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

Chemically activated CF3CH2CF3 was prepared with 104 kcal/mol of internal energy by the combination of CF3CH2 and CF3 radicals, and chemically activated CF3CH2CH3 was prepared with 101 and 95 kcal/mol by combination of CF3 and CH2CH3 radicals and by combination of CF3CH2 and CH3 radicals, respectively. The experimental rate constants for unimolecular 1,2-dehydrofluorination were 1.2 × 105 s-1 for CF3CH2CF3 and 3.2 × 106 s-1 for CF3CH2CH3 with 95 kcal/mol and 2.0 × 107 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, E0, of 73 kcal/mol for CF3CH2CF3 and 62 kcal/mol for CF3CH2CH3. Comparing these threshold energies to those for CF3CH3 and CF3CH2Cl illustrates that replacing the hydrogen of CF3CH3 with CH3 lowers the E0 by 6 kcal/mol and replacing with CF3 or Cl raises the E0 by 5 and 8 kcal/mol, respectively. The CF3 substituent, an electron acceptor, increases the E0 an amount similar to Cl, su...