Catechol, hydrogen peroxide determination

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

The hydroxylation reaction of phenol with hydrogen peroxide and zeolite encapsulated MePc has received considerable attention. With the perchlorinated phthalocyanine (ClnPc) and tetra-nitro ((N02)4Pc) substituted ligands, catalysts with superior activity have been obtained [32], Such catalysts have been prepared via the zeolite synthesis method around the individual complexes. With the former more bulky complex only the slimmer hydroquinone (HQ) has been obtained, while with the encapsulated perchloroPc equal ratios of catechol (CAT) and the para-isomer have been obtained (see table). The unsubstituted Pc in zeolite Y both with Co and Cu as metallating ion, show an excess of the ortho-isomer (CA T) [32J, corresponding to the approximate thermodynamic ratio. This points to the critical importance of the available space close to the encapsulated Pc as selectivity determining parameter when there is more space, the catalyst yields more catechol. 

The T-atoms that the framework is composed of play a major part in determining the nature of the zeolite. Transition metals can be incorporated into the structure to produce catalysts for selective oxidation reactions. An example of this behaviour is the zeolite TS-1 (titanium silicate-1) which is used in industry for the production of hydroquinone and catechol from phenol and hydrogen peroxide. 

---