ABSTRACT
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 30x10-3 – 130x10-3 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.
TABLE
OF CONTENTS
Title
page
Abstract
Table
of contents
List
of tables
List
of figures
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
1.8 Justification
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 Catalysis
2.4.1 General characteristics of catalysed reaction
2.4.2 Types of catalysis
2.4.3 Catalytic poisoning
2.4.4 Autocatalysis
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
3.1 Materials
3.1.1
Chemicals
3.1.2
Apparatus/Equipment
3.2 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
5.1 Conclusion
5.1 Recommendation
CHAPTER ONE
1.
INTRODUCTION
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.
1.2.2 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:
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).
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).
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.
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).
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).
1.5.2
STRUCTURES
OF CoIII HEDTA(H2O)
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
Aim
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.
Objective
The objectives of this research is to determine the
effects of :
1. Temperature
2. Dielectric constant
3. Catalysis, on the rate of reactions in the
oxidation-reduction reactions of [CO11HEDTAH2O]-
and Cu2+
Justification
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).
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