1.4- Dihydroxybenzene, from phenol

Polystyrenes cross-linked with divinylbenzene commercial resins, such as Lewatit VP-OC 1163 (Bayer AG, Leverkusen, Germany), Amberlite XAD-4 (Rohm Haas, Frankfurt, Germany), and Serdolite PAD I, PAD II, and PAD III (Serva AG, Heidelberg, Germany), have been applied in the adsorption of phenol, chlorophenols, and dihydroxybenzenes from water solutions [90]. Besides, commercial polymethacrylate/divinylbenzene resins, for example, Supelcogel TPR-100... 

Supelco, Deisenhofen, Germany) have been investigated in the adsorption of phenol, chlorophe-nols, and dihydroxybenzenes from water solutions [90],... 

TP and Ag + were found to react with 4-methoxyphenol and 3,5-dimethoxyphenol by 100% electron transfer (equation 2), whereas with TlOH the efficiency of electron transfer is only ca 75%. The ease of oxidation increases considerably in going from the neutral phenols to the phenolates even the weak oxidants Tl(OH)2 and Ag(OH)2 are able to oxidize the phenolates with 100% yield to give the corresponding phenoxyl radicals. In going from phenol to the dihydroxybenzenes the oxidizability increases hydroquinone and resorcinol are oxidized with 100% yield not only by TP but also by the weaker oxidant TlOH . Catechol forms a complex with Tl(II), which has the same structure as thaf produced by reaction of ortfjo-semiquinone radical with Tl or by reaction of ort/io-benzoquinone with XT . The rate constants for reaction of the Tl(II) and Ag(II) species are between 10 -10 M s (see Table 1). 

This process has been widely studied and led to the constmction of new and original industrial units. Interest in the reaction stems from the simplicity of the process as well as the absence of undesirable by-products. However, in order to be economically rehable, such a process has to give high yield of dihydroxybenzenes (based on hydrogen peroxide as well as phenol) and a great flexibiUty for the isomeric ratio of hydroquinone to catechol. This last point generated more research and led to original and commercial processes. 

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). 

In the case of low temperature tar, the aqueous Hquor that accompanies the cmde tar contains between 1 and 1.5% by weight of soluble tar acids, eg, phenol, cresols, and dihydroxybenzenes. Both for the sake of economics and effluent purification, it is necessary to recover these, usually by the Lurgi Phenosolvan process based on the selective extraction of the tar acids with butyl or isobutyl acetate. The recovered phenols are separated by fractional distillation into monohydroxybenzenes, mainly phenol and cresols, and dihydroxybenzenes, mainly (9-dihydroxybenzene (catechol), methyl (9-dihydtoxybenzene, (methyl catechol), and y -dihydroxybenzene (resorcinol). The monohydric phenol fraction is added to the cmde tar acids extracted from the tar for further refining, whereas the dihydric phenol fraction is incorporated in wood-preservation creosote or sold to adhesive manufacturers. Naphthalene Oils. Naphthalene is the principal component of coke-oven tats and the only component that can be concentrated to a reasonably high content on primary distillation. Naphthalene oils from coke-oven tars distilled in a modem pipe stiU generally contain 60—65% of naphthalene. They are further upgraded by a number of methods. 

One of the earliest reports of LO inhibition concerned the effects of ortho-dihydroxybenzene (catechol) derivatives on soybean 15-LO [58]. Lipophilic catechols, notably nordihydroguaiaretic acid (NDGA) (19), were more potent (10 /zM) than pyrocatechol itself. The inactivation was, under some conditions, irreversible, and was accompanied by oxidation of the phenolic compound. The orfAo-dihydroxyphenyl moiety was required for the best potency, and potency also correlated with overall lipophilicity of the inhibitor [61]. NDGA and other phenolic compounds have been shown by electron paramagnetic resonance spectroscopy to reduce the active-site iron from Fe(III) to Fe(II) [62] one-electron oxidation of the phenols occurs to yield detectable free radicals [63]. Electron-poor, less easily oxidized catechols form stable complexes with the active-site iron atom [64]. 

