Action of Oxidation Inhibitors

It is generally accepted that one class of oxidation inhibitors functions by removing the reactive free radicals which are the chain-carriers in the oxidation of a hydrocarbon. However there is some disagreement about the exact mechanism by which inhibitors of the phenolic type act. Bickel and Kooyman (1956) suggest that peroxy-radicals, R02, abstract the hydroxyl hydrogen from a hindered phenol, IH, to give a hydroperoxide and a radical from the inhibitor which is unreactive and does not continue the oxidation chain. The inhibitor radical may subsequently react with another peroxy-radical to give non-radical products. Thus 

In contrast Boozer and Hammond (1954) postulate that the inhibition is due to the formation of a complex between the peroxy-radical and the inhibitor, which subsequently reacts with a second peroxy-radical to give non-radical products. Thus 

Deuterium substitution of the hydroxyl group in a hindered phenol leads to a decrease in the rate of reaction which suggests that scission of the OH-bond is the rate controlling factor. However, the effects of deuterium substitution on the complex formation and its subsequent reactions are not clearly understood. 

Experiments have been carried out with the rotating cryostat to study the reaction of 2,6-di-t-butyl-4-methyl phenol (lonol) with n-heptyl and n-heptylperoxy radicals. When lonol was deposited on n-heptyl radicals the e.s.r. spectrum showed that some reaction had occurred at 77°K. When the deposit was warmed slowly the spectrum of the residual n-heptyl radicals disappeared and was replaced by that of the substituted phenoxy-radical, (4), formed by loss of the hydroxyl hydrogen. 

It is concluded that (i) both n-heptyl and n-heptylperoxy radicals react with lonol by abstraction of the hydroxyl hydrogen in accord with the mechanism postulated by Bickel and Kooyman (1956) and (ii) the resultant phenoxy radical does not react readily with oxygen and mil therefore not continue an oxidation chain. 

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An alluring field of research is the mechanism of action of oxidation inhibitors. This research will undoubtedly yield in the near future a theory for inhibition of undesirable oxidation processes. The relatively stable free radicals observed on such inhibition display extremely interesting properties. Of great interest are the effects of synergism, of inhibitor mixtures, and of mixtures of inhibitors with catalysts. A strictly quantitative and elegant description of all these phenomena may be made within the scope of the chain theory for slow oxidation. 

Synergism of action of oxidation inhibitors The mechanisms of action of inhibitors on the oxidation of organic compounds... 

FIGURE 21.30 The sites of action of several inhibitors of electron transport and/or oxidative phosphorylation. 

Thus, effectiveness of basic (alkaline) additives has been greatly improved by increasing their solubility in base stocks, by exploiting synergistic action between two similar additive types and by the use of a third additive to enhance (catalyse) the performance of the other two. Similar principles have been used to augment the performance of oxidation inhibitors. 

As noted above, the duration of the retarding action of an inhibitor is directly proportional to the / value. In systems with a cyclic chain termination mechanism, the / coefficient depends on the ratio of the rate constants for two reactions, in which the inhibitor is regenerated and irreversibly consumed. In the oxidation of alcohols, aminyl radicals are consumed irreversibly via the reaction with nitroxyl radical formation (see earlier) and via the following reaction [11] ... 

Consider now the case of a combined action of two inhibitors, one of which (InH) breaks the chains by reacting with peroxyl radicals and the other (S) breaks down ROOH [9]. The radical In formed in the reaction of InH with R02 is trapped by the peroxyl radical and, therefore, does not contribute to chain propagation (mechanism III, Chapter 14). If kSi -C k3, the inhibitor InH is consumed with a rate of v-Jf, where the initiation rate is Vi = Vio + A 3[ROOH]. After the time ts = ( s[S]o)-1> oxidation becomes quasistationary with the quasistationary hydroperoxide concentration (see Chapter 14)... 

