Quinols and Catechols

Oxidative de-methylation of Z-(a-hydroalkyl- and o -oxoalkyl-)-l,4,4,8-tetramethoxynaphthalene by [Ce(N03)6] produces two isomeric dimethoxynaphthaquinones. The oxidative addition of ortho- and para-quin-ones to four coordinate planar Co(II) complexes has also been studied. For the o-quinones, the reaction produces Co(III) complexes containing o-semi-quinone radical anions, although changes in the structure of the ligands such as to facilitate simple electron transfer results in competition between the oxidative 

Using rate constants derived from reaction of H2A and HA with [Co(ox)3] and [Fe(phen)3], comparisons of the Marcus-derived one-electron potentials for H2A /H2A and HA /HA with molecular orbital calculations for the homo energy confirm the greater reactivity of HA over H2A. It is pointed out that the Marcus-derived potential for HA /HA , 0.85-1.0 V, is greater than the best available measurement for this parameter, 0.68 V. The self-exchange rate for ascorbate radical is lO -lO M s and indicates a considerable barrier to electron transfer. The ascorbate radical A also has a high intrinsic barrier to electron transfer, and detection of second-order kinetics in the decomposition of A suggests a dimerization step with subsequent acid catalysis. 

The effect of Cl ion on the copper-catalyzed autoxidation of ascorbic acid has been examined. It is pointed out that the rate dependence on [O2] can be explained by a mechanism involving a copper(I) intermediate rather than the copper(III) proposed earlier. 

A study of the cerium(IV) oxidation of hydroquinone and hydroquin-one esters has been undertaken to investigate coupling of phosphorylation to two-electron oxidation. Reactions of substituted hydroquinones are thought to be inner sphere since there is little rate variation with structure (Table 3.2), and Marcus predictions lead to lower estimates of the rate. Although slower than other hydroquinones, the phosphate and sulfate esters show little difference in reactivity, implying that little P-O or S-0 stretching is required to obtain the semiquinone radical. 

Hydroquinone has been used to probe the reactivity of copper complexes with substituted phenanthroline ligands where it is thought that 

Rate constants in the cross-reactions between [Felphenls] and 1,2-, 1,3-, and 1,4-benzenediols have been used to calculate the reduction potentials of the corresponding H2At radicals. The values correlate with HOMO energies derived from molecular orbital calculations.  

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Figure 11.5 Elution of quinol and catechol on the boronate affinity monolithic column. Mobile phase (a) 250 mM ammonium acetate (pH 8.5), switched to 100 mM HAc (pH 2.7) at 6 min (b) baseline. (Reproduced from ref. 7 with permission. Copyright 2009, Elsevier BV.)...

In the rabbit, phenol gives rise to catechol and quinol. Hydroxyquinol is a minor metabolite of catechol but not of quinol, and neither quinol nor resorcinol is further oxidized to trihydric phenols.126 Aniline and its derivatives do not undergo meta substitution, but give rise to ortho- and para-directed hydroxylation products only.122... 

The activation parameters are similar to those for the overall reactions of quinol and its monomethyl ether (above). The pH dependence of 2 shows a gradual increase in the range pH 1-4, then a more rapid increase to a maximum at pH 8.5, followed by a rapid decrease. Kaiser and Weidman accounted for this quantitatively by assuming reactions of catechol with HsIOg and a periodate monoanion, and either reaction of catechol with or reaction of the... 

The fact that Weiss and Downs have been aide to isolate phenol in the products of their reactions with solid catalysts indicates a hydroxylation mechanism similar to that postulated in the case of vapor phase catalysis, in whidi the formation of the monohydroxylated derivative is the first step. The presence of the hydroxyl group as a substituent in the benzene molecule activates the para and ortho positions so that the introduction of a second oxygen molecule would be expected to result in the formation of quinol (C6H4(OH)2l 4) and catechol (C0H4(OH)21 2) with a preponderance of the former. Quinone which would result from the further oxidation of quinol has been found in the oxidation products from benzene for the case of the homogeneous catalytic reaction. 

Table 3.2, Rate Constants and Activation Parameters for Reactions of Quinols H2C and Catechols H2C at 25°C...

Methods which are applicable to relatively few phenols, include the bromine method (82) for phenol, the indophenol method (77) for p-amino-phenol, the phloroglucinol method (68) for quinol, and the cobalt method for catechols (2). 

