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The effects of temperature, dielectric constant and catalysis in the kinetics of the oxidation –reduction reactions (involving electron transfer) of N-(2-hydroxy-ethyl) ethylenediammine- N’,N’,N’-Triacetatocobalt (II) by Cu2+ cation were determined. The dielectric constant was decreased from 63.05 to 43.18 and it was found that the rates of the reaction did not show any appreciable change. This seems to mean that the change in the dielectric constant of the medium had no effect on the rates of reaction in this [CoIIHEDTAH2O]– and Cu2+ systems. At constant concentration of all the reactants, the effect of added ions on the rates of reaction was investigated by varying the concentration of acetate ion (CH3COO–) from 30×10-3 – 130×10-3 mol dm-3 and noting the rates of the reactions. The same was repeated for magnesium ion (Mg2+). For this system, the rates of reaction were found unaffected by the presence of either Mg2+ or CH3COO–The temperature dependence of rates on this reaction was investigated at 350C, 400C, 500C, 550C and 600C respectively. It was found that increase in temperature increases the rates of reaction. The plot of logkobs versus the reciprocal of the square of temperature is linear, hence the activation parameters were evaluated.
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Abstract – – – – – – – – – – – – – – – – – – – – – – – – vi
Table of contents – – – – – – – – – – – – – – – – – – – – vii
List of tables – – – – – – – – – – – – – – – – – – – – – ix
List of figures – – – – – – – – – – – – – – – – – – – – – x
CHAPTER ONE: INTRODUCTION
1:1 Electron Transfer ———————————————————————————–
1.2 Classes of Electron Transfer ———————————————————————
1.2.1 Inner sphere electron transfer ———————————————————————
1.2.2 Outer sphere electron transfer ———————————————————————
1.3 Mechanism of electron transfer reactions ——————————————————
1.3.1 Inner sphere mechanism —————————————————————————–
1.3.2 Outer sphere mechanism —————————————————————————–
1.4 Applications of Electron Transfer —————————————————————–
1.5 Chemistry of cobalt ———————————————————————————
1.5.1 Use of cobalt ——————————————————————————————
1.5.2 Structure of [Co¹¹HEDTAH2O] ——————————————————————–
1.6 Chemistry of Transition Metals
1.7 Aims and Objectives
CHAPTER TWO: LITERATURE REVIEW
2.1 Dielectric constant
2.1.1 Dielectric properties
2.2 Microscopic concept of polarization
2.3 Effect of variation of dielectric constant of a medium
2.4.1 General characteristics of catalysed reaction
2.4.2 Types of catalysis
2.4.3 Catalytic poisoning
2.4.5 Examples of catalytic process
2.5 Effect of Temperature on reaction velocity
2.6 The bioinorganic chemistry of copper
CHAPTER THREE: MATERIALS AND METHODS
3.2.1 Preparations of the complex [CoIIHEDTAH2O]
3.2.2 Preparation of 0.1m of perchloric acid
3.2.3 Preparation of standard solution of sodium perchlorate
3.2.4 Preparation of the standard solution of copper (II) teraoxosulphate (VI) salt
3.3 Determination of the λmax (510nm)
CHAPTER FOUR: RESULTS AND DISCUSSION
4.1 Determination of the rate constant of the reaction (kobs )
4.2 The effect of dielectric constant
4.3 The effect of added ions
4.4 Temperature dependence of rates of reaction.
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
1.0 BACKGROUND OF THE STUDY
1.1 ELECTRON TRANSFER
Electron transfer (ET) occurs when an electron moves from an atom or a chemical species ( e.g. a molecule) to another atom or chemical species. Electron transfer is a mechanistic description of the thermodynamic concept of redox, wherein the oxidation states of both reaction partners change.
Numerous biological processes involve electron transfer reactions. These processes include oxygen binding, photosynthesis, respiration, and detoxication. Additionally, the process of energy transfer can be formalized as two-electron exchange (two concurrent electron transfer events in opposite directions) in case of small distances between the transferring molecules.
1.2 CLASSES OF ELECTION TRANSFER
There are several classes of electron transfer, defined by the state of the two redox centers and their connectivity.
1.2.1 Inner sphere electron transfer
In inner sphere electron transfer, the two redox centers are covalently linked during the electron transfer (Burgees, 1978). This bridge can be permanent, in which case the electron transfer event is termed intermolecular electron transfer. More commonly, however, the covalent linkage is transitory, forming just prior to the electron transfer and then disconnecting following the electron transfer event. In such cases, the electron transfer is termed intermolecular electron transfer. A famous example of an inner sphere electron transfer that proceeds by a transitory bridged intermediate is the reduction of [CoCl(NH3)5]2+ by [Cr(H2O)6]2+ (Taub and Meyer, 1954). In this case the chloride ligands is the bridging ligands that covalently connects the redox partners.
