Introductory Chapter: Redox - An Overview

*Rozina Khattak, Murtaza Sayed, Muhammad Sufaid Khan and Hamsa Noreen*

## **1. Redox**

The term "redox" is the combination of two different terms that describe two totally different chemical processes, i.e., "reduction" and "oxidation." The abbreviation "red" has been taken to distinguish reduction from oxidation that is "ox." The reduction is a process wherein any chemical entity gets reduced. It is different from oxidation, which is opposite of the reduction because the chemical entities are oxidized. These two processes or reactions which simultaneously take place in a system are abbreviated as "redox."

### **1.1 General introduction**

Oxidation-reduction was primarily used to describe the reaction(s) of combination and/or removal of oxygen with or from chemical substances, respectively. Simultaneously, the removal and/or the addition of hydrogen were also used to differentiate among oxidation and reduction, respectively. The definitions were extended to a broader level, and the changes in the oxidation number or oxidation state of elements were considered to define oxidation and reduction. The increase in the oxidation number leads to oxidation and its alternative process yields reduction. This vast definition encompasses the recent and exact interpretation of "redox" reactions that is acceptance and donation of the electron(s) between the reacting entities. Consequently, the redox phenomenon indicates a simple reaction, formation of carbon dioxide as a consequence of the oxidation of carbon and/or formation of methane by the reduction of carbon, for example, and the complex reaction consisting of a number of electron transfer reactions during the oxidation of sugar in the human body to produce energy.

The redox reaction(s) involves an oxidant or oxidizing agent and a reductant or reducing agent. The oxidant takes the electron(s) and oxidizes the reductant. The reductant, however, donates the electron(s) and reduces the oxidant.

Redox reactions are the key to make many desired chemical changes and/or processes reality to get maximum benefit out of it. A simple overview to surface the vital need of these reactions revolves around combustion, metabolic reactions, extraction of metals from their ores, manufacture of countless chemicals, and reactions occurring in our natural environment. For example, a cell either battery and/ or biological cell involves redox processes [1–3]. Research that involves the biological systems interprets that the electronic and the structural environment of the substance(s) are the key factors that control chemical transformations such as electron transfer mechanisms in DNA molecules, which may appear through the exposure of cells to radiations that may have the power of ionization to cause

biologically deleterious effects such as inactivation, transformation, and mutagenesis [4–6]. The water molecules are ionized by some specific radiations and form radicals in the vicinity of DNA that contribute to the significant damage in DNA and chemical modifications to DNA, consequently. Of these radicals, the hydroxyl radical is thought to be the most damaging and produces the consequences for DNA strand breakage by the redox dependence of the rate of interaction of hydroxyl radical adducts of DNA nucleobases with oxidants [7].

Redox reactions mainly follow second-order kinetics with a series of intermediate reactions in a range of mechanisms such as entity transfer mechanisms that involve electron, atom, or group transfer and ligand addition, substitution, or dissociation.

Redox phenomenon in terms of electron transfer (ET) reactions and their mechanisms is catered for the interest of readers of this book. Essentials of only the electron transfer reactions of coordination or transition metal complexes and advanced oxidation processes for water treatment are being focused in a brief and narrative way.

#### **1.2 Redox reactions of transition metal complexes**

An enormous number of studies unfolded the characteristics and effect of the structural geometries on the kinetics and mechanisms of the redox reactions of various transition metal complexes [8–25]. The literature review helped to summarize that the redox reactions of the transition metal complexes undergo two types of mechanisms. These types are classified as the *outer-sphere and inner-sphere mechanisms*, which lead the electron transfer processes of the transition metal complexes.

#### **1.3 Schematic representation of the mechanistic pathways**

The fundamental distinction between the two mechanistic routes of electron transfer is the simplicity of the outer-sphere mechanism over the inner-sphere mechanism. The outer-sphere redox reactions are simple in nature and undergo electron transfer in a very simple way. The outer-sphere mechanism is further classified into the self-exchange and cross-exchange reactions. This classification is based upon the oxidation state of coordination compounds. In the self-exchange reactions, the same coordination compounds with different oxidation states reduce and oxidize each other. However, in the cross-exchange reactions, different coordination compounds with either of the same and/or different oxidation states or numbers reduce and oxidize each other in the vicinity. However, in the innersphere mechanism of electron transfer, the substitution of ligand or atom prior to electron transfer plays a key role. The difference between the two mechanisms is represented in **Figure 1** [13].

and the reducing agents. To suggest or propose an outer-sphere mechanism one needs credible evidence with proof of unavailability of the alternative inner-sphere mechanistic pathway. Consequently, there are a large number of reactions that are clearly defined to be operated through inner-sphere mechanism. However, many reactions follow outer-sphere mechanism and an uncomfortably big number of reactions operates by inner-/outer-sphere mechanism i.e., in between [26].

