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 x 10(5) s(-1) for CF3CH2CF3 and 3.2 x 10(6) s(-1) for CF3CH2CH3 with 95 kcal/mol and 2.0 x 10(7) s(-1) with 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RP,KM theory to the experimental values provided threshold energies, E-0, 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 E-0 by 6 kcal/mol and replacing with CF3 or Cl raises the E-0 by 5 and 8 kcal/mol, respectively. The CF3 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 beta-carbon. ils the HF transition state forms, it appears that electron density flows from the departing hydrogen to the beta-carbon and from the beta to the alpha-carbon, to the alpha-carbon from its substituents, but the alpha-carbon releases most of the incoming electron density to the departing fluorine. The present work supports this scenario because electron-donating substituents, such as CH3, 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 CF3 would raise the E-0 for HF elimination because they hinder the flow of electron density.