Outer sphere electron transfer
In outer-space reactions, the participating redox centers are not linked by any bridge during the electron transfer event. Instead, the electron “hops” through space from the reducing center to the acceptor. Outer sphere electron transfer can occur between different chemical species or between identical chemical species that differ only in their oxidation sate. The later process is termed self-exchange. As an example, self-exchange describes the degenerate reaction between permanganate and its one-electron reduced relative, manganese:
[MnO4]– + [Mn*O4]2– [MnO4]2– + [Mn*O4]– (Lavallee, et al; 1973) (1.1)
In general, if electron transfer is faster than ligands substitution, the reaction will follow the outer-sphere electron transfer. Often occurs when one/both reactants are inert or if there is no suitable bridging ligands.
1.3 MECHANISM OF ELECTRON TRANSFER REACTIONS
In a redox process, the oxidizing and reducing centers can react with or without a change in their coordination spheres. In some reactions, the electron transfer can only be accomplished by the transfer of ligands from reducing agent to the oxidizing agent.
There are two stoichiometric mechanism: the inner sphere mechanism involves a ligands transfer, and transient shared ligands, while the outer sphere mechanism includes the simple electron transfers, without the presence of shared ligands.
1.3.1 Inner sphere mechanism
The reduction of the non-liable Co complex by the aqueous Cr complex produces a reduced Co complex and an oxidized CrCl complex. The chloride ligands has been transferred between the metal centers as proven by the fact that addiction of 36Cl– to the solution results in no incorporation of 36 Cl– into the Cr complex (Wilkins, 1991).
The reaction is faster than reactions which remove Cl– from Co111 or introduces Cl– to Cr3+ (aq), and hence the Cl– ion must have moved directly from the coordination sphere of one complex to the other during the reaction.
N.B The intermediate has a bridging Cl– ligand.
The Cl– ion is a good bridging ligand as it has more than one pair of electrons, and so can form bonds to each of the metal centers simultaneously. Other good bridging ligands include SCN–, N2, N3– and CN– (Wilkins, 1991).
Fig 1.1 Shows SCN– as a good bridging ligand (Wilkins, 1991).
1.3.2 The Outer Sphere Mechanism
When both the species in the redox reaction have non-liable coordination spheres, no ligands substitution can take place on the very short time scale of the redox reaction. The electron transfer must proceed by a mechanism involving transfer between the two complex ions in outer-sphere contact.
If the redox reaction is faster than the ligands substitution, then the reaction has an outer-sphere mechanism.
When the reaction involves ligands transfer from an initially non-liable reactant to a non-liable product, there is no difficulty in assigning the inner-sphere mechanism.
When the products and reactants are liable, it is difficult to make an unambiguous assignment of either an inner or an outer-sphere mechanism (Richardson, 1984).
Fig 1.2: Shows a situation where both the reactants and products are labile (Richardson 1984).
1.4 APPLICATIONS/DEVELOPMENTS IN THE ELECTRON TRANSFER REACTIONS
Electron transfer experiment since the late 1940s (Marcus, 1956)
Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as it relation to the study of other kinds of chemical reactions, represents to us an intriguing history, one in which many threads have been brought together.
|ET at liquid-liquid interfaces|
|ET at polymer-liquid interfaces|
|ET at semiconductor electrodes|
|ET at modified electrodes|
|ET at metal electrodes|
|Eirrev and Eo|
|ET at colloids and micelles|
|Solar energy conversion|
|ET in solids and polymers|
| Electron transfer|
1950’s & 1960’s
|Organic ET’s and SN-2’s|
|Quantum chemistry calculation|
|Solvent dynamics and ET|
|Ion pairs, recombination,escape|
|Magnetic effects on ET|
|Coupled ET and proton transfer|
|ET in proteins|
|Methyl and other transfers|
|Long range ET in rigid media|
|ET across rigid organic bridges|
Fig. 1.3. Examples of the applications/developments in electron transfer reactions. (Marcus and Siddarth, 1992).
1.5 CHEMISTRY OF COBALT
Cobalt is a chemical element with the symbol Co and atomic number 27. Cobalt always occurs in nature in association with Ni and usually also with arsenic (Seyferth et al., 1989). The most important Co minerals are smaltite, (CoAs2), and cobaltite (CoAsS) but the chief technical source of Co are residue called “speisses” which are obtain in the smelting of arsenical ores of Ni, Cu and Pb,
Cobalt is a hard bluish-white metal (mp 14930c, bp 31000c). it dissolve slowly in dilute mineral acids, the Co2+/Co potential being -0.2227 V, but it is relatively unreactive. While it does not combine directly,with C,P and S on heating, it is attacked by atmospheric O2 and by water vapor at elevated temperatures, giving CoO. Very reactive finely divided metal particles can be made by reduction of CoCl2 with Li naphthalenide in glyme (Beattie et al., 1996).
