Ozonolysis of phenols in aqueous solution

Abstract

In the ozonolysis of phenol in aqueous solution at pH 3, 7 and 10 the following products were quantified: catechol, hydroquinone, 1,4-benzoquinone, cis,cis-muconic acid, H2O2, 2,4-dihydroxybiphenyl and 4,4-dihydroxybiphenyl. At pH 10, material balance (products vs. phenol consumption) is obtained. Singlet dioxygen, O2(1 delta g), and .OH are formed as short-lived intermediates. The precursor of the latter, O3.-, and a phenoxyl radical is suggested to arise from electron transfer from phenol/phenolate to ozone. Addition of .OH to phenol gives rise to dihydroxycyclohexadienyl radicals which add dioxygen and eliminate HO2. thereby forming catechol/hydroquinone. In competition and catalysed by H+ and OH-, the dihydroxycyclohexadienyl radical eliminates water yielding a phenoxyl radical. At pH 10, they readily oxidize catechol and hydroquinone. This reforms phenol (accounting for the low phenol consumption) and yields higher-oxidised products, eventually 1,4-benzoquinone. cis,cis-Muconic acid can be accounted for by the Criegee mechanism, while O2(1 delta g) is released on the way to (some of the) catechol and hydroquinone. Similar reactions proceed with hydroquinone (products: 1,4-benzoquinone, 2-hydroxy-1,4-benzoquinone and H2O2, with high yields of O2(1 delta g) and .OH) and with catechol (products: 2-hydroxy-1,4-benzoquinone, cis,cis-muconic acid, H2O2 with high yields of O2(1 delta g) and .OH). Material balance is not obtained for these two systems. Pentachlorophenolate, pentabromophenolate and 2,4,6-triiodophenolate ions give rise to halide ions, O2(1 delta g) (58%/48%/10%) and .OH (27%/2%/0%). It is suggested that together with O2(1 delta g) the corresponding ortho- and para-quinones plus a halide ion are formed. Further halide ion is released upon the hydrolysis of these and other products. For pentachlorophenolate the material balance with respect to the short-lived intermediates is 85%. With the bromo- and iodophenolates the O2(1 delta g) yields are substantially lowered, most likely due to release of triplet (ground state) dioxygen induced by the heavy atom effect.

Protein Cross-Linking and Oligomerization through Dityrosine Formation upon Exposure to Ozone

“…For instance, phenolate ions are much more reactive toward O 3 than phenols, i.e., by a factor of ∼10 6 . 39 However, it is difficult to determine the amounts of conjugated ions of Tyr in our flowtube experiments. Local pH and exact pK a (of the phenolic hydrogen) values for the individual Tyr residues are hard to determine under these experimental conditions.…”

“…According to this equation, ∼10 −5 to ∼10 −3 dissociated ions per residue should be present at pH 5−7, respectively. By a simple division of the rate constant of Tyr + O 3 calculated by the model in this study (∼6 × 10 5 −3 × 10 7 M −1 s −1 ) with the rate constant of phenolate + O 3 reported by Mvula and von Sonntag (1.4 × 10 9 M −1 s −1 ), 39 we may estimate that ∼4 × 10 −4 to ∼2 × 10 −2 of the reactive Tyr residues in BSA were present in the form of dissociated ions under the experimental conditions applied in this study, indicating neutral to slightly acidic pH in the flow tube experiments. Further, rate constants of Tyr + O 3 reported in the literature (0.7−2.8 × 10 6 M −1 s −1 in neutral pH, 52 and 7.2 × 10 7 M −1 s −1 in aqueous medium at 298 K calculated using variational transition state theory 37 ) are in fairly good agreement with our model results.…”

“…Further, during ozonolysis reactions, 1 O 2 (singlet oxygen) and H 2 O 2 (hydrogen peroxide) are known to occur. 39 These reactive oxygen species (ROS) likely induce secondary reactions, such as oxidation, ring opening of aromatic amino acid side chains, and protein backbone cleavage. 40−42 However, the role of this secondary chemistry in atmospheric protein modification and degradation needs to be investigated in follow-up studies.…”

Boron doped TiO2 catalysts for photocatalytic ozonation of aqueous mixtures of common pesticides: Diuron, o-phenylphenol, MCPA and terbuthylazine

“…It is known that hydrogen peroxide can be generated during photocatalytic treatments 32 and also through direct ozone reactions [35,36,37]. The role of H 2 O 2 and O 3 involved in 33 photocatalytic reactions has been analyzed through the results depicted in Fig.7.…”

“…The combined application 31 of ozone and TiO 2 photocatalysis, known as photocatalytic ozonation, leads to a 32 synergistic effect due to enhanced production of reactive oxygen species (ROS) such 33 as hydroxyl radicals in comparison with the application of either single ozonation or 34 single TiO 2 photocatalysis [7,8]. 35 In this study, the degradation of four herbicides and pesticides: diuron (DIU), o-36 phenylphenol (OPP), 2-methyl-4-chlorophenoxyacetic acid (MCPA) and terbuthylazine 37 (TBA), commonly found in water ecosystems, has been studied. Their molecular 38 structures can be found in Table S1 of the supplementary material.…”