N–H bond dissociation energies, reduction potentials and pK_as of multisubstituted anilines and aniline radical cations

Abstract

The one-electron reduction potential and the pKa of 10 ortho-, meta- and para-substituted aniline radical cations have been determined by means of pulse radiolysis. The N–H bond dissociation energies of the corresponding anilines were also determined using a thermodynamic cycle. All three properties of the aniline radical cations and the corresponding anilines were shown to be linearly dependent on the sum of the Brown substituent constants, Σσ+. A conditional scale for ortho substituents was also derived (σo+= 0.73σp+). The results from this work along with previously published results have been used to derive a linear free energy relationship between one-electron reduction potentials of benzene radical cations and the substituent pattern. In addition an equation for the calculation of X–Y bond dissociation energies of arbitrarily substituted molecules with the general formula Ph–X–Y is proposed.