Antioxidant activity aromatic amines

Although aminyl radicals are stable towards oxygen, they can oxidi2e other aromatic amines, phenols and thiols (10), and regenerate the diarylamine. Thus, mixtures of phenols and diarylamines frequendy show better antioxidant activity than either one alone. This is called synergism. 

Radical Trapping. Figure 2 shows some of the reactions of aromatic amines that contribute to their activity as antioxidants and to their tendency to form highly colored polyconjugated systems. 

Antioxidants that break chains by reactions with peroxyl radicals. These are reductive compounds with relatively weak O—H and N—H bonds (phenols, naphthols, hydro-quinones, aromatic amines, aminophenols, diamines), which readily react with peroxyl radicals forming intermediate radicals of low activity. 

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

The value of antioxidant protection by way of natural food sources has been pointed out in the literature with considerable frequency. Among the components of soy flour known to have some antioxidant properties are isoflavones and phospholipids. Amino acids and peptides in soybean flour also possess some antioxidant activity. There also may be some antioxidant impact from aromatic amines and sulfhydryl compounds. 

In addition, ceruloplasmin shows oxidase activity towards bifunctional aromatic amines and phenols, but with much higher Km values, and also to a group of pseudosubstrates that can rapidly oxidize Fe11. Ceruloplasmin may also have a role as a serum antioxidant. 

A sulfur moiety introduces a hydroperoxide deactivating effect into the stabilizer molecule. Moreover, the condensation of low molecular weight compounds with halides of sulfur represents an easy synthetic approach to the increase of the molecular weight and consequently to the improvement of the physical persistency of stabilizers. Efficient AO were prepared by this way from alkylphenols, aromatic amines or phenothiazine [21,174]. Most of them contain two active units connected with a sulfide/disulfide bridge and should be listed rather among high molecular weight stabilizers. Antioxidant 136 (n > m), prepared from pyrocatechol and... 

Attachment of B ansformation Products of Stabilizers. Up-to-date knowledge dealing with the chemistry of transformation products of phenolic [6, 15, 17, 20] and aromatic aminic [16, 43, 230] antioxidants and photoantioxidants based on hindered piperidines [10] indicates the possibility of attaching compounds having structures of quinone imine or quinone methide, or of radical species like cyclohexadienonyl, phenoxyl, aminyl or nitroxide to polymeric backbones. These reactions proceed mostly via reactivity of macroalkyl radicals derived fi-om stabilized polymers. Various compounds modelling this reactivity have been isolated [19, 230]. These results are of importance mainly for the explanation of mechanisms of antioxidant activity [6, 22, 24]. 

Antiozonant Properties. Aromatic secondary diamines are the only class of organic chemicals able to reduce efficiently the ozone crack growth of vulcanizates under dynamic conditions and be acceptable at the same time from both the technical and toxicological points of view. The presence of a secondary aromatic amine moiety itself in a molecule is not a sufficient condition to attain antiozonants efficiency. (E.g., secondary monomaines are only antioxidants and flex-crack inhibitors without appreciable antiozonant activity. On the other hand, all N,N -disubstituted PD antiozonants are also efficient antoxidants and most of them also act as flex-crack inhibitors (1). Both these stabilization activities have to be considered in the complex antiozonant mechanism, together with some metal deactivating activity. 

The antioxidant activity of aromatic amines gets stronger with increasing conjugation of the system in the series DHQ[Pg.101]

Antioxidant properties of aromatic nitroxides have been reported in polymers and model systems [3,24], The antioxidant activity is lower than that of the parent amines or derived hydroxylamines. 

Stabilizing activity is improved in mixtures of aromatic amines varying in their structures. This is due to different contributions to antioxidant, antifatigue or antiozonant effects. Combinations of DPA 9a with PD 11c, d, PNA with PD, ternary mixtures PD/DPA/PNA, PD lib with DHQ 16b or 16c, oligomeric 21 with 11 or 16 (R = NHCftHj), condensate 22 with lib, PT 20 with 9 or lib are examples of combinations effective in rubber protection. Mixtures containing... 

The constant potential amperometric detector determines the current generated by the oxidation or reduction of electoactive species at a constant potential in an electrochemical cell. Reactions occur at an electrode surface and proceed by electron transfer to or from the electrode surface. The majority of electroactive compounds exhibit some degree of aromaticity or conjugation with most practical applications involving oxidation reactions. Electronic resonance in aromatic compounds functions to stabilize free radical intermediate products of anodic oxidations, and as a consequence, the activation barrier for electrochemical reaction is lowered significantly. Typical applications are the detection of phenols (e.g. antioxidants, opiates, catechols, estrogens, quinones) aromatic amines (e.g. aminophenols, neuroactive alkaloids [quinine, cocaine, morphine], neurotransmitters [epinephrine, acetylcoline]), thiols and disulfides, amino acids and peptides, nitroaromatics and pharmaceutical compounds [170,171]. Detection limits are usually in the nanomolar to micromolar range or 0.25 to 25 ng / ml. 

The aromatic amines phenothiazine, phenoxa-cine, and iminostUbene (formula [276]) proved to be about two orders of magnitude more effective than common phenolic antioxidants in their protective activity against oxidative nerve cell death (Moosmann et al. 2001). This remarkable efficacy could be directly correlated to calculated properties of the compound by means of a novel, quantitative structure-activity relationship model. 

A dual tendency is manifested in the influence of carbon blacks on the rate of oxidation of raw and cured rubbers. In the absence of antioxidants, carbon blacks play the role of weak inhibitors in the presence of antioxidants, the initiating ability of carbon black, which substantially accelerates oxidations, is manifested. Evidently such behavior of the carbon black can be explained by the fact that its inhibiting action is negligibly small in comparison with the action of secondary aromatic amines, and hence is not manifested in the presence of the latter. In this case its initiating action becomes appreciable. The elucidation of which active centers on the surface of the carbon black particles so differently influence the character of the chemical processes in rubbers is of great interest however, these problems have as yet received insufficient study. 

---