Electrochemical studies on some carbonyl compounds and superoxide in aprotic organic solvents
Lauw, Sherman Jun Liang
Date of Issue2017-02-02
School of Physical and Mathematical Sciences
Carbonyl compounds are an undeniably essential class of molecules. This is exemplified by their prevalence in the myriad of biologically active natural products, pharmaceutical compounds, and/or chemical reagents that we come in contact with on a daily basis. As part of their biological functions, transformations, or decomposition, many of these compounds undergo complex (and often not fully understood) electron transfer and coupled chemical reactions. For these reasons, research pertaining to this area remains relevant and is also of continued importance. Therefore, the major theme of this thesis was directed toward the examination of the redox reactions, that occur at an electrode surface, of some organic carbonyl compounds. Studies were performed by utilizing a combination of electrochemical (e.g. cyclic, linear sweep, and rotating disk voltammetry, as well as controlled potential electrolysis), spectroscopic (ATR-FTIR), and digital simulation techniques. Where possible, the exact sequence of heterogeneous electron transfer and homogeneous chemical steps that occurred were determined, and the identities of intermediate species and/or end products elucidated. In particular, this thesis focused on investigating the electrochemical behaviors of biotin, cinnamaldehyde, and di-(2-ethylhexyl) phthalate in aprotic organic solutions such as acetonitrile, dimethyl sulfoxide, and N,N-dimethylformamide, and under various experimental conditions including varied scan rates, temperatures, and substrate concentrations. Overall, it was found that the 3 carbonyl compounds were all electrochemically reducible at fairly large negative potentials and were generally involved in a series of follow-up chemical reactions (e.g. dimerization or decomposition) following an electron transfer step. A second focus of this thesis was the voltammetric study of superoxide and its interactions with a variety of consumable food and vitamin molecules, and structurally varied aliphatic alcohols, with the aim of examining the possible scavenging reactions that take place. Superoxide is a free radical anion of the reactive oxygen species (ROS) family which has been demonstrated to be able to lead to the undesirable modification or damage of many important biological molecules if left unchecked. Therefore, analyses of antioxidants and their reactions are highly pertinent since they constitute one of the body’s main defenses against ROS; such as by scavenging free radicals like superoxide and thereby prevent the potential harm caused. In particular, electrochemical methods are suitable for these studies because superoxide can precisely be generated under mild conditions, with no by-products, while its coupled homogeneous reactions can be monitored in situ. In this thesis, examinations of the scavenging reactions of different compounds toward superoxide were carried out by carefully increasing the concentration of each substrate in a stepwise manner and concomitantly monitoring of the chemical reversibility of the one-electron voltammetric reduction of dioxygen to superoxide. This enabled the extent of radical inhibition and relative reactivities to be easily determined via measurements of the resultant oxidative peak current magnitudes on the reverse scan of CV, as well as the elucidation of the general mechanisms that were involved during the removal of electrochemically generated superoxide. Overall, it was also found that the relative reactivities of the different compounds toward superoxide were strongly influenced by structural features such as the presence of acidic/labile hydrogens, conjugation in the molecule, and number of hydroxyl groups available.