Anticipated products from the reaction of phenol with ozone or OH radicals in the atmosphere are dihydroxybenzenes, nitrophenols, and ring cleavage products (Cupitt, 1980). Reported rate constants for the reaction of phenol and OH radicals in the atmosphere 2.8 x 10 " cmVmolecule-sec at room temperature (Atkinson, 1985) and with NO3 in the atmosphere 2.1 x lO" cmVmolecule-sec at 296 K (Atkinson et al., 1984). 

Fig. 15.22 On-line REMPI-TOFMS (at 266 nm) analysis of roast gas while roasting 80 g Ara-bica coffee, a The full-time-mass-intensity three-dimensional plot as recorded during roasting, b A time-intensity cross-section from a at a fixed time (medium roast level). The three phenolic VOCs, phenol (m/z 94), giraiacol (m/z 124) and 4-vinylguaiacol (150 m/z), are efficiently ionised at 266 nm. In addition, firrfurylacohol (m/z 96), dihydroxybenzene (m/z 110), indol (m/z 117) and caffeine (m/z 194) were also detected. (Adapted from [203])...

Reaction between phenol and hydroxyl yields the dihydroxybenzenes, which can then undergo further oxidation (hydroquinone to benzoquinone, further hydroxylated to hydroxybenzoquinone, catechol and resorcinol to trihydroxybenzenes [79,100]). The condensation products, phenoxyphenols and dihydroxybiphenyls, most likely originate from the reaction between phenol and the phenoxyl radical [101]. Their presence indicates that some phenoxyl forms in the system, due to the reaction of phenol with OH or NO2. The possibility for NO2 to oxidise phenol to phenoxyl has been the object of a literature debate [102,103] in the context of nitration processes. The problem can be tackled upon consideration of the reduction potentials of the various species. The reduction potential of phenoxyl to undissociated phenol is E = 1.34 V - 0.059 pH [104], while for the reduction of nitrogen dioxide to nitrite it is E = 0.90 V [105]. Accordingly oxidation of phenol to phenoxyl would be possible above pH 7.5, and of course in the presence of phenolate (pH > 10 [106]). 

Hydroxyl radical generated from hydrogen peroxide in the presence of iron(II) salts hydroxylates most aromatic centres, and indeed a phenol hydroxylation process based on this chemistry was operated for nearly a decade.471 The process ran at similar conversions to the acid-catalysed route mentioned above, however selectivity to dihydroxybenzenes was somewhat lower, and some resorcinol was formed along with catechol and hydroquinone. 

Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxylbearing carbon. Among the coloured products from the oxidation of phenol by chromic acid is the dicarbonyl compound p-benzoquinone (also known as 1,4-benzoquinone or simply quinone). Dihydroxybenzenes, hydroquinone (7.30) and catechol (7.32) are oxidized to p-benzoquinone (7.31) and o-benzoquinone (7.33), respectively, by milder oxidants such as Jones reagent. Fremy s radical (7.34) is an excellent and very specific oxidizing agent for the oxidation of phenols to o- or p-benzoquinones. (m-Quinones do not exist.)... 

Dihydroxybenzene may be prepared from 2-hydroxybenzaldehyde by the Dakin reaction, which involves oxidation in alkaline solution by hydrogen peroxide (Scheme 4.15). The reaction involves a 1,2-shift to an electron-deficient oxygen and is similar to the cumene process used to synthesize phenol (Section 4.2). 

The naphthylamines may be prepared by reduction of the corresponding nitro compound, but they are readily accessible from naphthois by the Bucherer reaction The naphthol is heated, preferably under pressure in an autoclave, with ammonia and aqueous sodium hydrogen sulfite solution, when an addition-elimination sequence occurs. The detailed mechanism is not completely elucidated, but the Bucherer reaction is restricted to those phenols that show a tendency to tautomerize to the keto form, such as the naphthois and 1,3-dihydroxybenzene (resorcinol). Using 1-naphthol for illustration, the first step is addition of the hydrosulfite across the 3,4-double bond of either the enol or keto tautomer (Scheme 12.9). Nucleophilic attack by ammonia at the carbonyl group... 