*Schematic representation of the redox mechanism. (a) The inner-sphere mechanism. (b) The outer-sphere*

**1.4 Experimental approach: kinetics and mechanisms of some selected**

The redox reactions of a few selected coordination compounds of the transition metal, iron, in its two oxidation states, i.e., +2 and +3, are briefly discussed. The mixed ligand complexes such as dicyanobis(phenanthroline)iron(III) and

dicyanobis(bipyridine)iron(III) oxidize hexacyanoferrate(II), acetylferrocene, and 1-ferrocenylethanol by outer-sphere mechanism [8–11, 13] in the aqueous-organic media. The effect of optimized parameters on the kinetics of the redox reactions helped to propose the operated mechanism and rate laws (**Figures 2**–**5**) [8, 10].

**transition metal complexes of Fe(II) and Fe(III)**

**Figure 1.**

*Introductory Chapter: Redox - An Overview DOI: http://dx.doi.org/10.5772/intechopen.92842*

*mechanism.*

**5**

One cannot easily propose the operated mechanism of electron transfer under specific cases beside the simple and apparent difference between the two reaction pathways. There are two reasons for this. It may usually be possible to suppose without any doubt that the inner-sphere mechanism is operating the electron transfer process in favorable cases, but in many reactions where the reactants or the products and/or both of them are substitution labile, the mechanism through the inner-sphere process becomes suspected. In such reactions, the exact nature of the real reacting entities that are taking part in the reactions and the products which form initially becomes impossible to recognize without proper experimental and technical treatments. The other reason for ambiguity in recognizing the reliable electron transfer mechanism appears when the nature of the outer-sphere mechanism is considered, which does not need any re-arrangement of the structure of the reacting entities rather it only needs the transfer of an electron between the oxidizing biologically deleterious effects such as inactivation, transformation, and mutagenesis [4–6]. The water molecules are ionized by some specific radiations and form radicals in the vicinity of DNA that contribute to the significant damage in DNA and chemical modifications to DNA, consequently. Of these radicals, the hydroxyl radical is thought to be the most damaging and produces the consequences for DNA strand breakage by the redox dependence of the rate of interaction of hydroxyl

Redox reactions mainly follow second-order kinetics with a series of intermediate reactions in a range of mechanisms such as entity transfer mechanisms that involve electron, atom, or group transfer and ligand addition, substitution, or dissociation. Redox phenomenon in terms of electron transfer (ET) reactions and their mechanisms is catered for the interest of readers of this book. Essentials of only the electron transfer reactions of coordination or transition metal complexes and advanced oxidation processes for water treatment are being focused in a brief and

An enormous number of studies unfolded the characteristics and effect of the structural geometries on the kinetics and mechanisms of the redox reactions of various transition metal complexes [8–25]. The literature review helped to summarize that the redox reactions of the transition metal complexes undergo two types of mechanisms. These types are classified as the *outer-sphere and inner-sphere mechanisms*, which lead the electron transfer processes of the transition metal complexes.

The fundamental distinction between the two mechanistic routes of electron transfer is the simplicity of the outer-sphere mechanism over the inner-sphere mechanism. The outer-sphere redox reactions are simple in nature and undergo electron transfer in a very simple way. The outer-sphere mechanism is further classified into the self-exchange and cross-exchange reactions. This classification is based upon the oxidation state of coordination compounds. In the self-exchange reactions, the same coordination compounds with different oxidation states reduce and oxidize each other. However, in the cross-exchange reactions, different coordination compounds with either of the same and/or different oxidation states or numbers reduce and oxidize each other in the vicinity. However, in the innersphere mechanism of electron transfer, the substitution of ligand or atom prior to electron transfer plays a key role. The difference between the two mechanisms is

One cannot easily propose the operated mechanism of electron transfer under specific cases beside the simple and apparent difference between the two reaction pathways. There are two reasons for this. It may usually be possible to suppose without any doubt that the inner-sphere mechanism is operating the electron transfer process in favorable cases, but in many reactions where the reactants or the products and/or both of them are substitution labile, the mechanism through the inner-sphere process becomes suspected. In such reactions, the exact nature of the real reacting entities that are taking part in the reactions and the products which form initially becomes impossible to recognize without proper experimental and technical treatments. The other reason for ambiguity in recognizing the reliable electron transfer mechanism appears when the nature of the outer-sphere mechanism is considered, which does not need any re-arrangement of the structure of the reacting entities rather it only needs the transfer of an electron between the oxidizing

radical adducts of DNA nucleobases with oxidants [7].

**1.2 Redox reactions of transition metal complexes**

**1.3 Schematic representation of the mechanistic pathways**

narrative way.

*Redox*

represented in **Figure 1** [13].

**4**

#### **Figure 1.**

*Schematic representation of the redox mechanism. (a) The inner-sphere mechanism. (b) The outer-sphere mechanism.*

and the reducing agents. To suggest or propose an outer-sphere mechanism one needs credible evidence with proof of unavailability of the alternative inner-sphere mechanistic pathway. Consequently, there are a large number of reactions that are clearly defined to be operated through inner-sphere mechanism. However, many reactions follow outer-sphere mechanism and an uncomfortably big number of reactions operates by inner-/outer-sphere mechanism i.e., in between [26].
