THRESHOLD ENERGIES AND UNIMOLECULAR RATE CONSTANTS FOR ELIMINATION OFHF FROM CHEMICALLY ACTIVATED CF3CH2CH3 AND CF3CH2CF3 - EFFECT OF CH3 AND CF3 SUBSTITUENTS AT THE BETA-CARBON AND IMPLICATIONS ABOUT THE TRANSITION-STATE

Chemically activated CF3CH2CF3 was prepared with 104 kcal/mol of inter nal energy by the combination of CF3CH2 and CF3 radicals, and chemical ly activated CF3CH2CH3 was prepared with 101 and 95 kcal/mol by combin ation of CF3 and CH2CH3 radicals and by combination of CF3CH2 and CH3 radicals, respectively. The experimental rate constants for unimolecul ar 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) w ith 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RP,KM theory to the experimental values provided t hreshold energies, E-0, of 73 kcal/mol for CF3CH2CF3 and 62 kcal/mol f or CF3CH2CH3. Comparing these threshold energies to those for CF3CH3 a nd 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 electro n acceptor, increases the E-0 an amount similar to Cl, suggesting that chlorine substituents also prefer to withdraw electron density from t he beta-carbon. ils the HF transition state forms, it appears that ele ctron density flows from the departing hydrogen to the beta-carbon and from the beta to the alpha-carbon, to the alpha-carbon from its subst ituents, but the alpha-carbon releases most of the incoming electron d ensity to the departing fluorine. The present work supports this scena rio because electron-donating substituents, such as CH3, on either car bon would reduce the E-0 as they aid the flow of negative charge, whil e electron-withdrawing substituents such as Cl, F, and CF3 would raise the E-0 for HF elimination because they hinder the flow of electron d ensity.