Sophisticated isotope experiments were also performed using H2180 (Mildred Cohn) and 32P, and various exchange reactions identified between ATP, ADP, and Pr Analysis of the mode of action of two inhibitors was also relevant. Dinitrophenol (DNP) uncoupled the association between oxidation and ATP generation (Lardy and Elvejhem, 1945 Loomis and Lipmann, 1948). Oligomycin inhibited reaction (ii) above, blocking the terminal phosphorylation to give ATP, but not apparently the formation of A C. 

Research in this field is ongoing aiming to understand the mechanism of action of kinetic inhibitors. Lee and Englezos (2005) showed that inclusion of polyethylene oxide (PEO) to a kinetic inhibitor solution was found to enhance by an order of magnitude the performance of the hydrate inhibitor. Binding of inhibitor molecules to the surface of hydrate crystals was considered to be the key aspect of the mechanism of kinetic inhibition (Anderson et al.,... 

Mechanism of action of PDE5 inhibitors. Abbreviations NO, nitric oxide PDE5, phosphodieterase type 5. 

There is another group of inhibitors which act by adsorption onto either the metal or the oxide. These are usually organic materials and the most effective are either alcohols or amines. They are mainly used in specialized applications such as inhibition of acid corrosion during pickling or in mitigation of corrosion in acid oil wells. The exact action of these inhibitors is beyond the scope of this chapter but they are discussed by Hackerman and others (14). Some buffering inhibitors, such as sodium benzoate may also act by adsorption on the surface. 

Iron is necessary for enzyme activity the activity decreases with the amount of iron lost in the preparation. Therefore, it is tempting to assume that iron acts as an electron carrier. The successive reduction and oxidation of the iron in the course of succinic oxidation have never been demonstrated, but the action of many inhibitors on the succinic dehydrogenase system can be explained by assuming the formation of metal complexes. 

Since E of the inhibition reaction is smaller than that of the reaction of chain development, the ratio of the constants drops with increasing temperature, and the decelerating action of the inhibitor is reduced. The rate of nonproductive consumption of the inhibitor, related to its direct oxidation by molecular oxygen, also increases with increasing temperature. ... 

As the oil is used, the neutralization number may increase due to contamination (e.g., SO2 from combustion of S in the fuel, CO2 from combustion or that present in atmosphere) and/or oxidation of the oil. The oxidation of the oil results in the formation of oil soluble alcohols, ketones, acids and peroxides (which may polymerise to give insoluble resins) thereby increasing the acid number, viscosity and darkening the oil colour. The rate and extent of oxidation of the oil during use depends on temperature, length of exposure to air or oxygen, amonnt of moisture, catalysts present (formed by the action of oxidation products on the metal surface), type of oil and the inhibitors used. 

Fig. 53. Diagram of thyroid activity. Iodine ions are taken up from blood (left), oxidized in the gland by removal of electrons and bound organically. From the iodinated tyrosine residues in the thyroglobulin molecule arise tri- and tetraiodothyronine, which are freed by proteolysis and released to the blood stream. At right is the addition compound with a-globulins. At the bottom, the sites of action of the inhibitors (antithyroid substances) are indicated.

Mixtures of inhibitors often possess a combined action. For example, when phenol and sulfide are introduced into the oxidized hydrocarbon, the first one retards by chain termination in the reaction with RO 2, and the second one decreases the rate of degenerate chain branching decomposing hydroperoxide. When two inhibitors enhance the retardation action of each other, we deal with synergism. When their retardation action is simply summated (for example, the induction period under the action of a mixture is equal to the sum of the induction periods under the action of each individual inhibitor), we have their additive retardation action. If the retardation action of a mixture is smaller than the sum of the retardation actions of each inhibitor, we have antagonism of inhibitors. 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

The unique properties and actions of an inhibitory substance can often help to identify aspects of an enzyme mechanism. Many details of electron transport and oxidative phosphorylation mechanisms have been gained from studying the effects of particular inhibitors. Figure 21.29 presents the structures of some electron transport and oxidative phosphorylation inhibitors. The sites of inhibition by these agents are indicated in Figure 21.30. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

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