Mechanism of oxidation of L-ascorbic acid by the pentaamminechromatocobalt(III) complex ion in aqueous solution

Abstract

Oxidation of L-ascorbic acid by pentaamminechromatocobalt(III) nitrate has been investigated over the ranges 6.1 ⩽ pH ⩽ 8.7, 21.0 ⩽ θ ⩽ 30.0 °C, in excess of L-ascorbic acid at a constant ionic strength of 0.50 mol dm^-3 (NaClO₄). Overall the reaction occurs in two distinct stages. The first, involving saturation kinetics, proceeds with an increase in absorbance at wavelengths 350–390 nm. During this stage the pentaamminechromato complex and the ascorbate anion react via an outer-sphere mechanism to form an ion pair. The rate equation for this stage is (i) with [A]_T being total ascorbate, K₄ and k₅ are the equilibrium constant for adduct formation and first-order rate constant for its decomposition, respectively. At 25.0 °C, k₅ and K₄ have respective values of (3.5 ± 0.4) × 10^- s^-1 and (8.5 ± 1.7) × 10² dm³ mol^-1. The second stage involving a decrease in absorbance at similar wavelengths is the reduction of the free chromium(VI) ion which is postulated to have been formed during the first stage of the reaction. The pseudo-first-order rate constant for this stage can be expressed as in equation (ii) where K₁ and K₆ are the proton-dissociation constants for ascorbate and the hydrogenchromate anion. At 25 °C, the rate constant for oxidation of ascorbate by HCrO₄^-, k₇, is 5.15 ± 0.06 dm³ mol^- s^-1 while k₈, the corresponding value for oxidation by CrO₄^2-, is (4.6 ± 1.5) × 10^- . Cobalt(III) is reduced to cobalt(II), as >90% cobalt(II) was detected at the end of the second stage of the kinetic reaction. The activation enthalpies lie in the range 20–47 kJ mol^-, and the ΔS^‡ values range from -204 J K^- mol^-1 for the first stage to -50 J K^-1 mol^-1 for the k₈ step in the second stage. The mechanism is discussed with respect to the oxidising properties of both the cobalt and chromium centres, and a comparison is also made with the reaction of free chromate ion and L-ascorbic acid.

References

1. J. H. Espenson and E. L. King, J. Am. Chem. Soc., 1963, 85, 3328; E. L. King and J. A. Neptune, J. Am. Chem. Soc., 1953, 77, 3186.
2. G. P. Haight, D. C. Richardson and N. H. Coburn, Inorg. Chem., 1964, 3, 1777; J. Y. Tong and E. L. King, J. Am. Chem. Soc., 1953, 75, 6180; S. A. Frennesson, J. K. Beattie and G. P. Haight, J. Am. Chem. Soc., 1968, 90, 6018.
3. C. H. C. Briggs, J. Chem. Soc., 1919, 67.
4. A. Okumura, N. Takeuchi and N. Okazaki, Inorg. Chim. Acta, 1985, 102, 127.
5. N. P. Sadler and T. P. Dasgupta, Transition Met. Chem., 1992, 17, 317, 409.
6. P. H. Connett and K. E. Wetterhahn, Struct. Bonding (Berlin), 1983, 54, 95.
7. P. H. Connett and K. E. Wetterhahn, J. Am. Chem. Soc., 1985, 107, 4283.
8. P. H. Connett and K. E. Wetterhahn, J. Am. Chem. Soc., 1986, 108, 1842.
9. D. A. Dixon, N. P. Sadler and T. P. Dasgupta, J. Chem. Soc., Dalton Trans., 1993, 3489.
10. B. Banas, Inorg. Chim. Acta, 1981, 53, L13.
11. S. K. Ghosh, R. N. Bose and E. S. Gould, Inorg. Chem., 1987, 26, 3722.
12. F. Basolo and R. F. Murman, Inorg. Synth., 1953, 4, 171.
13. T. P. Dasgupta and G. M. Harris, J. Am. Chem. Soc., 1967, 90, 6360.
14. J. F. Endicott, G. Hugh, M. Hoffman and A. House, J. Chem. Educ., 1969, 46, 440.
15. D. A. Dixon, N. P. Sadler and T. P. Dasgupta, Transition Met. Chem., 1995, 20, 295.
16. B. M. J. Bielski, in Ascorbic acid: Chemistry, Metabolism and Uses, eds. P. A. Seib and B. M. Tolbert, American Chemical Society, Washington DC, 1980, ch. 4, p. 82.
17. D. E. Cabelli and B. H. J. Bielski, J. Phys. Chem., 1983, 87, 1809.
18. K. Abdur-Rashid and T. P. Dasgupta, J. Chem. Soc., Dalton Trans., 1996, 1393.
19. I. B. Afana'sev, V. Grabovetskii and N. S. Kubrianova, J. Chem. Soc., Perkin Trans. 2, 1987, 281.
20. K. Tsukahara and Y. Yamamoto, Bull. Chem. Soc. Jpn., 1981, 54, 2642.
21. R. A. Kirby and T. P. Dasgupta, unpublished work.
22. M. Kimura, M. Yamamoto and S. Yamabe, J. Chem. Soc., Dalton Trans., 1982, 423.
23. N. Oishi, Y. Nishida, K. Ida and S. Kida, Bull. Chem. Soc. Jpn., 1980, 53, 2847.
24. M. J. Akhtar and A. Haim, Inorg. Chem., 1988, 27, 1608.
25. R. B. Jordan, Reaction Mechanisms of Inorganic and Organometallic Systems, Oxford University Press, Oxford, 1991, p. 182.
26. P. Martinez, J. Zuluaga, P. Noheda and R. van Eldik, Inorg. Chim. Acta, 1992, 195, 249.
27. K. Tsukahara, T. Izumitani and Y. Yamamoto, Bull. Chem. Soc. Jpn., 1982, 55, 130.