One-Electron Reduction of Carbonium Ions. IV. A Kinetic Study on the Reduction of the Substituted Tropylium Ions with Cr(II)

Abstract

Abstract The one-electron reduction of the unsubstituted, methyl-, ethyl-, isopropyl-, t-butyl-, triphenylmethyl-, and phenyltropylium ions with Cr(II) in a 10% HCl solution gives, quantitatively, the dimer of the corresponding substituted tropyl radical. The measurements of the reaction rate at 25°C exhibit this order of reactivity: t-butyltropylium (k2=7.98 l/g·ion·sec), isopropyltropylium (8.22), ethyltropylium (10.3), methyltropylium (11.1), tropylium (74.0), phenyltropylium (144), and triphenylmethyltropylium (567) ions. The values of log k2 have a linear correlation with the transition energies of the charge-transfer bands observed for these carbonium ions with pyrene, and also with the polarographic half-wave potentials of the respective cations. These correlations indicate that the reactivity of the carbonium ion in the reduction with Cr(II) is determined mainly by the electron affinity inherent in the respective cations. The values of log k2 also have a good linear correlation with the pKR+ values, implying a parallelism between the electron affinity and the electrophilicity of these stable carbonium ions.