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

Abstract n ‐Butane‐1‐ 13 C is isomerized by “HF‐SbF 5 ” to n ‐butane‐2‐ 13 C without intermediate or concurrent isomerization to isobutane. The rate of the 13 C‐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‐SbF 5 catalysed and aluminium halide‐hydrogen halide catalysed isomerizations of butane is discussed.