KINETICS AND MECHANISM OF ELECTRON TRANSFER REACTIONS OF MIXED –VALENCE DI -μ- OXO TETRAKIS (1,10-PHENANTHROLINE) DI MANGANESE (III,IV) ION AND ROSANILINE HYDROCHLORIDE WITH SOME REDUCING AGENTS

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Date
2005-07
Authors
ABIODUN, Babatunde Oluwayemisi
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Abstract
- 10. My wonderful children Lekky and Timmy for turning the manuscript of this work into the beautiful typescript that it is. I also thank them for appreciating the fact that joining this academic train means a reduction in the level of the warmth and care they otherwise would have been disposed to. 11 My mother, Mrs Cecilia Modupe Omole, my brothers and sisters for their love and prayers, particularly Bidemi Ogunwale who took care of the house while most of this work was being done. - 7 - ABSTRACT The kinetics and mechanism of the electron transfer reactions of a mixed-valence manganese complex (MnIII O2 MnIV) with four different reductants ( Ascorbic acid (H2A), 1, 3 dihroxybenzene (H2R), SO3 2-and S2O3 2-) have been studied in acid medium at 28 ± 10C.The reactions were found to obey second order kinetics with the following general rate law dt [Mn O Mn ] d 2 3 IV 2 III = k2 [MnIIIO2MnIV] [Reductant]. The experimental data for three of the reaction systems were found to exhibit first order dependence on acid concentration with this general rate law. dt [Mn O Mn ] d 2 3 IV 2 III = (a +b [H+] ) [MnIIIO2MnIV] [Reductant]. For MnIIIO2MnIV / H2A system a = 3.50 x 10-4 dm3 mol-1 s-1 ,b = 2.13 x 10-3 dm-3 mol-1 s-1 at constant [H+] 0.50 mol dm-3, I = 0.5 mol dm3 (NaCl), T =30oC and λ = 580nm. For MnIIIO2MnIV / H2R system a = 0.70 x 10-3 dm3 mol-1 s-1 ,b = 0.02 x 10-2 dm6 mol-2 s-1 at [H+] = 0.50 mol dm-3, I= 1.0 mol dm-3 (NaCl), T= 28 ± 1oC and λmax = 580nm.. For MnIIIO2MnIV / SO3 2- system a = 1.22 x 10-6 dm3 mol-1 s-1 ,b = 7.83 x 10-7 dm6 mol-2 s-1at [H+] 5 x 10-2 mol dm-3, I= 0.5 mol dm-3 (NaCl), T= 28 ± 1oC and λmax = 580nm . while the rate law for the MnIIIO2MnIV / S2O3 2- system is dt [Mn O Mn ] d 8 3 IV 2 III = (a +b [H+] ) [MnIIIO2MnIV] [S2O3 2-] where a = 7.01 x 10-3 dm3 mol-1 s-1 and b = 0.92 x 10-3 dm6 mol-2 s-1at 28 ± 1oC [H+] = 2 x 10-3 mol dm-3 ,I= 0.5 mol dm-3 (NaNO3). - 8 - There were no spectroscopic and kinetic evidences for the formation of an intermediate complex in any of the reaction systems. This is not in favour of inner sphere mechanism. Therefore, the electron transfer must probably occur by the outer sphere mechanism in these systems. In MnIIIO2MnIV/H2A and MnIIIO2MnIV/SO3 2- systems, the reactions were catalysed by cation and anion species .This is in support of the outer sphere mechanism but in the MnIIIO2MnIV/H2R and MnIIIO2MnIV/S2O3 2- systems, no catalysis was observed and free radicals were also not important in the redox processes. Therefore, the results obtained are in support of proton coupled electron transfer (PCET) mechanism and is hereby proposed for the two reaction systems. The kinetics and mechanism of the electron transfer reaction of rosaniline hydrochloride (referred to as Ros) with reductants (OH-, NO3 -, IO4 -, SO3 2- and S2O6 2-) have also been studied in aqueous medium at 30oC, I =1.0 mol dm-3(NaCl, CH3COONa or NaClO4), [H+] = 1 x 10-4 mol dm-3 except for OH- system that does not involve [H+]. The stoichiometry is 1:1 in all the five systems investigated. The reaction is first order in both the [oxidant] and [reductant] for OH-, NO3 - and S2O6 2- systems respectively therefore the overall order for the system is second order with the following rate law -d/dt[Ros] = k2[Ros][reductant] In the IO4 - and SO3 2- systems, the reaction is first order in the [oxidant] and zero order in the [reductants] hence the overall order is first order with the rate law -d/dt[Ros] = ko [Ros] - 9 - The rates of the redox reactions showed direct dependence on acid concentrations for Ros/ NO3 - system and an inverse dependence on acid concentration for Ros/SO3 2- and Ros/S2O6 2- systems while in Ros/IO4 - system, acid concentration have no effect on the rate of the reaction. The overall rate equation for the reactions can be given as -d/dt[Ros] = (a+b [H+] ) [Ros][NO3 -] for Ros/NO3 -system -d/dt[Ros] = (a+b [H+]-1 ) [Ros][reductant] for Ros/SO3 2- and Ros/S2O6 2- systems The rate of the reaction displayed negative salt effect for Ros /OHsystem and a positive salt effect for Ros /NO3 - system and the rate also was sensitive to changes in the di-electric constants. Spectroscopic investigation showed the presence of short-lived intermediate complex formation in the Ros/IO4 -and Ros/ SO3 2- systems. This suggested that both systems are proposed to take place by inner sphere mechanism. In Ros/ OH-, Ros/NO3 -, and Ros/S2O6 2- systems, there was no evidence for the formation of an intermediate complex of significant stability and free radicals are absent therefore the mechanisms of their reactions are discussed in terms of outer sphere mechanism. On the basis of the experimental results obtained, the outer sphere mechanism has been proposed for Ros/ OH-, Ros/NO3 - and Ros/S2O6 2- systems respectively and inner sphere mechanism for Ros/IO4 - and Ros/SO3 2- systems.
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DEPARTMENT OF CHEMISTRY FACULTY OF SCIENCE, AHMADU BELLO UNIVERSITY. ZARIA, NIGERIA.
Keywords
KINETICS AND MECHANISM,, ELECTRON TRANSFER REACTION,, MIXED –VALENCE DI -μ- OXO TETRAKIS,, 1,10-PHENANTHROLINE,, DI MANGANESE (III,IV),, ROSANILINE HYDROCHLORIDE,, REDUCING AGENTS
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