Proelectrophile-Nucleophile Adduct Formation and Permanganate Oxidation of Alkenes: Use of Model Compounds to Probe Complex Reaction Pathways
Flanders, Phillip M.
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Laboratory studies of reactions between model compounds yield valuable insights into complex reaction chemistries that would be difficult if not impossible to observe directly in environmental media. We used model compounds to investigate two types of reaction pathways: adduct formation and permanganate oxidation of alkenes. First, we mimicked covalent bond forming reactions between nucleophilic contaminants and electrophilic sites within natural organic matter (NOM) or naturally occurring electrophilic toxicants. Initial studies modeled adduct formation at electrophilic NOM sites by reacting the model electrophiles p-benzoquinone and patulin with a suite of model nucleophiles. Further studies used MnO2(s, pyrolusite)-oxidized hydroquinone “proelectrophiles” to simulate the highly electrophilic moieties that can be generated when NOM reacts with oxidants in a sediment or soil. Overall, our model compound studies demonstrated that added nucleophiles compete with H2O\OH- to form monoadducts, which can then undergo further oxidization and addition to form di- and triadducts. Mass spectra consistent with monoadducts from reactions between nine nucleophiles and MnO2-oxidized gentisic acid were acquired by LC/MS. Separate experiments monitoring both the consumption of a model proelectrophile and a nucleophile (acetylhydroquinone and 4-ethylaniline, respectively) and the generation of Mn(II) from reduction of MnO2 provided indirect evidence for multiple oxidation-addition steps. Second, we used model compounds to study permanganate oxidation of alkenes. Competition between hydrolysis and oxidation of a common intermediate leads to a distribution of products. To collect direct evidence of the influence of pH and oxidant dose on this product distribution, we selected two model alkenes – cis-stilbenedicarboxylic acid and 3-cyclopentenecarboxylic acid – that yield permanganate oxidation products amenable to LC/MS analysis. High permanganate dose favors highly oxidized products, while alkaline pH favors products from hydrolysis. This dissertation demonstrates how strategic selection of model compounds can elucidate complex environmental contaminant removal pathways by providing an internally consistent system for the study of multiple reaction conditions and competitive reactions.