Dihydric phenols, oxidation

According to a general rule, not only dihydric phenols, but also those diamines of the p-series which still contain one hydrogen atom attached to each nitrogen, are dehydrogenated to quinone or quinonediimine with great ease. Hence in the oxidation solution emeraldine is also immediately converted into the doubly quinonoid chain... 

An excellent survey of the various methods of synthesis is available53 this section includes some of the illustrative methods of synthesis of the simpler analogues. These methods may be considered under the following headings (a) the oxidation of hydrocarbons (b) the oxidation of phenols and the oxidative coupling of phenols (c) the oxidation of dihydric phenols and aminophenols and (d) the cyclisation of aroylbenzoic acids. 

At a pH less than 6, molecular ozone directly attacks the phenolic ring. Then, the ozone further oxidizes the dihydric phenol to either o- or p-quinone. Because ozone is a relatively less powerful oxidant, the selectivity can be clearly demonstrated, as in Figure 8.8. The Hammett plot was first reported by Hoigne (1982) and confirmed by Gurol and Nekoulnalni (1984) (Figure 8.9). At a pH greater than 6, however, ozone is decomposed as hydroxyl radicals, and substituted phenols are ionized to form phenolate anions, which are much stronger electrophilic species than the protonated forms at low pH. As a result, the measured rate constants for some substituted phe-nolates approach the diffusion-controlled limits. 

Example, 2.1.3.1) with relatively large positive standard potentials. Dihydric phenols such as hydroquinone and catechol are moderately reducing substances that can be oxidized to the corresponding quinones (Example 2.1.2.2). For the case of PCE and catechol, the combination of these reactions gives ... 

New phenolic phosphites prepared from dihydric phenols and phosphorus halides prevent degradation of polypropylene by heat, oxidation, processing, and ultraviolet radiation. These products are active and synergistic with dithiopropionate esters. They seem to function as both free radical scavengers and peroxide decomposers. 

Oxidative transformations of hindered synthetic phenols and a-tocopherol are analogous to the oxidation of various mono- and dihydric phenols of plant origin. Natural phenols present in fruits or green tea leaves are oxidized on contact with air and/or during fermentation (a process characteristic of tea leaves) and are transformed into dark colored quinoid systems, not harmful for human beings. It may be extrapolated that trace amounts of discoloring quinones or quinone methides arising from phenolic antioxidants in plastics are harmless as well. 

Under milder conditions, brucine (121) and the corresponding dihydric phenol, bisapomethylbrucine (122), are oxidized to bruciquinone (CXXXV). 

Dihydroxy acids, preparation, 179 Dihydroxy compounds, alkylation in the Williamson reaction, 227 dehydration, 33, 236 dehydrogenation to lactones, 336 esterification, 481 preparation, by bimolecular reduction of ketones, 134 by cleavage of oxides, 172 by hydrolysis of esters, 169 by reduction of dihydric phenols, 138... 

The distinguishing feature of tyrosinase is that it catalyzes the oxidation of monohydric phenols, like tyrosine, to the dihydric form and dihydric phenols, like DOPA and catechol, to the corresponding quinones. The striking biological effects of this enzyme arise from quinones which polymerize to produce the darkening of various plants on injury and melanin in mammals. The relative oxidation rates of several dihydric phenols by tyrosinase are given in Table III. 

Laccase shares with tyrosinase the ability to oxidize dihydric phenols, like catechol, to the corresponding quinones. Whether the enzyme is active in the oxidation of monohydric phenols like cresol is a matter of controversy since the purity of preparations said to catalyze such oxidations has been questioned 107), Much more important is the ability of laccase to oxidize various aminophenols, like p-phenylenediamine, which is in fact the best substrate for the enzyme (Table III). The enzyme is important commercially because it oxidizes some complex... 

Two studies have appeared dealing with the phenolic oxidative coupling of the dihydric benzylisoquinoline LI. In the first study, oxidation of LI with potassium ferricyanide yielded two dienones (LII), one of which was obtained crystalline. The crystalline material was reduced with sodium borohydride to two noncrystalline dienols (LIII) which underwent dienol-benzene rearrangement in anhydrous methan-olic hydrogen chloride to ( )-corydine (XXVI). Rearrangement of the... 

Yamamura and coworkers used an oxygen absorption method to study the effects of a series of 46 dihydric phenols on inhibition of azo-initiated oxidation of tetralin . They reported activities in terms of the stoichiometric factor, n, and the rate of oxygen absorption, during induction periods. The 13 catechols studied all showed higher n factors (n = 2.0-2.3) and lower values than any other of the diols. Unfortunately, they were not able to obtain values. 

ANTIMONY(III) OXIDE (1309-64-4) (1309-64-4) OjSbj Ignites and bums in heated air above 420°F/215°C. Violent reaction with strong oxidizers bromine trifluoride. Reacts with chlorinated mbber, alcohols/glycols, organic and a-hydroxy acids (fruit acids), o-dihydric phenols polyethylene glycol and other polyhydroxy compounds. 

A generally applicable reaction is that due to Elbs302 in which phenols are oxidized to dihydric phenols by 1 mole of peroxydisulfuric acid in an alkaline medium. The second hydroxyl group enters the para-position to the first. A para-substituted monohydric phenol gives good yields of ortho-oxidation products on oxidation with 3 moles of H2S208. 

The strong antioxidation efficiency of dihydric phenols and, at the same time, their facile oxidation are connected with the presence of two hydroxyl groups in positions 1,2 or 1,4 of benzene nucleus. Generally, hydroquinones have lower values of redox potential (500—660 mV185 ) than pyrocatechols (765—773 mV186 ). The values decrease by the gradual alkylation of aromatic nucleus in both isomeric series and may be weE correlated with the steric and electronic factors of substituents. However, the redox potential value is not a sufficient condition to achieve the stabEiza-... 

Analysis of the oxidation products of chain-breaking antioxidants from the group of dihydric phenols reveal that quinoid compounds are involved in the mechanism of their action. Due to the formation of benzoquinones and hydroxybenzoquinones, the dihydric phenols are staining antioxidants. This fact reduces the technical importance of this in other ways very interesting group of antioxidants. 

The majority of the reactions of catechols relate to their monoalkyl and dialkylethers (guaiacol and veratryl systems respectively) and remarkably few to the parent dihydric phenol which alone is prone to facile oxidation. 

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