The reactions of ozone with compounds relevant to drinking-water processing: Phenol and its derivatives

A dissertation submitted to the University of Namibia in fulfilment of the requirements for the Degree of Doctor of Philosophy in Chemistry

Kinetic and mechanistic investigations on the reactions of ozone with phenol and some of its derivatives (hydroquinone, catechol, phloroglucinol, pentachlorophenol, pentabromophenol, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene and 1,3,5-trimethoxybenzene) have been carried out. These compounds were chosen, as they can serve as models for compounds which are abundant in surface waters and wastewater. The scope of the investigation was widened by including dihydrogen sulfide, often a contaminant of ground waters in arid areas, and its organic analogues. By employing various analytical techniques, such as high performance liquid chromatography (HPLC); ion chromatography (IC); nuclear magnetic resonance spectroscopy (NMR); gas chromatography (GC) and gas chromatography coupled to mass spectroscopy (GC-MS), it was possible to identify and quantify reaction products. The stopped-flow technique was employed to measure rate constants of ozone reactions. When strong substrate absorptions at 240–280 nm prevented the use of this technique, rate constants were measured by competition kinetics. Pulse radiolysis was used for the study of OH-induced reactions. Methods were developed for the detection and quantification of reactive intermediates in ozone reactions such as singlet dioxygen, OH, O2 and hydroperoxides. A germanium diode detector was used for the quantification of O2(1g) formation. For the detection of OH formation, 2-methyl-2-propanol and DMSO were used and tetranitromethane was applied for the detection of O2. Based on the products in the reactions of ozone with phenol and its derivatives, various reaction pathways have been identified. Ozone reacts with phenol and its derivatives by ozone addition (Criegee mechanism), electron transfer and O-atom transfer reactions. The Criegee mechanism, practically the only reaction with olefins, often occurs to only a small extent here. Instead, OH (up to 26%), O2 and O2(1g) are major intermediates. The occurrence of the Criegee mechanism was confirmed by the yields of hydrogen peroxide and their corresponding carbonyl compounds. The Criegee mechanism is shown to be more pronounced in methoxybenzenes as compared to phenols. For example, in the reaction of ozone with 1,4-dimethoxybenzene, the hydrogen peroxide yield (56%) and that of methyl(2Z,4E)-4-methoxy-6-oxo-hexa-2,4-dienoate (52%), which are the products of the Criegee type reaction, are much higher than that of hydrogen peroxide (5.6%) in the reaction of ozone with hydroquinone. A possible reason for this is that, i.e. the zwitterion formed in the reaction of ozone with hydroquinone may undergo a deprotonation reaction [reaction (1)] which competes with the 1,3- dipolar cyloaddition [reaction (2)] whereas in the case of the zwitterion formed in the reaction of ozone with 1,4-dimethoxybenzene, the 1,3-dipolar cycloaddition may occur without hindrance.