Benzene, This simplest aromatic compound is utilized by a variety of bacteria as the sole source of carbon and energy in a mineral salts solution (Beerstecher, 1954, p. 189). In the rabbit and the human several hydroxylated intermediates have been identified (Porteus and Williams, 1949) phenol, 1,2-dihydroxybenzene (catechol), 1,4-dihydroxybenzene (quinol), and 1,2,4-trihydroxybenzene (hydroxyquinol). In bacteria, the fissioned ring product has been isolated as trans,trans-m xcomc acid (Kleinzeller and Fend, 1952 Wieland et al., 1958). Catechol is the probable immediate precursor of muconate, this conversion having been demonstrated repeatedly by (1) isolation of muconic acid, (2) simultaneous adaptation studies, and (3) experiments with the cell-free, ferro-enzyme pyrocatechase (Hayaishi and Hashimoto, 1950 Evans and Smith, 1951 Evans et al., 1951 Stanier and Ingraham, 1954). Two atoms of oxygen are added per molecule of catechol. Hayaishi and co-workers (1955) have shown by means of experiments with and and... 

Phenol itself is converted to catechol, quinol, and hydroxyquinol in the rabbit (Porteus and Williams, 1949), and to catechol by a gram-negative vibrio (Evans and Happold, 1939 Evans, 1947). A modified sequential monohydroxylation scheme was proposed by Wieland and co-workers (19S8) for Nocardia sp. on the basis of results using the simultaneous adaptation technique (Eq. 9). 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Fe7-ric chloride reaclion.—Dissolve a drop of the free phenol in water and add a drop of ueutral ferric chloride. A green (catechol), blue (orcinol, pyiogallol) or purple (phenol, resorcinol) colouration is produced, which is often destroyed by acid or alkali. Quinol is o.xidised to quinone, and turns biown (p. 193). The naphthols give precipitates of dinaphthol (p. 220). 

Aromatic hydroxylation. Aromatic hydroxylation such as that depicted in Figure 4.3 for the simplest aromatic system, benzene, is an extremely important bio transformation. The major products of aromatic hydroxylation are phenols, but catechols and quinols may also be formed, arising by further metabolism. One of the toxic effects of benzene is to cause aplastic... 

Inoue O, Seiji K, Kasahara M, et al. 1988a. Determination of catechol and quinol in the urine of workers exposed to benzene. Br J Ind Med 45 487-492. 

Cupric salts in 0-00001 N-concentration can be detected by the formation of a turbidity with a mixture in solution of an alkali-metal thiocyanate and either gallic acid, tannic acid, catechol, or quinol.5... 

Disposition in the Body. Rapidly absorbed from the gastrointestinal tract and readily penetrates the skin. It is metabolised by conjugation to yield phenyl glucuronide and phenyl sulphate small amounts are oxidised to catechol and quinol which are also conjugated. Oxidation of these metabolites to quinones may tint the urine green. Acid-labile phenol conjugates are present endogenously in serum at concentrations of about 0.1 pg/ml. Phenol is a metabolite of benzene. 

The migration of oxygen from a quaternary center in a cyclohexadienone may be preferred to a carbon shift, when present as an ether or ester function rather than free hydroxy. Thus the p-quinol acetate (117) yields the orcinol monoacetate (118 79%) on treatment at room temperature with trifluoroacetic anhydride, and the p-quinol ether (119) forms the resorcinol diethyl ether (120 71%) in ethanolic sulfuric acid. In the second case, hemiketalization must intervene also some methyl shift (12%) is observed. With the quinol (121), treatment with acetic anhydride-sulfuric acid leads to the lactone (122) acetylation or lactonization probably precedes oxygen shift. A number of related examples can be found in the steroid area. - Thermal 1,3-shifts of p-quinol acetates can also be induced acetate (117) yields catechol acetate (123 50-60%, 45 °C) by way of isomerization of the first-formed acetate (124). In the o-quinol acetate series, 1,2-acetoxy shift is seen in (125) (126 92%) and in (127) (128 90%), both in... 

The further oxidation of phenol may also result in the formation of catechol, C,iH4(OH) (1 2). The transformation may be effected by fusion with sodinm hydroxide.85 The snbstance may also be obtained by oxidizing benzene with hydrogen peroxide in the presence of ferrous sulfate88 and by reducing o-benzoquinone with aqueous sulfurous acid in the cold.81 Quinol may be prepared from phenol by oxidation with potassium persulfate in alkaline solution.38 It can also be obtained directly from benzene by the electrolytic oxidation of an alcohol solution to which... 

Oxidation of phenols. p-Substituted phenols give p-quinols. Catechols and hy-droquinones are oxidized to the corresponding quinones. 

Chlorophenols.—The three inonochlorinated compounds are obtainable from the corresponding ohloranilins. Orthoohloro-phenol (1—2) is a colorless liquid, boils atl7o°-170° (347°-848°.8 P.), converted into catechol by KHO. Metachlorophenol (1—3) is a liquid, boiling at 214° (417°.2 F.). KHO converts it into resorcin. Parachlorophenol (1—4) is a crystalline solid, fusible at 37° (98°. C F.), converted into quinol by fusion with KHO. Di-, tri-, and penta-ohlorophenols are also known. 

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