FORMATION OF PHENOLS AND QUINONES

In order to simplify as far as possible the very complex data dealing with the oxidation of benzene and its various derivatives, the phenomena will be considered in the following order (a) oxidations involving the formation of diphenyl and its derivatives (b) oxidations involving the formation of phenols and quinones (c) oxidations accompanied by a break-down in the ring structure and (d) oxidations which primarily involve the side chains present in the homologs of benzene and their respective phenolic or other derivatives. In the case of each of these principal types of oxidation, it has been deemed advisable to review briefly the main facts which have been established as the result of investigations which have been carried out in the liquid phase. In this way, it is hoped that the important chemical relationships of the substances whose transformations are to be considered may be kept well in mind. 

Radicals RO play an important role in the oxidation mechanism of hydrocarbons and carbon-chain polymers. It was proved that phenoxyls are formed from phenols by their action44 49 85,143 144) followed by formation of phenolic and quinone methionide compounds in coupling reaction and by disproportionation. Also the reaction between RO and In and formation of alkoxycyclohexadienones of the type CIX may be presumed. Such compounds have not been isolated from the reaction mixtures yet. However, their formation was proved by spectra145, 179) and they probably appear only temporarily during the inhibited oxidation due to their low stability. According to1441, the following equilibrium reaction takes place ... 

Metabolism of BP mediated by the cytochrome P-450 monooxygenase system forms three classes of products phenols, dihydrodiols and quinones. Formation of phenols and dihydrodiols is obtained by an initial electrophilic attack of an enzyme-generated oxygen atom. 

This theory of Swaby and Ladd assumes that native lignin may be the source of phenols and quinones that combine with amino acids or ammonia to form polymers but, until decomposed, lignin is not active in humic acid formation. Since humic acid shows poor crystallinity it is probably not synthesized enzymatically but by heterogeneous... 

There is a difference in the behavior of benzenediolatoborate and naphthalenedio-latoborate solutions on the one hand, and lithium bis[2,2 -biphenyldiolato(2-)-0,0 ] borate (point 5 in fig. 8) lithium bis[ sali-cylato (2-) Jborate (point 6) or benzene-diolatoborate/phenolate mixed solutions on the other (Fig.8). This can be tentatively explained by the assumption of different decomposition mechanisms due to different structures, which entail the formation of soluble colored quinones from benzenediolatoborate anions and lithium-ion conducting films from solutions of the latter compounds (points 5 and 6) [80], The assumption of a different mechanism and the formation of a lithium-ion conducting, electronically insulating film is supported by... 

Notes An oxidizing agent useful for dehydrogenations particularly those resulting in aromatization, extended conjugation from aromatic systems, and the formation of enones. Oxidation of phenol provides quinones. 

Cumene oxidized relatively slowly, at about 1/13 the rate of p-xylene. This was not caused by the formation of phenol, as might be expected by an acid-catalyzed rearrangement of cumene hydroperoxide. No phenol or product clearly derived from phenol, as by radical attack or by oxidation to a quinone, was detected at any time in the reaction mixture. The two major products were a-methylstyrene and 2-phenylpropylene oxide their concentrations increased with time. The group at Shell also observed the formation of a-methylstyrene and 2-phenylpropylene oxide among the products of cumene oxidation in butyric acid at 140°C. with cobalt and manganese catalysts (30). 

PPO catalyses the dependent oxidation of phenolics to quinones. The secondary reactions of quinones lead to the formation of polymeric brown or black pigments, which are responsible for significant post-harvest losses of fruits and vegetables [72]. Finally, induced PPO activity consists of both systemic and localized components. Systemic induction of PPO in tissues in response to all types of injuries may represent a broad, defensive role for PPO in protection of juvenile tissues from subsequent attack by a broad spectrum of pathogens and pests [71]. 

The oxidation of diols by quinolinium dichromate (QDC) shows a first-order dependence on QDC and acid.5 The oxidation of phenols to quinones by quinolinium dichromate in aqueous acetic acid is acid catalysed rate-determining formation of a cationic intermediate is indicated by a p value of —3.79 and further analysis shows the rates to be influenced equally by both inductive and resonance effects of the substituents.6... 

A fairly general procedure for the synthesis of o- and p-quinones consists in coupling a phenol with a diazonium salt and reducing the resulting azo compound to an aminophenol with sodium dithionite. Mild oxidation with, for example, iron(m) chloride results in the formation of the corresponding quinone (e.g. the preparation of 1,2-naphthoquinone described and formulated in Expt 6.131). 

Based on the evidence obtained from the amount and nature of transformation products formed, a mechanism of melt stabilising action of tocopherol in PP and PE has been proposed, see Scheme 6 [34]. It is well known that, like other hindered phenols, a-tocopherol is rapidly oxidised by alkylperoxyl radicals to the corresponding tocopheroxyl radical (a-Toe, Scheme 6a). Further oxidation of the tocopheroxyl radical in the polymers leads to the formation of coupled and quinonoid-type products, e.g. SPD, TRI, DHD (see Figs. 8 and 9). Dimerisation of the intermediate o-quinone methide (QM) leads to the formation of the quinonoid-type dimeric coupled product, SPD (Scheme 6 reaction d). 

Free Phenolic Structures Containing /3-Ary I Ether Bonds The first step of the reaction involves the formation of a quinone methide from the phenolate anion by the elimination of a hydroxide, alkoxide, or phenoxide ion from the a-carbon (Fig. 7-25). The subsequent course of reactions depends on whether hydrosulfide ions are present or not. In the latter case (soda pulping), the dominant reaction is the elimination of the hydroxymethyl group from the quinone methide with formation of formaldehyde and a styryl aryl ether structure without cleavage of the /8-ether bond (Fig. 7-26). When hydrosulfide ions are present (strong nucleophiles) they react with the... 

A single enzyme is sometimes capable of many various oxidations. In the presence of NADH (reduced nicotinamide adenine dinucleotide), cyclohexanone oxygenase from Acinetobacter NCIB9871 converts aldehydes into acids, formates of alcohols, and alcohols ketones into esters (Baeyer-Villiger reaction), phenylboronic acids into phenols sulfides into optically active sulfoxides and selenides into selenoxides [1034], Horse liver alcohol dehydrogenase oxidizes primary alcohols to acids (esters) [1035] and secondary alcohols to ketones [1036]. Horseradish peroxidase accomplishes the dehydrogenative coupling [1037] and oxidation of phenols to quinones [1038]. Mushroom polyphenol oxidase hydroxylates phenols and oxidizes them to quinones [1039]. 

The equation for the formation of hydroquinone from quinone has been given above- All homologous quinones react in the same way. The hydroquinones are di-acid phenols, which dissqlve in alkalies and show all the properties of phenols. They are not volatile with steam. 

Possible atmospheric reaction products are oxy-, hydroxy-, nitro- and hydroxynitro-PAH derivatives (Baek et al. 1991). Photochemical oxidation of a number of PAHs has been reported with the formation of nitrated PAHs, quinones, phenols, and dihydrodiols (Holloway et al. 1987 Kamens et al. 1986). Some of these breakdown products are mutagenic (Gibson et al. 1978). Reaction with ozone or peroxyacetyinitrate yields diones nitrogen oxide reactions yield nitro and dinitro PAHs. Sulfonic acids have also been formed from reaction with sulfur dioxide. 

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