Ozonolysis of β-Caryophyllonic and Limononic Acids in the Aqueous Phase: Kinetics, Product Yield, and Mechanism

Photooxidation-Initiated Aqueous-Phase Formation of Organic Peroxides: Delving into Formation Mechanisms

Formation of highly oxygenated molecules (HOMs) such as organic peroxides (ROOR, ROOH, and H2O2) is known to degrade food and organic matter. Gas-phase unimolecular autoxidation and bimolecular RO2 + HO2/RO2 reactions are prominently renowned mechanisms associated with the formation of peroxides. However, the reaction pathways and conditions favoring the generation of peroxides in the aqueous phase need to be evaluated. Here, we identified bulk aqueous-phase ROOHs in varying organic precursors, including a laboratory model compound and monoterpene oxidation products. Our results show that formation of ROOHs is suppressed at enhanced oxidant concentrations but exhibits complex trends at elevated precursor concentrations. Furthermore, we observed an exponential increase in the yield of ROOHs when UV light with longer wavelengths was used in the experiment, comparing UVA, UVB, and UVC. Water-soluble organic compounds represent a significant fraction of ambient cloud-water components (up to 500 μM). Thus, the reaction pathways facilitating the formation of HOMs (i.e., ROOHs) during the aqueous-phase oxidation of water-soluble species add to the climate and health burden of atmospheric particulate matter.

Water Plays Multifunctional Roles in the Intervening Formation of Secondary Organic Aerosols in Ozonolysis of Limonene: A Valence Photoelectron Spectroscopy and Density Functional Theory Study

Although water may affect aqueous aerosol chemistry, how it intervenes in the formation of secondary organic aerosols (SOAs) at the molecular level remains elusive. Ozonolysis of limonene is one of the most important sources of indoor SOAs. Here, we report the valence electronic properties of limonene aerosols and SOAs derived from limonene ozonolysis (Lim-SOAs) via aerosol vacuum ultraviolet photoelectron spectroscopy, with a focus on the effects of water on Lim-SOAs. The first vertical ionization energy of limonene aerosols is measured to be 8.79 ± 0.07 eV. While water significantly increases the total photoelectron yield of Lim-SOAs, three photoelectron features attributable to Lim-SOAs each exhibit distinct dependence on the fraction of water in aerosols, implying that different formation pathways and molecular origins are involved in the formation of Lim-SOAs. Combined with density functional theory calculation and mass spectrometry measurements, this study reveals that water, particularly the water dimer, enhances the formation of Lim-SOAs by altering the ozonolysis energetics and pathways by intervening in its Criegee chemistry, acting as both a catalyst and a reactant. The atmospheric implication is discussed.

Ozonation of organic compounds in water and wastewater: A critical review

Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidation of inorganic and organic pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of organic contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in analytical and computational chemistry, which allowed to improve the mechanistic understanding of ozone reactions. This critical review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved organic matter, various functional groups (olefins, aromatic compounds, heterocyclic compounds, aliphatic nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.

Transformation of bromophenols by aqueous chlorination and exploration of main reaction mechanisms

Bromophenols (BPs) are ubiquitous phenolic contaminants and typical halogenated disinfection byproducts (DBPs) that are commonly detected in aquatic environments. The transformation of 2,4-dibromophenol (2,4-DBP) during chlorination process was fully explored in this research. It was found that active chlorine can react with 2,4-DBP effectively in a wide pH range of 5.0-11.0, with an apparent second-order rate constant (k_app) varying from 0.8 M^-1 s^-1 to 110.3 M^-1 s^-1. The addition of 5 mM ammonium ions almost completely suppressed the reaction via competitive consumption of free chlorine. With the concentration of HA increasing from 1.0 to 10.0 mg L^-1, the inhibition on the degradation of 2,4-DBP increased from 8.7% to 63.4%. By contrast, bromide ions at a concentration of 5 mM accelerated the process by about 4 times, due to the formation of hypobromous acid. On the basis of the eleven products (with eight nominal masses) identified by LC-TOF-MS, electrophilic substitution reactions and single-electron transfer reactions were mainly involved in the chlorination process. The concentration of primary chlorine-substituted products was about 4 times that of the dimer products, demonstrating that electrophilic substitution reaction was predominant during chlorination of 2,4-DBP. Density functional theory (DFT) based calculations revealed that HOCl is the dominant active oxidizing species for elimination of 2,4-DBP and coupling reaction occurs more easily at para and ortho position of hydroxyl group in the phenolic moiety. These findings could provide some new insights into the environmental fate of bromophenols during chlorine disinfection of water and wastewaters.

pH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase

Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid-base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.

