The Electrophilic Activation Pathways for N-Butane in Hf-Sbf5 That Compete With Fast Reversible Protonation

The relative importance of competitive activation pathways for n-butane in HF-SbF₅ superacid has been studied in a flow reactor over the -15 °C to +15 °C temperature range. In order to inhibit the reversible protide transfer between the hydrocarbon and the carbenium ions generated in the superacid solution, all experiments were run in the presence of carbon monoxide, thereby converting the reactive ions into stable oxocarbenium ions. Analysis of the product distribution between gas and acid phases identifies the main activation step as being protolytic cleavage of the central C-C bond, forming ethane and the ethyl cation. The secondary butyl cation is formed via hydride abstraction from n-butane by the primary ions formed in the first step. No isomerization of n-butane was noticed under these conditions. Fast reversible protonation of the σ-bonds of n-butane is evidenced by observation of significant protium-deuterium exchange in the "recovered" alkane when DF-SbF₅ was used as superacid.

Affiliations: 1: Laboratoire de Physico-Chimie des Hydrocarbures, Centre de Recherches Chimie, Université Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg, France