Journal of the Chemical Society, Dalton Transactions p. 1393 - 1398 (1996)
Update date:2022-08-11
Topics:
Abdur-Rashid, Kamaluddin
Dasgupta, Tara P.
Burgess, John
The kinetics of oxidation of L-ascorbic acid (H2A) by cobalt(III) hexols, [Co{CoL4(μ-OH)2}3]6+ [L4 =(NH3)4, (en)2, or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine], was studied as a function of pH, L-ascorbic acid concentration, temperature and ionic strength, using stopped-flow and conventional spectrophotometric techniques. The rate of the reaction is first order with respect to the concentration of each reactant and increases as [H+] decreases. The kinetic data indicate involvement of the monoprotonated and deprotonated ascorbate species (HA- and A2-) in the redox process. For L4 = (NH3)4 the rate constants, k2 and k3 are 0.22 ± 0.02 and (5.51 ± 0.09) × 105 dm3 mol-1 s-1 respectively at 25°C, and the corresponding activation parameters are ΔH?2 = 103 ± 7 kJ mol-1, ΔS?2 = 89 ± 22 J K-1 mol-1 and ΔH?3 = 46 ± 3 kJ mol-1 and ΔS?3 = 19 ± 11 J K-1 mol-1. The variations in rate constants and activation parameters for the series of complexes mentioned above are discussed. The Fuoss theory was applied to the redox process to estimate the ion-pair formation constant and the rate constant for the electron transfer.
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