The inhibitory effect of water on the Co2+ and Cu2+ catalyzed decomposition of methyl linoleate hydroperoxides.

The inhibitory effect of water on the decomposition of methyl linoleate hydroperoxides (MLHP) catalyzed by Co2+ and Cu2+ was studied in a model system using proton nuclear magnetic resonance (NMR) spectroscopy. MLHP were prepared by photoxidation and purified by chromatographic methods. Proton NMR spectroscopy was used to measure reaction rates by monitoring changes in the intensity of the OOH signal. The rate constant of the reaction was obtained by plotting the natural logarithm of MLHP concentration vs time. In the first part of the study, no transition metals were added to the model system, so that the effect of water could be attributed to the interaction between water and MLHP only. The rate constant of the reaction (K) was found inversely proportional to the concentration of water. There was a downfield chemical shift of both hydroperoxide and water peaks in the NMR spectra when water was added. As temperature increased to 40 degrees C, the difference in K between the systems with 0% and 2% water disappeared. It is proposed that the hydroperoxides were solvated with water which retarded their decomposition. When Co2+ was added to the model system, K decreased as the concentration of water increased from 0% to 1.5%. As temperature increased from 18 degrees C to 40 degrees C, differences between the K for 0% and 2% water disappeared. A similar phenomenon was observed in reactions catalyzed with Cu2+. These findings would support a mechanism in which the protective effect of water involves both the solvation of OOH and hydration of the metal catalyst.