Abiotic transformation of catechol and 1-naphthol in aqueous solution - Influence of environmental factors

The abiotic transformation of catechol and I-naphthol singly and in mixture s was tested in sterile Tris-HCl buffer with regard to several environmenta l factors including temperature (7 degreesC, 20 degreesC and 30 degreesC), lighting conditions, pH (between 7.0 and 9.5) and dissolved oxygen (at part ial pressures of 0.0, 220, 2200, 11000 and 22000 Pa). Irrespective of lighting conditions, catechol autoxidation was confirmed in aerated medium with a rate independent of the presence of 1-naphthol but p roportional to the dissolved oxygen concentration, to the pH (its half-disa ppearance occurred in 24 h at pH 8.5) and, to a lesser extent, to the incub ating temperature (at 20 degreesC, 20% disappeared in 10 days at pH 7.0). U nder alkaline conditions, the reaction of the anionic form (catecholate) wi th an equimolar concentration of molecular oxygen (O-2) led presumably to h ydrogen peroxide anion (HO2-) and coloured polymerization products. When tested alone, 1-naphthol was not significantly influenced either by li ghting conditions, incubating temperature or dissolved oxygen concentration . It was also found to be quite stable with respect to pH, with a 15-fold w eaker transformation rate than for catechol at the highest pH used. When tested in a mixture with catechol, 1-naphthol was found to be involved in a new chemical oxidation reaction catalyzed by catecholate. The transfo rmation of one mole of 1-naphthol consumes four moles of oxygen. In the pre sence of catechol, the stoichiometry of the I-naphthol transformation, unde r the influence of oxygen, suggests the possible formation of 2,5,6,8-tetra hydroxy 1,4-naphthoquinone via Lawsone (2-hydroxy 1,4-naphthoquinone) and n aphthopurpurine (2,5,8-trihydroxy 1,4-naphthoquinone) as hypothetic interme diates. This is the first report of the autoxidation of 1-naphthol, catalyz ed by catechol, in aqueous solution, in the absence of UV irradiation. (C) 2001 Elsevier Science Ltd. All rights reserved.