SECTION 1 Oxidation-Reduction Reactions

Several types of reactions are commonly used in analytical procedures, either in preparing samples for analysis or during the analysis itself. The most important of these are precipitation reactions, acid-base reactions, complexation reactions, and oxidation-reduction reactions. In this section we review these reactions and their equilibrium constant expressions. 

Oxidative substitutions at ring junction positions in various tetrahydro-5-deaza-pterins (79JA6068) and -flavins (77JA6721) have been studied, e.g. to give (13), and the oxidation-reduction reactions of 5-deazaflavins (e.g. 78CL1177, 80CPB3514) across the 1,5-positions, e.g. (19) (20), are involved in their co-enzymic role in enzymic oxidations (see Section... 

Oxidation-reduction reactions involving perchlorates have been mentioned in several of the preceding sections and the reactivity of aqueous solutions is similar to that of aqueous solutions of perchloric acid. 

Many oxidation-reduction reactions (nicknamed redox reactions) take place in aqueous solution. One of these was mentioned in Section 11-2.1 when we characterized acids ... 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

Oxidation-reduction reactions represent yet another type of reaction that titrimetric analysis can utilize. In other words, a solution of an oxidizing agent can be in the buret, and a solution of a reducing agent can be in the reaction flask (and vice versa). In this section, we review the fundamentals of oxidization-reduction chemistry and discuss the titrimetric analysis applications. 

The usefulness of determining the oxidation number in analytical chemistry is twofold. First, it will help determine if there was a change in oxidation number of a given element in a reaction. This always signals the occurrence of an oxidation-reduction reaction. Thus, it helps tell us whether a reaction is a redox reaction or some other reaction. Second, it will lead to the determination of the number of electrons involved, which will aid in balancing the equation. These latter points will be discussed in later sections. 

Nicotinic acid and nicotinamide are precursors of the coenzymes NAD+ and NADP+, which play a vital role in oxidation-reduction reactions (see Box 7.6), and are the most important electron carriers in intermediary metabolism (see Section 15.1.1). We shall look further at the chemistry of NAD+ and NADP+ shortly (see Box 11.2), but note that, in these compounds, nicotinamide is bound to the rest of the molecule as an A-pyridinium salt. 

Riboflavin (vitamin B2) is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), coenzymes that play a major role in oxidation-reduction reactions (see Section 15.1.1). Many key enzymes involved in metabolic pathways are actually covalently bound to riboflavin, and are thus termed flavoproteins. 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

This reaction takes place when zinc and copper cire in direct contact, but as we explain ecir-lier in this section, a voltaic cell is created by connecting the two reactants by an external pathway. Only the electrons created at the anode in the oxidation reaction can travel to the reduction half of the reaction along this external pathway. A voltaic cell using this Scime oxidation-reduction reaction between copper and zinc is shown in Figure 19-1, which we examine piece by piece. 

Combustion is an oxidation-reduction reaction between a nonmetallic material and molecular oxygen. Combustion reactions are characteristically exothermic (energy releasing). A violent combustion reaction is the formation of water from hydrogen and oxygen. As discussed in Section 9.5, the energy from this reaction is used to power rockets into space. More common examples of combustion include the burning of wood and fossil fuels. The combustion of these and other carbon-based chemicals forms carbon dioxide and water. Consider, for example, the combustion of methane, the major component of natural gas ... 

Aqueous reactions can be grouped into three general categories, each with its own kind of driving force precipitation reactions, acid-base neutralization reactions, and oxidation-reduction reactions. Let s look briefly at an example of each before studying them in more detail in subsequent sections. 

As will become evident in this section, in the net transformation from reactant —s-product transformations many of the synthetically useful reactions involving >C=C<"+ are analogous to those involving neutral, un-ionized carbon-carbon double bonds (e.g. the Diels-Alder reaction, oxidation/reduction reactions, nucleophilic addition etc.). However, many of the reactions involving a neutral >C=C< often require the presence of an activating substituent in order to make the alkene more electron-deficient. In a sense, one-electron oxidation of an alkene to its radical cation provides a simple and unique mechanism for increasing the electrophilic (and, of course, radical) properties of... 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

Surface oxidation-reduction reactions involve electron-transfer processes between surface species. Examples of these reactions are considered in Section... 

In addition to their role in the formation of DNA and RNA (see Section 27.2), nucleotides have other important biological functions. For example, adenosine triphosphate (ATP) is an important energy carrier in biochemical reactions, and nicotinamide adenine dinucleotide is a coenzyme that is often involved in biochemical oxidation-reduction reactions. 

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

The bleaching process also proceeded on steady-state irradiation with visible light in the presence of moderately reductive compounds.33 Since this oxidation-reduction reaction is not possible in the dark, the reactions have been well studied with a variety of combinations of reductants such as the Fe(II>—Fe(III) system in aqueous solutions.1 This section discusses reversible spectral changes using a combination of two functions, i.e., oxidative (thionine) and reductive ones in a polymer matrix.34,35... 

OXIDATION AND REDUCTION All the reactions mentioned in the previous sections were ion-combination reactions, where the oxidation number (valency) of the reacting species did not change. There are however a number of reactions in which the state of oxidation changes, accompanied by the interchange of electrons between the reactants. These are called oxidation-reduction reactions or, in short, redox reactions. 

In this section only such oxidation-reduction reactions are dealt with as have importance in qualitative analysis. Other processes, with technological or historical importance, such as combustion or extraction of metals are not treated here as these fall outside the scope of this book. 

REDOX SYSTEMS (HALF-CELLS) Although all oxidation-reduction reactions are based on the transfer of electrons, this cannot always be seen immediately from the reaction equations. These processes can be better understood if they are split into two separate steps, the oxidation of one substance and the reduction of another one. Let us look into the examples quoted in the previous section. 

REDOX REACTIONS IN GALVANIC CELLS When discussing oxidation-reduction reactions we have not mentioned ways in which the directions of such reactions can be predicted. In other words, discussions in the previous chapters were aimed at understanding how oxidation-reduction reactions proceed, but there was no mention of why they take place. In this and the next few sections the problem will be dealt with in some detail. 

There is another common way to classify chemical reactions acid-base reactions, oxidation-reduction reactions, and reactions of more complicated types (beyond the scope of this book). Acid-base reactions are considered to involve the reactions of hydrogen ions with hydroxide ions. The reactions of acids and bases will be taken up in this section, and a more sophisticated view of these reactions is presented in Chapter 19. Oxidation-reduction reactions involve the transfer of electrons from one substance to another. Many combination reactions, many decomposition reactions, all single substitution reactions, and all combustion reactions are of this type, but more complex examples are presented in Chapters 16 and 17. 

Equivalents are measures of the quantity of a substance present, analogous to moles. The equivalent is defined in terms of a chemical reaction. It is defined in one of two different ways, depending on whether an oxidation-reduction reaction or an acid-base reaction is under discussion. For an oxidation-reduction reaction, one equivalent is the quantity of a substance that will react with or yield 1 mol of electrons. For an acid-base reaction, one equivalent is the quantity of a substance that will react with or yield 1 mol of hydrogen ions or hydroxide ions. Note that the equivalent is defined in terms of a reaction, not merely in terms of the formula of a compound. Thus, the same mass of the same compound undergoing different reactions can correspond to different numbers of equivalents. The ability to determine the number of equivalents per mole is the key to calculations in this section. 

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