Pulse Radiolysis and Electron Spin Resonance Studies on the Formation of Phenoxyl Radicals by Reaction of OH Radicals with Methoxylated Phenols and Hydroxybenzoic Acids

Abstract

OH radicals react with methoxylated phenols and methoxylated hydroxybenzoic acids by addition to the aromatic ring to yield hydroxycyclohexadienyl radicals (OH adducts) (k = (1—2) · 1010 M−1 s−1). With respect to the sites of attachment, OH shows a preference for ring positions activated by the electron donating OH or OCH3 groups. Only those hydroxycyclohexadienyl radicals which are produced by addition of OH to ring positions not occupied by methoxyl groups yield phenoxyl type radicals. Formation of phenoxyl radicals is enhanced in the presence of acid or base. Acid catalysis of phenoxyl formation is suggested to proceed via cationic intermediates similar to those proposed for formation of radical cations. In alkaline solution, OH− is eliminated from deprotonated hydroxycyclohexadienyl radicals to yield phenoxyl radicals (k > 106 s−1).—The phenoxyl radicals undergo reaction with a) each other (2k ∼ 1 · 109 M−1 s−1), b) OH adducts (k ∼ 1.3 · 109 M−1 s−1), and c) H adducts (k ∼ 4 · 109 M−1 s−1). The absorption maxima of the isomeric methoxyphenoxyl radicals are blue‐shifted relative to those of radical cations from structurally analogous di‐ and trimethoxybenzenes.

Kinetic and Theoretical Study of the Atmospheric Aqueous-Phase Reactions of OH Radicals with Methoxyphenolic Compounds

“…The rate constant of the reaction of 2-MP with OH radicals at T = 298 K is k 298K = (1.1 ± 0.1) × 10 10 L mol –1 s –1 . O’Neill and Steenken reported a second-order rate constant of k = 1.5 × 10 10 L mol –1 s –1 using a pulse radiolysis method at a similar temperature (293 K). This value is in close agreement with our measured value and is similar to the diffusion-limited rate constant of k D,298K = 1.4 × 10 10 L mol –1 s –1 (cf.…”

“…The rate constant of the para-substituted compound 4-MP was reported to be between that of 2-MP and 3-MP. 26 On the other hand, Cre, Syr, and 3-MC undergo faster reactions with OH radicals than 2-MP and 3-MP. Compared to monosubstituted phenols, each of the three compounds has an additional electron-donating group (EDG), i.e., −CH 3 , −OCH 3 , or −OH.…”

“…To the best of our knowledge, only two studies exist on OH reactivity toward methoxyphenols at room temperature. O’Neill and Steenken report rate constants ( k ) of OH reactions with nine methoxyphenolic compounds through the use of pulse radiolysis. Rate constants of OH radicals with guaiacol, syringol, and resorcinol were determined by Smith et al using a relative rate method, and the probe rate constants were taken from the results of O’Neill and Steenken .…”

“…To the best of our knowledge, only two studies exist on OH reactivity toward methoxyphenols at room temperature. O'Neill and Steenken 26 report rate constants (k) of OH reactions with nine methoxyphenolic compounds through the use of pulse radiolysis. Rate constants of OH radicals with guaiacol, syringol, and resorcinol were determined by Smith et al 27 using a relative rate method, and the probe rate constants were taken from the results of O'Neill and Steenken.…”

Supplementary material to "Photooxidants from Brown Carbon and Other Chromophores in Illuminated Particle Extracts"

“…1 Syringol 9.6E-11 (Lauraguais et al, 2012) 4.0E-19 b (Zein et al, 2015) 2.6E+10 (O'Neill and Steenken, 1977) 3.6E+07 (Tratnyek and Hoigne, 1991a) 1.3E+04 c (Hoigné and Bader, 1983) 3.7E+09 (Kaur and Anastasio, 2018), (Smith et al, 2015) 2…”