Reactivity of gas-phase anions with fully halogenated alkanes

Abstract

Gas-phase reactivity of negative ions with fully halogenated alkanes is reviewed, restricted to rate constant measurements under thermal conditions. Reactivity generally increases upon substitution by heavier halogens. Perfluorocarbons (PFCs) are generally unreactive with negative ions, but the highly reactive O⁻ anion reacts with the larger PFCs through multiple product channels. The trifluoromethyl halide series CF₃X (X = Cl, Br, I) has been well-studied, and the reactions tend to be fast when nondissociative electron transfer to the halocarbon would be exothermic, regardless of the actual resulting product channels. For many of the reactions of CF₃X and other perhaloalkanes, reactivity correlates with both the electron affinities and the electron attachment rates of the perhaloalkanes, though notable exceptions exist. This and other evidence suggests that many of the reactions are initiated by electron transfer to the halocarbon, which tends to explain why the reactivity of CF₃X generally increases in the order Cl < Br < I for second-order reactions. The same reactivity trend in third-order association reactions is explained by the corresponding trends in CF₃X dipole moment and polarizability. The results of several studies show that negative ion reactions are not a significant removal process for halocarbons in the atmosphere.