but also feasible under the reaction conditions. In this work, we develop heuristic kinetic feasilibity criteria, which correctly predict feasible reaction pathways for a wide range of simple polar (substitutions, additions, and eliminations) and pericyclic organic reactions (cyclizations, sigmatropic shifts, and cycloadditions). In contrast to knowledge-based reaction mechanism prediction methods, the same kinetic heuristics are successful in classifying reaction pathways as feasible or infeasible
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