Pulse radiolytic study of the acid dissociation of OH protons in radicals related to salicylic acid

Journal Article · Thu Aug 13 00:00:00 EDT 1987 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j100301a032· OSTI ID:5674123

Sun, Q; Schuler, R H

The deprotonation of carboxylated benzosemiquinone radicals prepared by pulse radiolytic oxidation of dihydroxybenzoic acids has been examined by time-resolved absorption spectrophotometry. The pK/sub a/ for dissociation of the OH proton in 3-carboxyl-1,4-benzosemiquinone is found to be 6.47 or 2.4 units higher than that in the unsubstituted radical. This pK/sub a/ is, however, well below that of the OH proton in salicyclic acid (13.6) so that hydrogen bonding is appreciably decreased by the delocalization of the unpaired spin in this radical. Protonation of the basic form of the radical occurs at the diffusion-controlled rate. The rate constant for deprotonation by OH/sup -/ is relatively low, 4.7 X 10/sup 7/ M/sup -1/ s/sup -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-..mu..s 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/sub a/ as 7.9. In spite of this high pK/sub a/, which indicates the rate constant for spontaneous dissociation of this radical to be > 10/sup 3/ s/sup -1/, the rate constant for deprotonation by OH/sup -/, 4.9 X 10/sup 8/ M/sup -1/ s/sup -1/, is considerably higher than in the case of 3-carboxy-1,4-benzosemiquinone.

• Research Organization: Univ. of Notre Dame, IN
• OSTI ID: 5674123
• Journal Information: J. Phys. Chem.; (United States), Vol. 91:17
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
REACTION INTERMEDIATES
DISSOCIATION
SALICYLIC ACID
RADIOLYSIS
ABSORPTION SPECTROSCOPY
CHEMICAL RADIATION EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
PH VALUE
THERMODYNAMIC ACTIVITY
THERMODYNAMICS
CARBOXYLIC ACIDS
CHEMISTRY
DECOMPOSITION
HYDROXY ACIDS
KINETICS
ORGANIC ACIDS
ORGANIC COMPOUNDS
RADIATION CHEMISTRY
RADIATION EFFECTS
REACTION KINETICS
SPECTROSCOPY
400600* - Radiation Chemistry