Temperature Dependence of the Substrate and Positional Selectivity of the Aromatic Substitution by Gaseous tert-Butyl Cation

Article abstract of DOI:10.1021/ja00265a006

The temperature dependence of the substrate and positional selectivity of the alkylation of benzene and toluene by radiolytically formed t-Bu⁺ ions has been investigated in isobutane gas at 720 torr in the range 0-140 deg C.The Arrhenius plot of the empirical k_T/k_B ratio is linear over an extended (0-100 deg C) interval, and its slope corresponds to a difference of 3.6 +/- 0.4 kcal mol^-1 between the activation energy for the tert-butylation of benzene and of toluene.The positional selectivity of the alkylation is affected by the temperature as well, the p:1/2 m ratio decreasing from ca. 27 at 0 deg C to 7.5 at 140 deg C.The results are interpreted as evidence that under conditions typical of radiolytic experiments the activation mechanism of the addition of t-Bu⁺ to arenes, a typical ion-molecule reaction, is essentially thermal, as expected at the high-pressure limit of gas-phase ionic processes.