Colloidal catalysts based on iron(III) oxides. 2. Peculiarities of catalyzed oxidation of palm oil

The catalytic activity of an iron(III)-oxide-based colloidal catalyst with respect to oxidation of palm oil with atmospheric oxygen at 60°C is investigated. In a microheterogeneous system of palm oil containing phospholipids and large amounts of ??-tocopherol and ??-carotene, oxidation in the presence of the catalyst proceeds by a latent radical mechanism. This mechanism manifests itself as oxygen uptake against a background of almost constant concentrations of natural antioxidants, i.e., ??-tocopherol and ??-carotene. Using a model reaction of oxidation of an individual natural olefin, limonene, it is shown that, in the presence of the catalyst and trace amounts of hydroperoxide, oxidation of the hydrocarbon occurs through the chain free-radical mechanism. The presence of a phospholipid (egg lecithin) dramatically decelerates the catalytic oxidation of limonene and makes the process proceed through the latent radical mechanism, the rate of which is unaffected by oil-soluble inhibitors.     

Specific features of radical generation in the reaction of thiols with hydrogen peroxide

Russian Chemical Bulletin - Using the method of inhibitors it was found that the reduction of H2O2 with natural thiols in aqueous solutions is accompanied by the formation of radicals. The reaction...     

Features of α-phenylisopropyl hydroperoxide decomposition catalyzed by cetyltrimethylammonium bromide

The features of cumene oxidation, α-phenylisopropyl hydroperoxide (ROOH) decomposition, and free radical formation in the presence of cetyltrimethylammonium bromide (CTAB) were studied by the kinetic methods and TLC product analysis. It was found that CTAB catalyzes ROOH decomposition to free radicals. The efficiency of an initiating system CTAB—ROOH is by a factor of 2.5 higher than that of a combination CTAB with α-phenylethyl hydroperoxide. In addition, CTAB catalyzes the reactions of ROOH with α-tocopherol and 2,2′-bis[2-(p-dimethylaminophenyl)indane-1,3-dione]. The kinetic characteristics of these reactions were determined. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1802–1807, August, 2005.     

The influence of cholesterol on the generation of radicals in mixed reverse micelles of cationic surfactants with hydroperoxides

Cationic surfactants (S+) and acetylcholine (ACh), the most important neurotransmitter playing an essential role in the neuromuscular and cognitive activity of living beings, form mixed reverse micelles with hydroperoxides (ROOH) in organic media, where ROOH decay into free radicals is catalytically accelerated. Adding cholesterol (Chol, 30 mol.%) to pyridinium (СРВ) and cetyltimethylammonioum (СТАВ) bromides, reduces the radical generation rate in ROOH catalytic decay several times. However, a higher radical initiation rate is observed in the case of less ordered and larger ACh–ROOH reverse micelles. A Chol additive does not essentially affect the size of СТАВ and СРВ micelles with hydroperoxides but results in their decrease in the case of ACh–ROOH.

     

Decomposition kinetics of free radical initiators in micellar solutions

The kinetic parameters of thermal decomposition of initiators, azo-bis-isobutyronitrile (AIBN) and dicyclohexyl peroxydicarbonate (PC), have been measured in aqueous micellar solutions of sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), and egg lecithin. The inhibitor technique and acceptors of free radicals have been used to estimate the efficiency of the initiation during AIBN decomposition. The rate constant of PC decomposition in a SDS micellar solution is the same as inn-decane. CTAB and lecithin accelerate the rate of PC decomposition. Mechanisms of surfactant effect on the PC decomposition are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1752–1757, October, 1994.The work was financially supported in part by the International Science Foundation (ISF) (Grant No. MAQ 000).     

Interaction between hydrogen peroxide and cobalt(II) acetylacetonate in the system of reverse micelles of triton X-100 nonionic surfactant in cyclohexane

The yield of free radicals upon the decomposition of hydrogen peroxide catalyzed by cobalt acetylacetonate (Co(acac)₂) in the systems of reverse micelles of TX-100/n-hexanol and AOT in cyclohexane at 37°C was studied with the inhibitor method using a stable nitroxyl radical as a spin trap. It is shown that, in micellar AOT solutions in cyclohexane as well as in n-decane, H₂O₂ and Co(acac)₂ in practice do not react, because H₂O₂ is localized in a micelle water pool and Co(acac)₂, in the organic phase. Therefore, the generation of radicals is not observed in AOT solutions in cyclohexane, whereas, in aqueous solution, Co(acac)₂ catalyzes the radical decomposition of H₂O₂. In the system of mixed reverse micelles of TX-100 and n-hexanol in cyclohexane, at equal overall concentrations of H₂O₂ and Co(acac)₂, the rate of radical formation is much higher than in aqueous solution; i.e., the micellar catalysis of the radical decomposition of H₂O₂ takes place. It follows from measurements of UV and ESR spectra and the kinetics of changes in the content of peroxides in the reaction mixture that TX-100 and n-hexanol react with free radicals formed upon H₂O₂ decomposition and with atmospheric oxygen.