Threshold energy and unimolecular rate constant for elimination of HF from chemically activated CF3CF2CH3: Effect of the CF3 substituent on the alpha-carbon

Methyl and CF3CF2 radicals were combined to form chemically activated CF3CF2CH3 with 104 kcal/mol of internal energy, and the experimental rate constant for unimolecular 1,2-dehydrofluorination was 4.5 x 10(5) s(-1). Fitting the calculated rate constant for HF elimination from RRKM theory to the experimental value provided a threshold energy, E(0), of 68.5 kcal/mol. Comparing this threshold energy to those for CF2HCH3, CH3CF2CH3, CF2ClCH3, and CF3CH3 shows that replacing the alpha-H of CF2HCH3 with CH3 lowered the E(0) by 7 kcal/mol and replacing with CF3, Cl, or F raises the E(0) about 8 kcal/mol. The CF3 substituent, an electron acceptor, increased the E,by an amount nearly equal to that with F and Cl substituents, suggesting that halogen substituents exert a similar inductive effect at the alpha-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.