Electrochemical oxidation of artificial and natural hydroxyl containing compounds in aprotic organic solvents
Chan, Ya Yun
Date of Issue2016
School of Physical and Mathematical Sciences
Nanyang Environment and Water Research Institute
Electrochemical oxidation studies were performed in aprotic organic solvents on two types of artificial hydroxyl containing emerging contaminants; namely bisphenol A and phenolic benzotriazoles (UV234 and UV327) as well as on the natural hydroxyl containing vitamin D compounds (vitamins D3 and D2). The techniques utilized to study the compounds included CV, CPE as well as combining spectroscopic methods such as UV-vis and EPR with electrochemistry and chemical oxidation. All 3 types of hydroxyl containing compounds were found to undergo chemically irreversible oxidation processes on the short voltammetric timescale, indicating that the initial heterogeneous electron transfer steps produced oxidized intermediates that were not long-lived. Further investigations enabled the determination of the exact electron transfer and chemical steps involved in the oxidation process. In this thesis, the oxidative mechanism for the compounds are proposed along with the identity of the intermediate species and final oxidized products which can provide useful insights into the reactions that can occur in lipophilic environments during oxidative metabolism as well as highlight the possible role of the intermediate species as reactive metabolites in oxidative degradation pathways. The electrochemical oxidation of the two artificial phenolic classes of compounds occurred at the phenolic moiety in a mechanism that is similar to phenols in general, while the electrochemical oxidation of the natural hydroxyl containing vitamin D most likely occurred at the triene moiety. Bisphenol A is chemically oxidized via a fourelectron/two-proton process (‒4e‒/‒2H+) in a 2 × ECE mechanism to form a relatively unstable dication which reacts quickly in the presence of water in acetonitrile. UV234 andUV327 displayed similar behavior and can be electrochemically oxidized via a twoelectron/one-protonprocess (‒2e‒/‒1H+) in an ECE mechanism under neutral and acidic conditions. In basic conditions, the phenolic benzotriazole is electrochemically oxidized in a CECdim process to form neutral benzotriazolyl-substituted phenoxyl radicals that immediately undergo a reversible dimerization (Cdim-step) to form a dimer which exists in equilibrium with the radicals. Vitamin D is electrochemically oxidized in an ECCE mechanism initially via one-electron to form a cation radical, which reacts homogeneously in two chemical steps [where one chemical step is fast (< 1 s) and one is relatively slow (> 1 s)]. On the longer electrolysis timescale, the oxidized product undergoes a second one-electron transfer at a less positive potential. Surface coverage of drop cast nanomaterial coatings formed on electrodes were assessed after investigations at a MWCNT-coated GC electrode suggested that electrode modification with nanomaterials was not useful in resolving the electrode fouling problem caused by adsorption of oxidized products during oxidation. It was demonstrated that the drop casting procedure is unable to prevent solution phase ions from diffusing through the nanomaterial coating to the base electrode and the contribution from the base electrode on the current-potential trace must be taken into account to fully understand the current response.