SUBSTITUENT EFFECTS ON THE STABILITIES OF PHENOXYL RADICALS AND THE ACIDITIES OF PHENOXYL RADICAL CATIONS

Oxidation potentials for 35 phenoxide ions and 3 naphthoxide ions have been combined with their pK(HA) values to estimate homolytic bond dissociation energies (BDEs) for the O-H bonds in phenols. Comparison with literature values shows that there is remarkably good agreement between DELTA-BDE values determined by different methods. A plot of oxidation potentials for m-phenoxides vs pK(HA) values was found to be linear over a range of 18 kcal/mol, pointing to the presence of an inherent group electronegativity factor, related to basicity, that strengthens the O-H bond in phenols. Deviations of points for para substituents from this line have provided a measure of their radical-stabilizing ability that is devoid of such inherent bond-strengthening effects. A good correlation of E(ox)(A-) values for p-GC6H4O- phenoxide ions with sigma+ constants was observed over a range of greater than 40 kcal/mol. The acidities of 35 phenoxyl radical cations have been estimated from pK(HA), E(ox)(A-), and E(ox)(HA) values. A good correlation of E(ox)(HA) vs pK(HA).+ was observed for m-GC6H4OH.+ radical cations, but the points for para donors were found to deviate from the line.