Hydroquinone phenol oxidization

At alkaline conditions (pH=l 1) no phenol, hydroquinone were detected. This is possibly due to the fact that the rate of phenol oxidation increases under alkaline conditions with optimum pH between 9.5 and 13 [17] and so once it is formed, it is readily oxidised. The absence of phenol and increased concentration of p-hydroxybenzoic acid could be also explained by reduced decarboxylation rates under conditions of high pH, which would result in the oxidation of p-hydroxybenzaldehyde to form p-hydroxybenzoic acid. 

Metal complexes catalyze oxidation of compounds having mobile hydrogens, such as ascorbic acid, hydroquinone, phenols, and amines, in the presence of molecular oxygen [Eq. (16)]. In this reaction, a substrate coordinates to the metal catalyst,... 

Phenols are more easily oxidized than alcohols, and a large number of inorganic oxidizing agents have been used for this purpose. The phenol oxidations that are of the most use to the organic chemist are those involving derivatives of 1,2-benzenediol (pyrocate-chol) and 1,4-benzenediol (hydroquinone). Oxidation of compounds of this type with silver oxide or with chromic acid yields conjugated dicarbonyl compounds called quinones. 

However, because catechol and hydroquinone are oxidized at less positive potentials ( + 1.32 V and +1.18 V, respectively), the only detectable products are the o- and p-quinones via an overall four-electron (ECECECEC) oxidation. The data for substituted phenols (Table 12.4) indicate that election-donating substituents cause phenol to become more nucleophilic (a stronger Lewis base) and easier to oxidize. Anisole (PhOMe) is more resistant to election removal ( + 1.75 V vs. SCE) than phenol ( + 1.55 V vs. SCE), but undergoes electron removal via a similar path [Eq. (12.38)] to yield the same quinone products. 

Oxidation. Bis(p-methoxyphenyl)telluroxide (1) is a mild and selective oxidant for conversion of xanthates, thiocarbamates, thioamides, and nonenolizable thiones into the corresponding oxo derivatives, and also of thiols into disulfides. Typically these reactions afford products in 70-100% yield. 1,2- and 1,4-Hydroquinones are oxidized by 1 to o- and p-quinones, respectively. Phenylhydroxylamine is oxidized to nitrosobenzene (90% yield). Phenols, amines, enamines, alcohols, oximes, dithiolanes, isonitriles, and 2,4-dinitrophenylhydrazones are unreactive. 

Derivatives of phenol or aniline can be oxidized to quinones, the yield and ease of oxidation depending on the substituents. If an amino or hydroxyl group is in the para position, the reaction proceeds readily, as illustrated by the synthesis of quinone from hydroquinone by oxidation with a sodium chlorate-vanadium pentoxide mixture (5>6%) or with chromic-sulfuric acid mixture (92%). A para halogen atom usually has a favorable effect. Any group in the para position is eliminated or oxidized. o-Quinones are usually prepared from the corresponding catechols. A survey of procedures for the synthesis of benzoquinones by oxidation has been made. ... 

The autoxidation of hydroquinone is accompanied by the fonnation of the p-semiquinone radical this was shown as early as 1938 by Michaelis and coworkers Since then numerous additional examples of this type of reaction have been described, relating not only to hydroquinones but also to catechols, resorcinols, pyrogallols, naphthols and substituted phenols. Most of this material has been reviewed including that relating to synthetic application of phenol oxidation and to phenoxyl radicals involved in the biosynthesis of natural products -... 

Phenoxyl radicals are converted to quinones and products of phenol dimerization. Phenol oxidizes in water giving pyrocatechol and hydroquinone in the presence of metal ions [242], The best yields were obtained with Cu2+ and Fe3+ ions (100—150° C). 

The drug resembles very similar to chloroquine mechanistieally and it does not possess any added advantages over the other 4-aminoquinoline drugs. It has been demonstrated amply that the hydroquinone (phenol) amine system rapidly gets oxidized to a eorresponding quinone-imine system, either accomplished via antioxidatively and/or metabolically and the resulting product may be solely responsible for the ensuing amodiaquine toxicity. 

At around the time of maximum catechol and hydroquinone, benzoquinone began to appear and went thorough maximum. The disappearance of benzoquinone was then followed by the formation of maleic acid and fiimaric acid. These acids were then transformed into the lower molecular weight carboxylic acids such as glycolic acid, acetic acid, formic acid and oxalic acid. On the b2isis the sequence in which the reaction intermediates were formed reaction pathways of phenol oxidation could be proposed as the sequential steps in Figure 10. 

Without going into detail we simply note that oxidation of alkylaromatic hydrocarbons with polymer-immobilized catalysts can be considered as a system of successive-parallel reactions including mononuclearic hydroxylation to phenols, oxidation of phenols, oxidative dehydrogenation of hydroquinones, oxidative decomposition of quinones and thermal and catalytic decomposition of peracids. 

Simple phenols that can be converted readily to quinones by enzymatic oxidation are used by arthropods as defensive secretions. The biosynthetic origin of the phenolic substrates is not known. Probably the most remarkable of these is the defense of bombardier beetles, of the genus Brachynus. A secretion as hot as 100°C is produced by a reaction among hydroquinone, a phenolic substrate, H2O2 and the enzyme catalase. A highly exothermic reaction occurs as hydroquinone is oxidized to benzoquinone, the major product of defense. The beetle, when endangered, discharges a hot explo-... 

Conversely, nucleophilic molecules Nu) [Lewis bases e.g., catechols, hydroquinones, phenols, alcohols, and thiols (and their anions) aromatic hydrocarbons and amines (benzene, toluene, pyridine, bipyridine, etc.)] can be oxidized (1) by direct electron-transfer oxidation [Eq. (161)] [with the electron coming from the Highest-Occupied-Molecular-Orbital (HOMO)] or (2) by coupling with the oxidation product of H2O (or HO ), hydroxyl radical (HO ) [Eq. (162)]. 

In fact, the preparative bulk electrolysis of phenol on BDD anodes under galvanostatic conditions has shown that, depending on the experimental conditions, it is possible to obtain partial oxidation of phenol to aromatic compounds or its complete oxidation to CO2 [6]. In particular, at low-current density and low phenol conversion, only aromatic compounds (benzoquinone, hydroquinone and catechol) are formed during phenol oxidation [6],... 

The progress curves of the phenol oxidation in the presence of phenolase and hydroquinone were calculated according to Eq. (16)... 

Biochemical Routes. Enzymatic oxidation of benzene or phenol leading to dilute solution of dihydroxybenzenes is known (62). Glucose can be converted into quinic acid [77-95-2] by fermentation. The quinic acid is subsequently oxidized to hydroquinone and -benzoquinone with manganese dioxide (63). 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

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