PMR spectroscopic investigation of structures, stabilities, and rearrangement reactions of C4‐C7tertiary carbonium ions in HF‐SbF5

Solutions in HF‐SbF5 of the tertiary butyl, pentyl, hexyl, methylcyclopentyl and heptyl cations have been investigated by PMR spectroscopy. A variety of procedures, conventional as well as novel ones, for preparing stable solutions of these cations are briefly described. Tertiary hexyl and heptyl cations rearrange rapidly to equilibrium mixtures of the isomeric tertiary hexyl and heptyl cations, respectively. The equilibrium compositions show the differences in thermodynamic stability of isomeric tertiary carbonium ions to be small. Equilibrations effected by a single 1,2‐hydride or methyl shift between equivalent positions have half‐lives of less than 10−3 s at ‐ 60 °C. Rearrangements of the type tert‐2‐methylpentyl to tert‐3‐methylpentyl cation have a half‐life of about 0.1 s at 0° (Eact = ∼ 14 kcal/mole, and Sact = ∼ 0). For these rearrangements a stepwise mechanism by successive hydride, methyl and hydride shifts via secondary carbonium ions is in agreement with the experimental data. Rearrangements of tertiary carbonium ions involving a change in the degree of chain branching (monobranched to linear or to dibranched cations) have half‐lives of the order of 10 s at 0°.