Phenol inhibition mechanism

Equations (in the form T = A/B) for the Bounday Mechanisms of the Phenol-Inhibited Oxidation of RH [69]... 

FIGURE 14.2 Domains of realization of various mechanisms of phenol-inhibited hydrocarbon oxida-... 

This second phase of the inhibition mechanism, which is proved by the isolation of a similar peroxide with quinol structure from monohydric phenols (13,15) is unfavorably affected by steric influence of the present substituent R however, even physical factors will probably influence the activity—e.g., the change in the solubility of the antioxidant in the substrate caused by substitution. The possible resonance of radical VUIb is also illustrated in the scheme. In this case the second radical R OO may be fixed in position 5 (IXb). The reaction may be used to help... 

It was shown with two types of phenolic sulphides that both reaction centres take part in transformation processes which proceed under the conditions of inhibited oxidation of polymers. The phenolic part of the molecule participates in the inhibition mechanism by processes which follow from its chain-breaking function. Not only the starting phenolic sulphides react in this way, but also the phenolic compounds which were formed by reactions at the second centre, i. e., at sulphidie sulphur, and the products of their consecutive transformations. Transformations of the sulphidie part of the molecule are connected with preventive mechanism of antioxidation action, which is characterized by the decomposition of hydroperoxides. 

Lashgari, M., Theoretical challenges in understanding the inhibition mechanism of aluminum corrosion in basic media in the presence of some p-phenol derivatives. Electrochimica Acta, 2011. 56(9) p. 3322-3327. 

The kinetic model of reactions. The inhibition mechanism of liquid-phase oxidation of hydrocarbons (RH) by phenolic compounds (InH) has been repeatedly discussed [1-16], The kinetic scheme of inhibited ethylbenzene oxidation in the presence of ora-substituted phenols, in general, includes a set of reaction steps as shown in Table 7.1. The scheme is... 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

Kharitonov, V. V., Psikha, B. L., Zaikov, G. E. The mathematical modelling of the inhibition mechanisms of sterically-hindered phenols in oxidizing low density polyethylene melt. Polymer Degradation and Stability, 51 (1996), p. 335 -341... 

Kharitonov, Zaikov and Gurevich [65] have established a correlation between the structure of the molecules and the mechanism of their oxidation by means of a high sensitive differential manometric device linked to a computer. A detailed study of all kinetic values of the oxidation process and its mechanism has been developed. This allows one to predict the oxidation behavior of compounds, the antioxidative activity and inhibition mechanism. The potential of the beginning of anodic oxidation of phenolic stabilizers in PP is in good accordance with induction period results. 

Fig. 1. Schematic representation for the three main CA inhibition mechanisms (A) Sulfonamides (and their isosteres, sulfamate, and sulfamide) substitute the fourth zinc ligand and bind in tetrahedral geometry of the metal ion (Alterio et al., 2009) (B) Inorganic anion inhibitors (thiocyanate as an example) add to the metal ion coordination sphere leading to trigonal bipyramidal adducts (Alterio et al., 2009) (C) Phenols anchor to the Zn(II) coordinated water molecule/hydroxide ion (Nair et al., 1994) (D) Coumarins (hydrolyzed in situ to 2-hydroxycinnamic acids) occlude the entrance of the active site cavity, interacting both with hydrophilic and hydrophobic amino acid residues. The inhibitor does not interact at all with the catalytically crucial Zn(II) ion which is coordinated by three His residues and a water molecule (Maresca et al, 2009 Maresca et al., 2010).

The functionalized phenaceturates 16 (Fig. 11.10) are substrates of class A and C [3-lactamases, especially the class C enzymes, as observed with the parent unfunctionalized phenaceturates 15. They are also modest inhibitors of these enzymes and the serine DD-peptidase of Streptomyces R61. The inhibition of class C [3-lactamases is turnover dependent, as expected for a mechanism-based inhibitor. Inhibition is not very dependent on the nature of the leaving group, suggesting that the QM is generated in solution after the product phenol has been released from the active site. It therefore... 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

Although several allelochemicals (primarily phenolic acids and flavonoids) have been shown to inhibit mineral absorption, only the phenolic acids have been studied at the physiological and biochemical levels to attempt to determine if mineral transport across cellular membranes can be affected directly rather than indirectly. Similar and even more definitive experiments need to be conducted with other allelochemicals that are suspected of inhibiting mineral absorption. Membrane vesicles isolated from plant cells are now being used to elucidate the mechanism of mineral transport across the plasma membrane and tonoplast (67, 68). Such vesicle systems actively transport mineral ions and thus can serve as simplified systems to directly test the ability of allelochemicals to inhibit mineral absorption by plant cells. 

A) Phenols of this group react with peroxyl radicals, hydroperoxide, and dioxygen, while respective phenoxyl radicals can react with RH and ROOH. Reactions of these phenols with R02 most commonly give rise to quinones the breakdown of phenoxyls does not produce active radicals. This group includes all phenols, except 2,6-di-/er/-alky I phenols and alkoxy-substituted phenols. Phenols of this group can inhibit oxidation by mechanisms I-VII. 

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. 

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