Catalyzed oxidation reactions of 1,2-dihydroxybenzene (catechol) in aerated aqueous solutions of Al3+
1990; Elsevier BV; Volume: 39; Issue: 3 Linguagem: Inglês
10.1016/0162-0134(90)84008-d
ISSN1873-3344
AutoresMurray B. McBride, F. J. Sikora,
Tópico(s)Analytical Chemistry and Sensors
ResumoThe slow oxidation of catechol by O2 in the pH range of 4.5−7.0 generated intensely-colored oligomeric and polymeric products when Al3+ (as well as Ga3+ or Sc3+) cations were present in solution. A soluble blue polymeric cation with an intense visible absorption at 620–650 nm and a structureless electron spin resonance (ESR) signal near the free electron position was formed in acidic solution. This product is believed to be a partially oxidized quinone formed by oxidative polymerization of aromatic rings via metal-promoted CC bonding. ESR identified an apparent radical intermediate of this process, a ring-coupled dimer complexed to two metal ions. ESR also identified Al3+-complexed o-benzosemiquinone monomers in aqueous solution near pH 7, an indication that the radical is stabilized in the metal-chelated form. At the highest pH and catechol concentrations studied, air oxidation produced intensely green solutions in which ESR could identify a radical monomer bonded to one Al3+ ion, and possibly a radical trimer bonded to two Al3+ ions. A mole excess of Al3+ appeared to suppress the formation of CO bond-coupled products relative to the more darkly colored CC bond-coupled products. The possible involvement of intermolecular charge transfer processes in these products is discussed here.
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