Diaryl carbonates are made from the reaction of phosgene with two molar equivalents of the particular sodium phenolate [3]. However, the direct conversion of phenols can be achieved by reaction of phosgene in the presence of quaternary ammonium salts, or acid acceptors (such as pyridine), at ambient temperatures [2016]. The three dihydroxybenzenes... 

The ionized dihydroxybenzenes 39" behave similarly (Scheme 17). Note that, in this series, one of the phenolic hydroxyl groups acts as a hydrogen acceptor and the other as an H donor. The El mass spectrum of catechol (o-39) exhibits a significant ortho effect. While the intensity of the [M — H20]" peak in the El spectrum of o-39 is no greater than ca 15%B, the spectra of resorcinol (m-39) and hydroquinone (p-39) both show negligibly smah [M — H20] + peaks ( 2%B). It is likely that water loss from the intermediate 47 generates again bicyclic [M — H20]" ions, i.e. ionized benzoxirene 48. And, notably, the CO losses does not parallel the ortho effect of the water elimination in this series, as it is the most pronounced ion the case of m-2>9. 

Nitrated hydroxyaromatics may enter into the atmosphere from both primary and secondary sources. The formation of nitrophenols and nitrocresols in die combustion processes of motor vehicles has been reported by Tremp et al. (1993). Others primary sources may be combustion of coal, wood, manufacture of phenol-formaldehyde resins, pharmaceuticals disinfectants, dyes and explosives (Harrison et al., 2005). Studies in our and other laboratories have shown that an additional important source of diese compounds in the atmosphere could be the gas-phase OH-radical initiated photooxidation of aromatic hydrocarbons such as benzene, toluene, phenol, cresols and dihydroxybenzenes in the presence of NOx during the daytime as well as the reaction of NO3 radicals widi these aromatics during the night time (Atkinson et al., 1992 Olariu et al., 2002). Once released or... 

The questions raised by Kekule s interpretation of the mechanism of this reaction led to a number of investigations which involved a further study of the oxidation of aromatic phenols and quinones. Thus, for example, Zincke in collaboration with Kuster48 and Rabinowitch 4 undertook a systematic investigation of the action of chlorine (potassium chlorate plus hydrochloric acid) on various derivatives of the three dihydroxybenzenes. The results of these experiments showed that these classes of benzene derivatives split up under conditions to give aliphatic compounds. The intermediate compounds which were formed from tetrahydrobenzene during these transfonnations, were identified as the hexachloro-o-diketone derivative (I) and the pentachloro-m-diketone derivative (II). 

Vapor phase catalytic alkylation of phenols with methanol was carried out on various phosphates as catalysts. The best activity and selectivity was observed on boron, rare-earth and niobium phosphate. With boron phosphate, the reaction is very selective for O-alkylation even at high temperature. On this catalyst o-methoxy-phenol is selectively obtained from 1-2-dihydroxybenzene. With rare-earth phosphate calcinated at 400°C and with niobium phosphate, O-alkylation selectivity decreases with an increase of reaction temperature. For rare-earth phosphates it is possible to improve the selectivity by calcination at higher temperature or by a wetness impregnation of cesium hydrogenophosphate. An explanation of these results is proposed. 

The discoveries of the isomeric dihydric phenols were as old as that of phenol itself. Catechol,(1,2-benzenediol, 1,2-dihydroxybenzene, o-dihydroxybenzene) was first obtained in 1839 essentially from the dry distillation of tannin. Resorcinol (1,3-dihydroxybenzene), was isolated in 1864 from the alkaline fusion of galbanum, and of asafoetida, resins, repectively from Iranian species of Ferula and Narthex asafoetida. In 1820 hydroquinone was recovered from the dry distillation of quinic acid although it was not investigated structurally until 1844 by Wohler. 

Antioxidants derived from cardanol such as 2-pentadecyl-1,4-dihydroxybenzene have been referred to (ref. 2). Hindered phenols are generally more efficient and... 

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