Threshold Energy and Unimolecular Rate Constant for Elimination of HF from Chemically Activated CF₃CF₂CH₃:  Effect of the CF₃ Substituent on the α-Carbon

Methyl and CF 3 CF 2 radicals were combined to form chemically activated CF 3 CF 2 CH 3 with 104 kcal/mol of internal energy, and the experimental rate constant for unimolecular 1,2-dehydrofluorination was 4.5 × 10 5 s -1 . Fitting the calculated rate constant for HF elimination from RRKM theory to the experimental value provided a threshold energy, E o, of 68.5 kcal/mol. Comparing this threshold energy to those for CF 2 HCH 3, CH 3 CF 2 CH 3, CF 2 ClCH 3, and CF 3 CH 3 shows that replacing the α-H of CF 2 HCH 3 with CH 3 lowered the E o by 7 kcal/mol and replacing with CF 3, Cl, or F raises the E o about 8 kcal/mol. The CF 3 substituent, an electron acceptor, increased the E o by an amount nearly equal to that with F and Cl substituents, suggesting that halogen substituents exert a similar inductive effect at the α-carbon that loses electron density as the transition state forms. These proposals will be compared to recent calculations of the carbon's atomic charges in the reactant and transition state.