A Pulse Radiolytic Study of the Acid Dissociation of OH Protons in Radicals Related to Salicylic Acid

Article abstract of DOI:10.1021/j100301a032

The deprotonation of carboxylated benzosemiquinone radicals prepared by pulse radiolytic oxidation of dihydroxybenzoic acids has been examined by time-resolved absorption spectrophotometry.The pK_a for dissociation of the OH proton in 3-carboxy-1,4-benzosemiquinone is found to be 6.47 or 2.4 units higher than in the unsubstituted radical.This difference presumably reflects internal hydrogen bonding by the carboxy group.This pK_a is, however, well below that of the OH proton in salicylic acid (13.6) so that hydrogen bonding is appreciably decreased by the delocalization of the unpaired spin in this radical.In the absence of proton acceptors equilibration is relatively slow and in near neutral solutions is controlled by spontaneous loss of the proton from the acidic form with a half-period of 23 μs.Protonation of the basic form of the radical occurs at the diffusion controlled rate.The rate constant for deprotonation by OH^- is relatively low, 4.7E7 M^-1 s^-1, so that reaction with base becomes important only above pH 10.As a result this radical provides an excellent system for studying acid-base equilibration processes in near neutral solutions.Azide ion is shown to be an efficient catalyst which allows the acid-base equilibrium to be examined on the 10-μs time scale.Deprotonation is also catalyzed by the dihydroxybenzoic acid used as the radical source.Analogous studies on 4-carboxy-1,3-benzosemiquinone give the pK_a as 7.9.In spite of this high pK_a, which indicates the rate constant for spontaneous dissociation of this radical to be > 1E3 s^-1, the rate constant for deprotonation by OH^-1, 4.9E8 M^-1 s^-1, is considerably higher than in the case of 3-carboxy-1,4-benzosemiquinone.This latter comparison again indicates that the nature of the spin delocalization has an appreciable effects on the reaction kinetics.