Oxidation of Phenol in Aqueous Acid:  Characterization and Reactions of Radical Cations vis-à-vis the Phenoxyl Radical

Aqueous sulfuric acid containing up to ∼14 M acid (H 0 ≥ −7.0) was used as solvent in pulse radiolytic redox studies to characterize cationic transients of phenol (C6H5OH) and map their reactions. The primary radical yields were first measured to correlate the variation in various radical concentrations as a function of increasing acid fraction in the solvent. Compared to their respective values at pH 2, the G(Ox • ) increased with almost a linear slope of ∼0.024 μmol J-1 for H 0 -1 (or pH-1) up to H 0 −6.0 (Ox • = •OH + SO4 •-), whereas G(H•) increased with a slope of ∼0.033 μmol J-1 for H 0 -1 (or pH-1) up to H 0 −5.0. In the presence of >10 M acid (H 0 < −5.0), phenol was oxidized to its radical cation, C6H5OH•+, which further reacted with phenol and generated the secondary, dimeric radical cation, (C6H5OH)2 •+, following an equilibrium reaction C6H5OH•+ + C6H5OH ⇋ (C6H5OH)2 •+, with K eq = 315 ± 15 M-1. The two cationic radicals were characterized from their individual UV−vis absorption spectra and acidity. The C6H5OH•+ absorption peaks are centered at 276 and 419 nm, and it was found to be more acidic (pK a = −2.75 ± 0.05) than (C6H5OH)2 •+ (pK a = −1.98 ± 0.02), having its major peak at 410 nm. On the other hand, in the presence of