A Concise Bioinspired Semisynthesis of Rumphellaones A-C and Their C-8 Epimers from β-Caryophyllene

The first semisynthetic route toward rumphellaones B (2) and C (3) and their C-8 epimers as well as the shortest synthesis of rumphellaone A (1) and its C-8 epimer from the most accessible sesquiterpene, β-caryophyllene (4), is presented. Synthetic routes involved caryophyllonic acid as a key intermediate, which was converted to rumphellaone A (and epimer) via acid-catalyzed lactonization and rumphellaone C (and epimer) using one-pot epoxidation-lactonization. Rumphellaone B (2) and its epimer were obtained from rumphellaone A (1) and its epimer, respectively, using Saegusa-Ito oxidation. The absolute configuration at C-8 was confirmed by single-crystal X-ray analysis of rumphellaone B (2) and an acylated derivative of rumphellaone C.

Superfast degradation of refractory organic contaminants by ozone activated with thiosulfate: Efficiency and mechanisms

Thiosulfate (S₂O₃^2-) is frequently used as an ozone (O₃) quenching agent when investigating the ozonation of organic contaminants and the kinetics thereof. In this study, however, O₃ is activated by S₂O₃^2-, resulting in a superfast degradation of O₃-refractory contaminants. Therefore, the focus of this study is the exploration into the enhancing role of S₂O₃^2- in the degradation of refractory organic contaminants by O₃, which has been overlooked thus far. Results obtained from scavenging experiments and electron paramagnetic resonance (EPR) spectra verify that ^•OH generated from the reaction of S₂O₃^2- with O₃ is mainly responsible for the superfast degradation of O₃-refractory contaminants. The ^•OH yield from the O₃/S₂O₃^2- process is determined to be 0.216. A plausible mechanism for the generation of ^•OH from the O₃/S₂O₃^2- process is proposed with the implementation of density functional theory (DFT). Initially, ozone reacts with a sulfur of S₂O₃^2- to form OOOSSO₃^2-. The adduct then rearranges to OO(O)SSO₃^2- or HOO(O)SSO₃^2- in the presence of H⁺, which cleaves to give a sulfoxide radical cation and O₂^•-/HO₂^•. O₂^•-/HO₂^• is rapidly transformed into ^•OH by O₃ through a series of steps. Degradation efficiency of O₃-refractory contaminants of this process highly depends on the molar ratio of S₂O₃^2- and O₃ ([S₂O₃^2-]:[O₃]). The optimal [S₂O₃^2-]:[O₃] is pH dependent in synthetic water (e.g. 0.3 at pH 7). The presence of bicarbonate inhibits the degradation of refractory contaminants by the O₃/S₂O₃^2- process. Humic acid exhibits a slight enhancing effect at low concentrations (0.1-0.2 mg-C/L), and an inhibiting effect at higher concentrations (≥0.4 mg-C/L). In addition, the efficacy of the O₃/S₂O₃^2- process in real water matrices is also confirmed.

Physicochemical Properties of Terebic Acid, MBTCA, Diaterpenylic Acid Acetate, and Pinanediol as Relevant α-Pinene Oxidation Products

The physicochemical properties and the synthesis of four α-pinene oxidation products, terebic acid, 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), diaterpenylic acid acetate (DTAA), and pinanediol, are presented in this study. The physicochemical properties encompass thermal properties, solubility in water, and dissociation constant (pK _a) for the investigated compounds. It was found that terebic acid exhibits a relatively high melting temperature of 449.29 K, whereas pinanediol revealed a low melting temperature of 329.26 K. The solubility in water was determined with the dynamic method and the experimental results were correlated using three different mathematical models: Wilson, NRTL, and UNIQUAC equations. The results of the correlation indicate that the Wilson equation appears to work the best for terebic acid and pinanediol. The calculated standard deviation was for 3.79 for terebic acid and 1.25 for pinanediol. In contrast, UNIQUAC was the best mathematical model for DTAA and MBTCA. The calculated standard deviation was 0.57 for DTAA and 2.21 for MBTCA. The measured water solubility increased in the following order: pinanediol > DTAA ≥ MBTCA > terebic acid, which affects their multiphase aging chemistry in the atmosphere. Moreover, acidity constants (pK _a) at 298, 303, and 308 K were determined for DTAA with the Bates-Schwarzenbach spectrophotometric method. The pK _a values obtained at 298, 303, and 308 K were found to be 3.76, 3.85, and 3.88, respectively.