Phenolate ions, oxidation

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Peroxodisulfate ions oxidize aromatic amines md phenols to colored derivatives, paitic ularly under the catalytic influence of silver ions [1-4]. 

A qualitative and preliminary picture (Fig. 11.16) of the mechanism of oxidation that emerges from our studies is the following Under the reaction conditions (pH = 6.5), the phenols exist in the phenolate form. Two phenolate ions coordinate to the two Cu(II) ions of the copper acetate dimer, reducing them to the Cu(I) oxidation state. Next, dioxygen reacts with the copper-phenolate adduct. The latter undergoes an 0-0 bond scission concomitant with the hydroxylation of the substrate. The acetate... 

Phenols (1.8 V vs Ag/Ag+) [62], in particular phenolate ion (0.55 V vs Ag/Ag+) [63], are easily oxidized by the electrochemical method to give cationic intermediates, which react with nucleophilic solvents such as MeOH, MeCN, or H2O or with the phenol itself, yielding dimers. 

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

Recently, Behiman and coworkers discussed the mechanism of the Elbs oxidation reaction and explained why the para product predominates over the ortho product in this oxidation. According to the authors, semiempirical calculations show that the intermediate formed by the reaction between peroxydisulfate anion and the phenolate ion is the species resulting from reaction of the tautomeric carbanion of the latter rather than by the one resulting from the attack by the oxyanion. This is confirmed by the synthesis of the latter intermediate by the reaction between Caro s acid dianion and some nitro-substituted fluorobenzenes. An example of oxidative functionalization of an aromatic compound is the conversion of alkylated aromatic compound 17 to benzyl alcohols 20. The initial step in the mechanism of this reaction is the formation of a radical cation 18, which subsequently undergoes deprotonation. The fate of the resulting benzylic radical 19 depends on the conditions and additives. In aqueous solution, for example, further oxidation and trapping of the cationic intermediate by water lead to the formation of the benzyl alcohols 20 (equation 13) . ... 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

Phenolate ions with propylene oxide in ethanol at 70° -0.77 0.129 -0.252 j... 

Tratnyek and Hoigne (1991) measured kr for 22 substituted phenols (mostly m- and p-substituted) in water over a pH range, to evaluate lq separately for the phenol and phenolate ion. Phenolate ions are 10 to 400 times more reactive toward 102. Satisfactory SARs for oxidation of all phenols were developed using Hammett equations with o (Shorter, 1978). Rates for all phenols correlate with cr through the relation ... 

The base promotes the formation of a phenolate ion, which undergoes a one-electron oxidation to form Cu(I) and a phenoxy radical. Two of these radicals combine to give the 4,4/-dihydroxybiphenyl compound, which can be further dehydrogenated to give the diphenoquinone. Within the detection limit of atomic absorption spectroscopy no Cu was observed in solution. Cu retention on the molecular sieve in this case is favored by the apolarity of the solvent, the absence of competing anions (e.g., acetate in solution), and the presence of base, with the latter promoting formation of copper hydroxides. 

In the field, a quick test for the general level of total phenolics in a plant can be the starting point for a more detailed quantitative analysis of the nature and amounts of specific compounds. Various oxidation-reduction methods have been exploited to analyze total phenolics in plant extracts. Many, but not all, phenols form complexes with ferric chloride (FeCy. Ferric chloride gives a color reaction with phenolic compounds. While the phenolate ion is oxidized, the ferric ions are reduced to the ferrous state. 

Oxidation rate constant k, for gas-phase second order rate constants, koe for reaction with OH radical, k os with NOj radical and ko3 with O3 or as indicated, data at other temperatures see reference aqueous photooxidation ty, = 66-3480 h in water, based on reported reaction constants for OH and ROj radicals with the phenol class (Mill Mabey 1985 Gtiesten et al. 1981 selected, Howard et al. 1991) k(aq.) > 10 M- s at pH 8, and k < 3 x 10 M- s for non-protonated species, k > 10 M- s for phenolate ions for the reaction with ozone in water using 3 mM f-BuOH as scavenger at pH 1.2-1.5 and 20-23°C (Hoigne Bader 1983b)... 

The rate coefficient 2 is proportional to the concentration of phenolate ion thus for 2-hydroxy-pyridine (pA A = 11-6) oxidation below pH 8 is slower than the spontaneous decomposition of peroxodisulphate. For the reaction of 2-hydroxy-P5rridine in 2 M sodium hydroxide, the variation of 2 with temperature is expressed by... 

The predominance of one species over another has been shown to be a function of a number of experimental variables including medium pH, electrode potential, solvent-electrolyte system, and electrode material [7,8]. Voltammetry studies of suitably substituted phenolate ions at elevated pH show the expected reversible one-electron process leading to formation of phenoxy radicals (V), with peak potentials on the order of —0.16 to —0.31 vs. SCE [8,9]. Oxidation of nonionized phenols in media sufficiently acidic to suppress ionization of the resulting cation radicals (II) (pXa —5 to 0 [10-13]) reveals positive peak potentials in the range of 1.3-1.65 V versus SCE [14,15]. As indicated earlier, the principal product-producing species from these oxidations are the phenoxy radical (V) and the phenoxonium ion (IV). 

In neutral or acidic medium, phenols are oxidized to phenoxonium ions, which can undergo an electrophilic aromatic substitution, for example with anisole to para-meih-oxyphenylcyclohexadienones (Table 2, number 14). In the oxidation of X [Eq. (5a)], the intermediate phenoxonium ion disproportionates to phenoxy radicals, which couple in 95% yield to the quinhydrone XIa [47a] ... 

Oxidations with sodium persulfate are markedly catalyzed by silver ions the actual reagent is presumed to be SjOg Ag". Toluene is oxidized to benzaldehyde (50% yield) together with coupled products, and benzyl alcohol is oxidized to benzaldehyde (75% yield). Phenols are oxidized to resinous polyphenols. ... 

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