Kinetics of rearrangements of methylbenzenium ions in HF

Rate constants and activation parameters of the following methylbenzenium ion rearrangements have been determined by direct measurement in HF containing SbF5: o‐xylenium and p‐xylenium to m‐xylenium, 1,2,3,4‐ to 1,2,3,5‐tetramethylbenzenium, 1,2,3‐ to 1,2,4‐ and 1,2,4‐ to 1,3,5‐trimethylbenzenium ions. The rate constants at 25° are 80, 335, 155, 8.9 and 1.4 × 10−5 s−1, respectively. The differences in rate of these rearrangements, which take place by successive 1,2‐hydrogen and methyl shifts with the latter as rate‐determining steps, are entirely due to differences in heat of activation, which varies from 19.8 to 23.3 kcal · mole−1, the activation entropy being about −3 cal · mole−1 · deg−1. These differences can be fully rationalized by considering the stabilities of the intermediate cations. Data on the relative stabilities of isomeric methylbenzenium ions are presented. In methylbenzenium ions the 1,2‐methyl shifts proper, the free energies of reaction of which vary from −6.5 to +6.5 kcal · mole−1, have free energies of activation of 15 to 22 kcal · mole−1, corresponding to rate constants of 35.5 to 0.00065 s−1 at 25°.