CHEMISTRY OF THE PERFLUORO AND POLYFLUORO ORGANIC COMPOUNDS——THE RELATIVE EASE OF FORMATION OF TWO PERFLUOROALKYL R ADICALS

Abstract

The relative rates of formation of perfluoro iso-propyl radicals (i—C_3F_7,5(i)) and perfluoro n-propyl radicals (n—G_3F_7, 5(n)) are reported. The relative rate constants of β-scission and of bromine abstraction by perfluoro-(α-alkoxyethyl)radicals(RFOCFCF_3:R_F=i—C_3F_7, 3(i); R_F=C_3F_, 3(n)), designated as kr(i) and k,(n), were measured at 7 temperatures in the range of 50—80℃, thence the ratio of the relative rate constants of formation of perfluoro iso-propyl radicals (5(i) and perfluoro n-propyl radicals (5(n)), K_β(i):K_β(n), were obtained. These ratios range from 540—680 at 50℃ to 400—590 at 80℃, much larger than those of the corresponding hydrocarbon radicals. Finally, the differences of activation-free-energy differences between 3(i) and 3 (n), △G~≠(i—n), together with the corresponding enthalpy and entropy terms, △H~≠(i—n), △S~≠(i- n), were calculated. These kinetic parameters indicate that the much higher rate of formation of the perfluoro iso-propyl radicals is caused by both the enthalpy and entropy terms, but quite likely the entropy term is the more important contributor. Apparently, the △S~≠(i—n) value is a manifestation of restricted rotation of the radicals 3(i) caused by the additional CF_3 side group. This restriction is partly relieved in the transition state of β-scission. Therefore, in homolysis, relative ground-state free energies of the reactants (as well as the relative stabilities of the radical intermediates) can affect the relative ease of the cleavage reactions, and the entropy term may sometimes become the underlying cause of these freeenergy differences.