Metal Ion Catalysis and Polarity of Environment in the Aerobic Oxidation of Unsaturated Fatty Acids

Abstract

IT is frequently stated that the intense activity of certain metallic compounds as oxidation catalysts is a matter of great technological interest and that knowledge of how these compounds react is still obscure in many details¹. The foundations of a better understanding of the mechanism of metal ion catalysis were laid by Haber and Weiss². The most notable recent advances in this field were made by C. E. H. Bawn³ and his school. It is now generally accepted that the aerobic oxidation of linoleic acid and its esters proceeds through a chain reaction with the free radicals R· and RO₂ ^˙ as chain carriers, namely: The most difficult aspect is the initial production of free radicals. It is this aspect which is most directly related to metal catalysis. The following heavy-metal stearates were prepared and their catalytic effect examined: cupric, ferric, (partially oxidized) ferrous, cobaltous, manganous, cerous, vanadyl, thallous and stannous stearates. The latter two were investigated with the object of determining whether two-electron transfer would be more efficient than one-electron transfer in the initiation process. In the polar solvents ethyl alcohol, ethyl acetate and ethyl caprylate, none of the heavy metal stearates had any appreciable catalytic activity at 25° C., approximately 0.1 M methyl linoleate (or linoleic acid) and a catalyst concentration equal to or less than M/5,000. In dilute benzene solutions, cobaltous, manganous and cerous stearates were the only active catalysts leading to hydroperoxide formation. At a catalyst concentration of iM/5,000, 25° C. and 0.1 M methyl linoleate, the rates of formation of hydroperoxide were approximately seven, five and four milliequivalents methyl linoleate hydroperoxide per mole methyl linoleate per hour with cobaltous, manganous and cerous stearate, respectively. Cupric stearate did not catalyse the formation of hydroperoxide but appeared to be an active catalyst in its decomposition into unsaturated aldehydes and ketones. The activation energy of hydroperoxide formation catalysed by cobaltous stearate was determined as 20 k.cal.