Stability of premethylated aromatic model compounds of constituents of humic substances toward KMnO 4 oxidation

Abstract

When analyzing aromatic constituents of humic substances, should potassium permanganate (KMnO 4) be used directly for the release of these constituents, aromatic rings possessing hydroxyl groups may easily be decomposed. As a means for stabilizing these compounds, methylation before KMnO 4 oxidation (premethylation) was conducted and the results were investigated. 28 different aromatic model compounds having hydroxyl and/or carboxyl groups were used as aromatic model compounds of constituents of humic substances. The recovery of these compounds and their methylated compounds after KMnO 4 oxidation was assessed quantitatively. The reaction temperature of KMnO 4 oxidation was 50°C. In the oxidation of non-methylated compounds, those having at least one hydroxyl group were easily decomposed by KMnO 4 oxidation, but for those with only a carboxyl group, stability was retained, except for terephthalic acid. In the oxidation of methylated compounds, many of those possessing carboxyl and hydroxyl groups could be stabilized by methylation, while those possessing only the hydroxyl group or substituted by two hydroxyl groups at the mutually meta- and para-positions with no intervening substitution groups, easily underwent decomposition even if methylated. Assessment of the applicability of the premethylation technique was made for more complex compounds. Two tannic compounds, (+)-catechin and penta-G-glucose, were used for this purpose, since their chemical structures are well known. Following premethylation, they were decomposed by KMnO 4 and then remethylated by dimethyl sulfate and potassium carbonate. GC/MS was used for the analysis of oxidation products. Compounds which could be predicted from the structure of original compounds were obtained in high yields.