Phenol hydroxylation benzoquinone conversion

Figure 11. The influence of residual H2O2 on the reaction testing of phenol hydroxylation to catechol (CAT), hydroquinone (HQ), and para-benzoquinone (BQ) [61], Reaction conditions 4 g phenol 50 mL water solvent 0.2 g a-Fe203 catalyst inner standard ethanol reaction temperature 70°C. Aliquots were sampled at different times and analyzed by (a) HPLC and (b) GC to determine the conversions of PHE (A) and yields of CAT + HQ + BQ ( ), CAT ( ), and BQ (T). Aliquots were also analyzed by (c) iodometric titration to determine the conversion of H2O2 (o). [Reproduced by permission of Elsevier from Ma, N. Ma, Z. Yue, Y. H. Gao, Z. J. Mol. Catal. A 2002, 184, 361-370.]...

Phenol is catabolized by liver microsomal monooxygenases to hydroxylated products (e.g., 1,4-dihydroxybenzene) that can undergo further conversion to a variety of electrophilic substances (e.g., benzoquinones). Such reactions may be involved in generating reactive toxic intermediates in the liver (Eastmond et al. 1986 Lunte and Kissinger 1983 Subrahmanyam and O Brien 1985). Based on the available data, hepatic effects are unlikely to occur at the exposure levels found in the environment or near hazardous waste sites. 

The hydroxylation of benzene on TS-1 produces phenol as the primary product and, by consecuhve oxidation, hydroquinone, catechol, benzoquinone and tarry products (Equation 18.6) [5, 30, 51]. The selectivity to phenol generally falls below 50% even at only 3-5% benzene conversion. Hydroxylation with a mixture of hydrogen and oxygen on Pd/TS-1 proved to be even less effective [52]. 

Hydroxylation is one of the most widespread conversions of phenols in redox reactions. This conversion occurs under a wide range of conditions, namely, at various pH, in organic and aqueous solutions as well as in the solid phase, due to the participation of quinoid intermediates that are prone to both ionic and radical transformations. Thus, the oxidation of 3,6-di-tcrf-butylpyrocatechol 107 in protic media is accompanied by the formation of 3,6-di-tcrt-butyl-2-hydroxy-para-benzoquinone 108 (equation 44). Hydroxylation of the 3,5-isomer 109 results in dealkylation (by an ionic or a radical route) and... 

The hydroxylation of benzene on TS-1 produces phenol as the primary product. Conversion is generally kept low, because introduction of a hydroxyl group activates the aromatic nucleus to further oxidation to hydroquinone, catechol, and eventually to tarry products (Eq. 2). Acetone, methanol, 2-butanone or just water are suitable reaction media [2,16,17]. In aqueous solution, benzoquinone was also found, in appreciable amounts, among the products. Hydroxylation of benzene with a mixture of hydrogen and oxygen, an in situ source of hydrogen peroxide, can be achieved on Pd-containing TS-1 [18]. This is, in principle, an easier route to phenol than that based on the preformed oxidant [19]. In practice, it proved less effective, because of faster catalyst decay (maximum TON... 

The copper halide/amine (pyridine) catalyst system promotes the conversion of phenol to o-benzoquinone via intermediate formation of a copper-catecholato species [107,108]. The proposed hydroxylation mechanism involves initial complexation of copper(I) with phenolate, followed by reaction with dioxygen (Scheme 6). 

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