The reactions of CF3 radicals with cyclanes and the determination of C?H bond dissociation energies in cyclanes

The reactions of CF₃ radicals with the C₃ to C₇ cyclanes and spiropentane were studied and the following Arrhenius parameters were obtained for the reaction CF₃ + c-RH → CF₃H + c-R:

c-RH	log A (cm3mole?1sec?1)	E (kcal/mole)	D(c-R—H) (kcal/mole)
Cyclopropane	11.54	8.73	100.7
Cyclobutane	11.66	6.48	95.7
Cyclopentane	12.30	6.18	94.3
Cyclohexane	12.12	6.26	94.9
Cycloheptane	12.43	5.89	94.0
Spiropentane	11.91	8.12	98.8

The CF₃ radicals were generated by photolysis of hexafluoroacetone or CF₃I and a comparison is made of the utility of the two compounds as radical sources. The Arrhenius parameters are compared with those for corresponding reactions of CH₃ radicals with cyclanes, and the general reactivity of cyclic compounds toward free radicals is discussed. An Evans-Polanyi treatment is used to derive C? H bond dissociation energies in cyclanes; and these results, based on the reactions of CF₃ with cyclanes, agree well with those previously obtained using CH₃ plus cyclanes. The final mean values are shown above.