Stereochemistry of the four-centered gas-phase dehydrohalogenation reaction

Paisley, Steven D.; Holmes, Bert E.

doi:10.1021/j100239a017

Cited by 9 publications

(9 citation statements)

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“…It is interesting to compare our data with published pyrolysis studies of unimolecular elimination experiments on chemically activated mixed halogenated species [37][38][39][40][41][42][43][44][45][46][47][48]. In all these experiments HCl loss was dominant.…”

Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 64%

“…This class of reactions has been subjected to intensive investigations [35,36]. The related unimolecular process has been assumed to involve a planar and polar four-centred transition state, and it has been shown [37] that syn elimination governs the removal of the hydrogen halide. Common methods for determining experimental reaction rates are to provide the unimolecular reactants with energy in excess of the minimum that leads to HX formation.…”

Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 99%

“…These include thermal heating, photolytic excitation, and chemical activation. In the last 20 years, this last technique has been applied intensively by Holmes and co-workers [37][38][39][40][41][42][43][44] for determining the rate constants for elimination of HCl and HF from hydrofluorocarbons and hydrochlorofluorocarbons. The resulting experimental data have been matched to rates calculated using the Rice-Rampsberger-Kassel (RRK) unimolecular theory [45] and the modified RRK version (RRKM) [36] to determine E o (HX), the threshold energy for HF or HCl removal (see Table 5).…”

Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 99%

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Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Giannetti

2005

Journal of Fluorine Chemistry

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Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 64%

Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 99%

Section: Hx Elimination From Partially Fluorinated Polymersmentioning

confidence: 99%

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Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Giannetti

2005

Journal of Fluorine Chemistry

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“…The second order elimination mechanism, E 2 , is assisted by a base and involves a planar transition state with an anti -configuration; i.e., the halogen and hydrogen are removed from opposite sides of the C−C bond. In the gas phase, the 1,2-dehydrohalogenation reaction has been assumed , to involve a planar and polar four-centered transition state, and it has been shown that syn -elimination governs the removal of the hydrogen halide. For the past 3 decades − experiments have been designed to better understand the polarity of the 1,2-dehydrohalogenation transition state, primarily by measuring the effect of different substituents on the rate constant and on the threshold energy barrier, E 0 .…”

Section: Introductionmentioning

confidence: 99%

Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl₂CH₃ and CFCl₂CD₃

and

Holmes

1996

J. Phys. Chem.

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Combination of CFCl2 and methyl-d 0 and -d 3 radicals form CFCl2CH3-d 0 and -d 3 with 100 and 101 kcal/mol of internal energy, respectively. An upper limit for the rate constant ratio of disproportionation to combination, k d/k c, for Cl transfer is 0.07 ± 0.03 for collision of two CFCl2 radicals and 0.015 ± 0.005 for CH3 and CFCl2 radicals. The chemically activated CFCl2CH3 undergoes 1,2-dehydrochlorination and 1,2-dehydrofluorination with rate constants of 3.9 × 109 and 4.9 × 107 s-1, respectively. For CFCl2CD3 the rate constants are 8.7 × 108 s-1 for loss of DCl and 1.1 × 107 s-1 for DF. The kinetic isotope effect is 4.4 ± 0.9 for HCl/DCl and appears to be identical for HF/DF. Threshold energies are 54 kcal/mol for loss of HCl and 68 kcal/mol for HF; the E 0's for the deuterated channels are 1.4 kcal/mol higher. Comparison of these threshold energies with other haloethanes suggests that for HF and HCl elimination the transition states are developing charges of different signs on the carbon containing the departing halogen and that chlorine and fluorine substituents exert similar inductive effects.

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“…For several years we have been measuring − unimolecular rate constants for the 1,2-dehydrohalogenation of chemically activated hydrofluorocarbons and hydrochlorofluorocarbons, and by fitting rate constants calculated using the RRKM theory to the experimental values, threshold energies, E 0 's, have been determined. By altering substituents and measuring their effect on the rate constant and on the threshold energy barrier, it should be possible to better understand the flow of electron density as the four-centered transition state for chloro- and fluoroalkanes forms and also to understand whether substituents act as electron donors or acceptors.…”

Section: Introductionmentioning

confidence: 99%

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State

Ferguson

Holmes

1998

J. Phys. Chem. A

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Chemically activated CF 3 CH 2 CF 3 was prepared with 104 kcal/mol of internal energy by the combination of CF 3 CH 2 and CF 3 radicals, and chemically activated CF 3 CH 2 CH 3 was prepared with 101 and 95 kcal/mol by combination of CF 3 and CH 2 CH 3 radicals and by combination of CF 3 CH 2 and CH 3 radicals, respectively. The experimental rate constants for unimolecular 1,2-dehydrofluorination were 1.2 × 10 5 s -1 for CF 3 CH 2 CF 3 and 3.2 × 10 6 s -1 for CF 3 CH 2 CH 3 with 95 kcal/mol and 2.0 × 10 7 s -1 with 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RRKM theory to the experimental values provided threshold energies, E 0 , of 73 kcal/mol for CF 3 CH 2 CF 3 and 62 kcal/mol for CF 3 CH 2 CH 3 . Comparing these threshold energies to those for CF 3 CH 3 and CF 3 CH 2 Cl illustrates that replacing the hydrogen of CF 3 CH 3 with CH 3 lowers the E 0 by 6 kcal/mol and replacing with CF 3 or Cl raises the E 0 by 5 and 8 kcal/mol, respectively. The CF 3 substituent, an electron acceptor, increases the E 0 an amount similar to Cl, suggesting that chlorine substituents also prefer to withdraw electron density from the β-carbon. As the HF transition state forms, it appears that electron density flows from the departing hydrogen to the β-carbon and from the β to the R-carbon, to the R-carbon from its substituents, but the R-carbon releases most of the incoming electron density to the departing fluorine. The present work supports this scenario because electron-donating substituents, such as CH 3 , on either carbon would reduce the E 0 as they aid the flow of negative charge, while electron-withdrawing substituents such as Cl, F, and CF 3 would raise the E 0 for HF elimination because they hinder the flow of electron density.

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Stereochemistry of the four-centered gas-phase dehydrohalogenation reaction

Cited by 9 publications

References 5 publications

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl₂CH₃ and CFCl₂CD₃

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State

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Stereochemistry of the four-centered gas-phase dehydrohalogenation reaction

Cited by 9 publications

References 5 publications

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl2CH3 and CFCl2CD3

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF3CH2CH3 and CF3CH2CF3: Effect of CH3 and CF3 Substituents at the β-Carbon and Implications about the Transition State

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Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl₂CH₃ and CFCl₂CD₃

Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF₃CH₂CH₃ and CF₃CH₂CF₃: Effect of CH₃ and CF₃ Substituents at the β-Carbon and Implications about the Transition State