Table 1.1: OXIDATION STATE AND STEREOCHEMISTRY OF COBALT
(Lippard et al., 1992)
|Oxidation state||Coordination number||Geometry||Example|
|Co-1, d10||4||Tetrahedral||[Co(Co)4-, Co(Co)3 No|
|Co0, d9||4||Tetrahedral||K4[Co(CN)4], Co(PMe3)4|
[Co(CO)3(PR3)2]+, HCo(PR3)4, [Co(NCMe5]+
[CoCl4]2- ,CoBr2(PR3)2,Co11 in Co3O4
|In a 12-heterpolytungstate;in the garnets|
[Co(en)2Cl2]+, [Cr(CN)6]3-, ZnCo2O4, CoF3,
1.5.1 USES OF COBALT
Cobalt plays an important biological role for instance;
Coenzymes B12; A vitamin known as coenzymes B12 is a known organometallic compound in nature (Crossnoe et al., 2002). It incorporates cobalt into a corrin ring structure. This compound is known to prevent anemia and also has been found to have many catalytic properties (Morales et al., 2003). Methylcobalamin can methylate many compounds, including metals. The reactions of alkylcobalamine depends on cleverage of the
alky-cobalt bond, which can result in Co(I) and an alkyl cation, Co(II) and alkyl radical, or Co (III) and alkyl anion (Abeles, 1977). Cobalt also contains other enzymes and proteins like glutamate mutase, dioidehydrase, methionime synthetase, and dipeptidase (Frieden, 1985).
1.6 CHEMISTRY OF TRANSITION METALS
In chemistry, the term transition metal (or transition element) has three possible meanings:
The IUPAC definition (IUPAC, 2006) defines a transition metal as “an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell”.
Many scientists describe a “transition metal” as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table (petrucci et al., 2002), (Housecroft and Sharpe, 2005). In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called “inner transition metals”.
Cotton and Wilkinson (cotton and Wilkinson, 1988) expand the brief IUPAC definition by specifying which elements are included. As well as the elements of groups 4 to 11, they add scandium and yttrium in group 3 which have a partially filled d sub-shell in the metallic state.
These last two element are included even though they do not (so far) seem to possess the catalytic properties which are so characteristic of the transition metals in general. Lanthanum and actinium in group 3 are however classified as lanthanides and actinides respectively.
English chemist Charles Bury (1890-1968) first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32. (Jensen,2003), (Bury, 1921), (Bury, 2008) These elements are now known as the d-block.
1.6.1. AIM AND OBJECTIVE
The study is aimed at generating kinetic data which will expand our knowledge of the actions and functions of vitamin B12, enzymes, coenzymes and proteins, mostly as it pertains to cobalt and copper in the kinetics of oxidation – reduction reactions involving electron transfer of [N-(2-hydroxy-ethyl) ethylenediamine –N’,N’,N’,- Triacetatocobalt (11)] by Cu2+ cation.
The objectives of this research is to determine the effects of :
An enormous amount of electron transfer chemistry goes on biological systems, and nearly all of them critically depend on metal-containing electron transfer agents (cotton et al, 2004).
Biologically iron is the most important transition element as its complexes play very important biological roles (Lead, 1996).
We have come to note that cobalt and copper also play important biological roles for instance, coenzymes B12; A vitamin known as coenzymes B12 is the only organometallic compound in nature (Crossnoe et al; 2002). It incorporates cobalt into a corrin ring structure. This compound is known to prevent anemia and also has been found to have many catalytic properties (Morales et al; 2003). Methylcobalamin can methylate many compounds, including metals. The reactions of alkylcobalamine depends on cleverage of the alkyl-cobalt bond, which can result in Co(I) and an alkyl cation, Co(II) and alkyl radical, or Co(III) and alkly anion (Abeles, 1977).
Cobalt also contains other enzymes and proteins like Glutamate mutase, dioidehydrase, methionime synthetase, and dipeptide. (Frieden, 1985).
Copper also contains important enzymes and proteins like Tyrosinase, amine oxidase, laccase, ascorbate oxidase, ceruloplasmin, superoxide dismutase, plastocyanim, nitric redutase, e.t.c (Gary and Donald,2011).
In many organisms, such as arthropods and molluses, oxygen is transported by copper protein known as haemocyamin, which unlike haemoglobin, is extracellular (Alkins et al; 2006).