Autoxidation of Methyl Linoleate in Freeze‐Dried Model Systems. II. Effect of Water on Cobalt‐Catalyzed Oxidation

The oxidation of methyl linoleate catalyzed by various salts of cobalt was studied in a model system based on microcrystalline cellulose. The freeze‐dried model system was adjusted to various water activities, and the effect of water on the oxidation kinetics was determined using manometric measurements and measurements of diene conjugation. It was found that water had an inhibitory effect on the metal‐catalyzed oxidation of the fatty ester, as well as on oxidation in the absence of added metals. The kinetics of the reactions were evaluated in terms of the previously established hydroperoxide decomposition mechanisms. The effect of water on the metal‐catalyzed oxidation was found to exist in the monomolecular decomposition period as well as in the more rapid phase of the reaction, during which the hydroperoxide decomposition is known to follow bimolecular decomposition kinetics. The inhibition of the reaction by water is interpreted as due to deactivation of added, as well as of originally present, metal catalysts by hydration of the coordination shells; and also possibly as due to hydrogen bonding between hydroperoxides and water, and therefore to interference with the normal bimolecular decomposition reaction.