H/D exchange, protolysis and oxidation of C3–C5alkanes in HF–SbF5. σ-Basicity vs. reactivity of C–H bonds

Abstract

The relative reactivity and basicity of CH bonds in C₃–C₅ alkanes was studied using the strongest deuterated superacid, DF–SbF₅, at various concentrations of SbF₅. In all cases, at concentrations up to approximatively 18 mol % SbF₅ in DF(HF), the predominant reaction, beside minor protolytic cleavage, was rapid isotope exchange of all C–H bonds via reversible protonation. The relative rate of exchange follows the Olah σ-basicity concept: tertiary > secondary > primary C–H bonds independently of the further reactivity of these bonds. At higher concentrations of SbF₅ the exchange process gives way to increasing ionization of the alkane, first via protolysis and later via an oxidative process with concomitant reduction of SbF₅.