Mechanism of inhibited oxidation

Depending on the oxidation conditions and its reactivity, the inhibitor InH and the formed radical In can participate in various reactions determining particular mechanisms of inhibited oxidation. Of the various mechanisms, one can distinguish 13 basic mechanisms, each of which is characterized by a minimal set of elementary steps and kinetic parameters [38,43 15], These mechanisms are described for the case of initiated chain oxidation when the initiation rate v = const, autoinitiation rate fc3[ROOH] -C vy and the concentration of dissolved dioxygen is sufficiently high for the efficient conversion of alkyl radicals into peroxyl radicals. The initiated oxidation of organic compounds includes the following steps (see Chapter 2). 

The mechanisms by which an inhibitor adds to an oxidized hydrocarbon exerts its influence may differ depending on the reaction conditions. If the rate constants of the elementary reactions of RH, InH, R02 , In, ROOH, and 02 are known, the kinetics of the inhibited oxidation of RH can mathematically be described for any conditions. However, such an approach fails to answer questions how the mechanism of inhibited oxidation is related to the structure and reactivity of InH, RH, and R02 or what inhibitor appears the most efficient under the given conditions, and so on. At the same time, these questions can easily be clarified in terms of a topological approach whose basic ideas are the following [43-45,70-72] ... 

Inhibited oxidation of RH occurs through a relatively large number of reactions (see Chapter 4), but the primary mechanism of inhibited oxidation is determined by a few key reactions. For example, cumene is oxidized in the presence of p-cresol at 320-380 K... 

The region of implementation of a particular mechanism of inhibited oxidation in the coordinates T—[RH]—[InH] is a combination of oxidized substances, inhibitors, and reaction conditions for which this mechanism is appropriate. 

It is apparent that the mechanism of inhibited oxidation can be changed by varying the reaction conditions (7, vi5 [InH], etc.). The question arises whether this is possible for any RH and InH. The formulae for the mechanism boundaries provide a clearly defined answer to this question. For instance, the region of mechanism II is limited from the left and from the right. By invoking equations for boundaries I—II and II-VII, we get for the upper limit of /)(R—H), (under fixed conditions)... 

ET Denisov. Reactions of Inhibitor Radicals and Mechanism of Inhibited Oxidation of Hydrocarbons, vol 17. Moscow VINITI, Itogi Nauki i Tekhniki, 1987, pp. 3-115 [in Russian]. 

This transition may exhibit a critical behavior when, at a certain concentration of inhibitor known as the critical concentration [InH]cr, the dependence of the induction period on [InH] drastically changes, so that di-/d[InH] at [InH] > [InH]cr becomes much higher than dr/d[InH] at [InH] < [InH]cr. In the literature this problem has been treated only with reference to mechanisms II, III, and VIII [61-68], while all the known mechanisms of inhibited oxidation of RH will be envisaged here (see earlier) [69]. The equations for the chain length, critical antioxidant concentration [InH]cr, stationary concentration of hydroperoxide [ROOH]st, and induction period are given in Table 14.3 and Table 14.4. 

Important results are achieved in this field when studying the oxidative transformations of organic substances inhibited by phenols [4-16], It was demonstrated with these examples that the reaction mechanisms of inhibited oxidation are nonlinear dynamical systems. In this situation they failed to obtain analytical solutions in the general case that would allow to select the key parameters of the reaction. Recommendations for the selection of inhibitor s structure and the operating conditions of materials on the basis of these parameters would be the next step. 

E. T. Denisov, The reactions of radical inhibitors and the mechanism of inhibited oxidation of hydrocarbons, Itogi Nauki i Techniki, Kinetika kataliz, Vol. 17, Moscow, Russia, 1987, pp. 3-115 (Russian). 

Table 11.1. Equations of reaction rate v and oxygen absorption in time A[02](t) at different mechanisms of inhibited oxidation...

For each mechanism of inhibited oxidation, [RO ] can be related to InH and ROOM, expressing this relationship mathematically and solving the system of two differential equations, which describe oxygen absorption and inhibitor consumption, and to express the amount of absorbed oxygen through the amount of the consumed inhibitor. For example, in the case of mechanism V when reactions (8), (31), (-31), and (33) are key (see Table 11.1), we obtain... 

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

The mechanism of inhibition by the salts of the long chain fatty acids has been examined . It was concluded that, in the case of the lead salts, metallic lead was first deposited at certain points and that at these points oxygen reduction proceeded more easily, consequently the current density was kept sufficiently high to maintain ferric film formation in addition, any hydrogen peroxide present may assist in keeping the iron ions in the oxide film in the ferric condition, consequently the air-formed film is thickened until it becomes impervious to iron ions. The zinc, calcium and sodium salts are not as efficient inhibitors as the lead salts and recent work has indicated that inhibition is due to the formation of ferric azelate, which repairs weak spots in the air-formed film. This conclusion has been confirmed by the use of C labelled azelaic acid, which was found to be distributed over the surface of the mild steel in a very heterogeneous manner. ... 

Little work has been carried out on the mechanism of inhibition of the corrosion. of copper in neutral solutions by anions. Inhibition occurs in solutions containing chromate , benzoate or nitrite ions. Chloride ions and sulphide ions act aggressively. There is evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies which permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

Ethanol also inhibits ADH-catalyzed retinol oxidation in vitro, and ethanol treatment of mouse embtyos has been demonstrated to reduce endogenous RA levels. The inhibition of cytosolic RolDH activity and stimulation of microsomal RolDH activity could explain ethanol-mediated vitamin A depletion, separate from ADH isoenzymes. Although the exact mechanism of inhibition of retinoid metabolism by ethanol is unclear, these observations are consistent with the finding that patients with alcoholic liver disease have depletedhepatic vitamin A reserves [review see [2]. 

The mechanisms responsible for inhibited oxidation depend on the experimental conditions and particular properties of RH and antioxidant (see earlier). Let us assume that hydroperoxide is relatively stable, so that it virtually does not decompose during the induction period (kdr -c 1). Actually, this means that the rate of ROOH formation is much higher than the rate of its decomposition, / 2[RH] [RO]2 ] 3> d[ROOH]. For each of the mechanisms of inhibited autoxidation, there is a relationship between the amounts of the inhibitor consumed and hydroperoxide produced (see Tablel4.2). For example, for mechanism V with key reactions (2), (7), (—7), and (8), we can get (by dividing the oxidation rate v into the rate of inhibitor consumption) the following equation ... 

As described earlier, the mechanism of inhibited chain oxidation depends on the structural features of RH and InH, as well as on the reaction conditions (T, v,[RH], [InH], [O2], and [ROOH]). In this section we present data illustrating this approach with reference to the autoxidation of hydrocarbons inhibited by sterically nonhindered phenols of group A. 

Inhibition mechanisms by A/-cyclopropyl MPTP analogues are also discussed in terms of two catalytic pathways, one of which is based on an initial SET step from the nitrogen lone pair, as proposed by Silverman, and the second is based on an initial a-carbon hydrogen atom transfer (HAT) step, as proposed by Edmondson, leading to a radical and dihydropyridinium product formation. The observation that MAO B catalyzes the efficient oxidation of certain 1-cyclopropyl-4-substituted-1,2,3,6-tetrahydropyridines to the corresponding dihydropyridinium metabolites suggests that the catalytic pathway for these cyclic tertiary allylamines may not proceed via the putative SET-generated aminyl radical cations [122], Further studies will be necessary to clarify all the facets of the mechanism of inhibition of MAO by cyclopropylamines. 

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