Journal of Physical Chemistry B p. 9567 - 9575 (2002)
Update date:2022-08-11
Topics:
Waugh
Farrokhnia
Sakakini
A kinetic and mechanistic study of the Cl/F exchange reaction of CCl3F, CCl2F2, and CClF3 with prefluorided chromia was conducted. The activation energies of 53 and 54 kJ/mol for the simultaneous mono- and bi-exchange of the chlorine atoms of CCl3F with the surface fluoride ions of a prefluorided chromia were lower than that found for CCl4 (65 kJ/mol). This was due to the stronger C-F bond formed in CCl2F2 and CClF3, the C-Cl bond strengths in CCl4 and CCl3F were roughly the same. Mono-exchange occurred through a four-center intermediate in which the breaking of a C-Cl bond is compensated for by the formation of a C-F bond. Bi-exchange occurred by the CCl3F adsorption by the bonding of two Cl atoms to two appropriately spaced Cr ions on the CrF surface. Mono-exchange only was observed in the reaction of CCl2F2, the activation energy for which was 84 kJ/mol. This higher activation energy was due to the higher C-Cl (~ 318 kJ/mol) bond strength in CCl2F2 than in CCl3F (D(C-Cl) = 305 kJ/mol). The absence of the occurrence of bi-exchange producing CF4 was due to the instantaneous formation of CClF3 on the intermediate, which was adsorbed through two chlorine atom where bi-exchange should have been possible. No exchange of the C-Cl bond of CClF3 with the surface CrF was observed.
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