HF‐SbF5 catalysed isomerization of n‐butane‐1‐13C

Abstract

Abstractn‐Butane‐1‐13C is isomerized by “HF‐SbF5” to n‐butane‐2‐13C without intermediate or concurrent isomerization to isobutane. The rate of the13C‐scrambling reaction is only slightly lower than that of the isomerization of n‐pentane to isopentane. This result strongly supports the hypothesis that alkylcarboniumion rearrangements that involve a change in the degree of chain branching proceed via a protonated cyclopropane ring. That no isobutane is formed can be explained by the ring opening of protonated methylcyclopropane to the isobutyl structure being “slow” because it involves the formation of an (incipient) primary ion. This is not contradictory to the fact that t‐butyl ions are formed in strong acids from n‐butyl precursors under different conditions. The difference between HF‐SbF5 catalysed and aluminium halide‐hydrogen halide catalysed isomerizations of butane is discussed.