Phenols oxidation potential

Phenol oxidations by metal complexes employ the (a- /2 2-peroxo)dicopper(II) and bis(u-oxo)-dicopper(III) complexes shown in Table 17.1(e) and (f), respectively as the oxidants [75]. In this study, the rate of phenol oxidation was correlated with phenol oxidation potential (Pqx)- We note that the use of the oxidation potential is more appropriate than the lEs of Figs. 17.7 and 17.8, since the latter approximates a gas phase internal energy for oxidation whereas the oxidation potential is the free energy for the solution reaction. Reaction of the substrates shown in Table... 

Most phenolic compounds are readily oxidized at carbon electrodes. The oxidation potentials vary widely depending upon the number of ring hydroxyl groups and their positions on the ring. Many compounds of biomedical and industrial interest are phenolic and LCEC based trace determinations are quite popular. 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

Hendrickson HP, Sahafayan M, Bell MA, et al. 1994. Relationship of flavonoid oxidation potential and effect on rat hepatic microsomal metabolism of benzene and phenol. J Pharm Biomed Anal 12 335-341. 

Oxidation peak potentials of phenol derivatives were measured with cyclic voltammetry 0.53, 0.47, 0.47, 0.28, and 0.77 V vs. Ag/ AgCl for phenol, 2,6-dimethyl-, 2,6-diphenyl-, 2,6-dimethoxy-, and 2,6-dichlorophenol respectively. The oxidation potential of phenol and 2,6-dichlorophenol are relatively high and this high potential is one of the reasons why phenol and dichlorophenol could not he polymerized by the oxidation with copper catalyst or lead dioxide. On the other hand, for the electro-oxidative polymerization the potential can he kept slightly higher than the oxidation potential of phenols and the polymerization proceeds. 

Since alcohols and ketones generally possess high oxidation potentials, the indirect method is often applied to those compounds, while phenols and carboxylic acids are mainly oxidized by... 

Both inter- and intramolecular [5 + 2] cycloaddition modes have been utilized in the synthesis of natural products. Successful intermolecular cycloaddition depends on making an appropriate selection of solvent, supporting electrolyte, oxidation potential, and current density. This is nicely illustrated in Schemes 23 to 25. For example, in methanol the controlled potential oxidation of phenol (101) affords a high yield (87%) of (102), the adduct wherein methanol has intercepted the reactive intermediate [51]. In contrast, a constant current electrolysis conducted in acetonitrile rather than methanol, led to an 83% yield of quinone (103). 

Oxidizing agents have been shown to be extremely effective for removing many complex organics from wastewater, including phenols, cyanide, selected pesticides such as ureas and uracils, COD, and organo-metallic complexes [11]. Many oxidants can be used in wastewater treatment. Table 9 shows the oxidation potentials for common oxidants [24]. The most widely used oxidants in the... 

Another facet of the reactivity of aminoxyl radicals has been brought to attention by recent studies of the oxidation of substrates endowed with low redox potential. Some aminoxyl radicals, depending on the reduction potential value of the couple >N—O / >N—O, can behave as moderate one-electron abstractor towards substrates endowed with appropriate oxidation potential. This rather unprecedented reactivity feature, outlined in Scheme 10, has been substantiated by the oxidation of aniline or phenol derivatives, whose redox potential is conveniently located in the 0.4-1.0 V/NHE range ". 

Amines such as diethylamine, morpholine, pyridine, and /V, /V, /V, /V -tetramethylethylene-diamine are used to solubilize the metal salt and increase the pH of the reaction system so as to lower the oxidation potential of the phenol reactant. The polymerization does not proceed if one uses an amine that forms an insoluble metal complex. Some copper-amine catalysts are inactivated by hydrolysis via the water formed as a by-product of polymerization. The presence of a desiccant such as anhydrous magnesium sulfate or 4-A molecular sieve in the reaction mixture prevents this inactivation. Polymerization is terminated by sweeping the reaction system with nitrogen and the catalyst is inactivated and removed by using an aqueous chelating agent. 

The phenol must be relatively easily oxidizable substituents that raise the oxidation potential lead to an inhibition of the dehydrogenation reaction (2,6-dichlorophenol gives only a low-molecular-weight polymer and 2,6-dinitro-phenol does not react at all). 

The reactivity of phenols in the Cu-complex-catalyzed oxidation was studied by measuring the oxidation rate, the rate constant ke, and the redox potentials of the Cu complex and of the phenol (Fig. 29)158). The logarithm of the oxidation rate is proportional to log ke, which supports the assumption that the electron-transfer step is rate-determining. A linear relationship is observed between log ke and Hammett s o value of the phenol, which is proportional to the oxidation potential of the phenol. 

With respect to the kinetics of oxidation reactions, the same comments as made in Section 14.2 are, of course, valid. To illustrate, we consider the oxidation of substituted phenols and anilines by Mn02 and of substituted phenols by HC1O4. By analogy to the type of LFER used to evaluate NAC reduction (Eq. 14-38), we can relate oxidation reaction rates to the one-electron standard oxidation potentials of... 

That is, hox(AtXH)= - / (ArX ). For this type of reaction, E (ArX ) is positive. Hence, the more positive this value, the more difficult it is to oxidize the compound. For many phenols and anilines, polarographic half-wave potentials, Zs1/2(ArX"), determined at pH values where the compound is present in its neutral form, are available. These values should reasonably parallel the oxidation potentials of the compounds, and therefore can also be used to relate oxidation rate constants ... 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

In the former case the diphenoquinone is formed exclusively while in the latter case small amounts of low molecular weight polymer have been observed. As would be expected, substituents which raise the oxidation potential of the phenol retard the polymerization. Thus whereas 2.6-dimethylphenol polymerizes readily at room temperature, temperatures in the neighborhood of 60° C are required to polymerize 2-chloro-6-methylphenol at comparable rates and even higher temperatures are necessary to oxidize 2.6-dichlorophenol. 

Phenolic substances are for the most part readily oxidized at a graphite electrode. The oxidation potentials for phenols vary widely with structure, and some (hydroquinones and catechols) are far more readily oxidized than others (cresols). Many compounds of biological interest (catecholamines, pharmaceuticals, plant phenolics) [32] and industrial interest (antioxidants, antimicrobials, agricultural chemicals) [33] are phenolic, and trace determination based on LCEC is now quite popular. 

Like phenols, aromatic amines are oxidized at a graphite electrode over a wide range of oxidation potentials. Some compounds (phenylenediamines, benzidines, and aminophenols) are ideal candidates due to their very low oxidation potentials numerous applications have been developed. 

In studies on the OH-induced aromatic hydroxylation, the oxidation of hy-droxycyclohexadienyl radicals by Fe(CN)63 has often been used for the determination of the yield of a given precursor radical (Volkert et al. 1967 Volkert and Schulte-Frohlinde 1968 Klein et al. 1975). Other oxidants such as Cu2+, Ag+, Fe3+ or Cr3+ give lower yields, and complications are apparent, since, for example, the oxidation potential of Ag+ (0.8 V) is higher than that of Fe(CN)63 (0.36 V Bhatia and Schuler 1974). The substituent has a strong influence on the rate of oxidation (Table 6.2), and quantitative oxidation to the corresponding phenol [reaction (11)] is only observed with electron-donating substituents (Buxton et al. 1986). Even the terephthalate ion OH-adduct requires the stronger oxidant... 

---