Chain-breaking antioxidants kinetics

We thought that the resumption of oxidation after a few min was likely due to the depletion of NO since subsequent additions of NO inhihited oxidation with kinetics similar to the first addition. In order to prove this, we repeated the experiment, hut this time determined the [ NO] at periodic time intervals. We initiated the experiment with 20 pM and then added 0.9 pM NO 1 min later. At this 1 min time point, oxidation was inhihited (Figure 3). This inhibition continued until about 4.5 min, which is about when the [ NO] fell to below the limit of detection. At this time there was still sufficient Fe (7.2 pM) to reinitiate oxidation. These results demonstrate that NO is acting as a chain-breaking antioxidant during cellular lipid peroxidation. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Barclay, L.R.C., Locke, S.J., MacNeil, J.M., VanKessel, J., Burton, G.W. Ingold, K.U. (1984). Autooxidation of micelles and model membranes. Quantitative kinetic measurements can be made by using either water-soluble or lipid-soluble initiators with water-soluble or lipid-soluble chain-breaking antioxidants. Journal of American Chemistry Society, 106, 2479-2481. 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

The present paper reports the results of a kinetic study of the inhibition of the azobisisobutyronitrile-initiated autoxidation of cumene at 60 °C. and of Tetralin at 70 °C. by zinc diisopropyl dithiophosphate, undertaken to test the validity of the chain-breaking inhibition mechanism proposed above. In addition, the effectiveness of several metal dialkyl dithiophosphates as antioxidants in the autoxidation of squalane... 

The analysis of the thermodynamic and kinetic characteristics of these reactions will allow a better understanding of the biological importance of chain breaking in the antioxidant actions of polyphenols. 

Studies of the kinetic deuterium isotope effects which established the chain-breaking mechanism of antioxidant action by hydrogen donation were carried out in our laboratories by E. T. McDonel, J. C. Crano, and D. N. Vincent. Studies of sulfoxides, sulfenic acids, thiolsulfinates, and their reactions with hydroperoxides which illustrate the chemistry of the processes involved in their activity as preventive antioxidants were done by K. E. Davis, J. V. Webba, E. R. Harrington, and D. M. Kulich. 

Modern kinetic investigations of antioxidant action began with the investigations of Bolland and ten Haave [151,152] on inhibited oxidation of ethyl linoleate and with the broad theoretical and experimental studies of Waters and his coworkers [153—155]. Bolland and ten Haave proposed that inhibition resulted from chain-breaking by the faster reaction of R02 with antioxidant, AH, than with hydrocarbon RH to give an unreac-tive radical A which then terminates with R02- or A, viz. 

The hindered phenol antioxidants Formula 4.4 and aromatic amines (e.g., p-phenylene diamines, Formula 4.1) both operate by kinetic chain-breaking processes. They donate a hydrogen to an alkyl peroxide radical and break the free radical chain with reactions such as below. 

The chemical mechanisms involved in the action of antioxidants have been discussed in standard texts [9,63-66] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor, (CB-D) mechanism in which chain-propagating peroxyl radicals (POO ) are preferentially reduced to hydroperoxide by the antioxidant (AH). 

The classical inhibition of oxidation processes is based on kinetic chain break and deactivation of branching, intermediate products. In the case of TP, chain type of the process is not obvious. The chain break in chemical increments suggests the inertness of residual inhibitor radical. These radicals are active above 200°C. Therefore, classical antioxidants are ineffective at high temperatures. 

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