Kinetic investigations of azo dye oxidation in aqueous media

Abstract

A detailed kinetic and spectroscopic investigation has been made of the oxidation of 4-(4-sulfophenylazo)-1-naphthol (Orange I, 4) and 1-(4-sulfophenylazo)-2-naphthol (Orange II, 5) and a series of specially synthesised, substituted 1-arylazo-2-naphthol dyes (6a, 6b) by hypochlorite, peracids and hydrogen peroxide in aqueous media. In all cases, observed second-order rate constants exhibit maxima in alkaline media at a pH equal to the midpoint of the pK_A values of dye and oxidant. It is found that substituents upon the aryl ring ortho to the azo group suppress dye reactivity towards hypochlorite and, in addition, it is shown that peracids exhibit similar trends. In contrast, oxidation by hydrogen peroxide is enhanced by ortho substituents, suggesting that it functions via a different mechanism. Surprisingly, the dye common anion has been identified as the form of the dye most susceptible to oxidation by peracids and hypochlorite, with the undissociated oxidant molecules acting as electrophiles. In contrast, dye oxidation by hydrogen peroxide proceeds via the perhydroxyl anion and the hydrazone tautomeric form of the dye. The main function of ortho substituents is to increase the dye pK_A, thereby influencing observed rates (k_2obs) by changing the equilibrium concentration of reactive species. Substituents also exert an electronic effect on absolute rate constants, k₂, giving good Hammett plots for both ortho and para substituents. Reactive oxygen species, e.g. singlet oxygen, superoxide or hydroxyl radicals, are not implicated in the rate-controlling step of the reaction.

References

1. P. Gregory and C. V. Stead, J. Soc. Dyers Colour, 1978, 94, 402.
2. K. Bredereck and C. Schumacher, Dyes Pigments, 1993, 21, 45.
3. T. Omura, Y. Kayane and Y. Tezuka, Dyes Pigments, 1992, 20, 227.
4. T. Omura, Dyes Pigments, 1993, 23, 179; 1994, 24, 125; 1994, 26, 33.
5. R. Ohura, A. Katayama and T. Takagishi, Textile Res. J., 1991, 61, 242; 1992, 62, 552.
6. K. Bredereck and C. Schumacher, Dyes Pigments, 1993, 23, 121.
7. D. M. Davies and P. Jones, J. Soc. Dyers Colour, 1983, 98, 17.
8. D. Phillips, M. Duncan, E. Jenkins, G. Bevan, J. Lloyd and J. Hoffmeister, J. Soc. Dyers Colour, 1996, 112, 287.
9. S. Goszcynski, A. Paszczynki, M. B. Pasti-Grigsby, R. L. Crawford and D. L. Crawford, J. Bacteriol., 1994, 176, 1339.
10. M. Chivukula, J. T. Spadaro and V. Renganathan, Biochemistry, 1995, 34, 7765; J. T. Spadaro and V. Renganathan, Arch. Biochem. Biophys., 1994, 312, 301.
11. J. T. Spadaro, L. Isabelle and V. Renganathan, Environ Sci & Tech., 1994, 28, 1389.
12. H. Zollinger, Colour Chemistry, VCH Publications, Weinheim Germany, 1987.
13. P. F. Gordon and P. Gregory, Organic Chemistry in Colour, Springer-Verlag, New York, 1983.
14. J. Griffiths, J. Soc. Dyers Colour, 1971, 801; 1972, 106.
15. K. Bredereck and C. Schumacher, Dyes Pigments, 1993, 21, 23.
16. K. Hamada and M. Mitsuishi, Dyes Pigments, 1992, 19, 161.
17. J. Oakes, P. L. Gratton, R. J. Clark and I. P. Wilkes, to J. Chem. Soc., Perkin Trans. 2, submitted; J. Oakes and P. L. Gratton, J. Chem. Soc., Perkin Trans. 2, submitted.
18. D. Swern, Organic Peroxides, Wiley Interscience, vol I, 1970, p. 313.
19. D. M. Davies and P. Jones, J. Org. Chem., 1978, 43, 769.
20. S. B. Brown, P. Jones and A. Suggett, ‘Inorganic Reaction Mechanisms, ed. J. O. Edwards, J. Wiley and Sons, Inc., 1970, p. 159.
21. pK_A Prediction for Organic Acids and Bases, eds. D. D. Perin, B. Dempsey and E. P. Serjeant, Chapman and Hall, 1981.
22. J. Hine, Physical Organic Chemistry, McGraw-Hill, New York, 1962.
23. J. O. Edwards, Peroxide Reaction Mechanisms, ed. J. O. Edwards, J. Wiley, Interscience, New York, 1982.
24. P. Robson, J. Chem. Soc., 1964, 5170.
25. D. M. Davies and M. E. Deary, J. Chem. Soc., Perkin Trans. 2, 1991, 1549.
26. W. P. Jencks, Acc. Chem. Res., 1976, 9, 425; 1980, 13, 161.
27. N. A. Johnson and E. S. Gould, J. Org. Chem., 1974, 39, 407.
28. P. Ball and C. H. Nicholls, Dyes Pigments, 1982, 3, 5.
29. J. Oakes, P. L. Gratton and I. Weil, J. Chem. Soc., Dalton Trans., 1997, 3805.
30. J. Oakes, G. J. Welch and P. L. Gratton, J. Chem. Soc., Dalton Trans., 1997, 3811.