Redox and Acid-Base Reactions

Industrial applications inclnde the production of petrochemicals, fine chemicals and pharmacenticals (particnlarly throngh asymmetric catalysis), hydrometallurgy, and waste-treatment processes. Many life processes are based on metallo-enzyme systems that catalyse redox and acid-base reactions. 

Applying the Nernst equation for such a process, which is a combined redox and acid-base reaction, we can write... 

The variety of reactions which have been described indicate the large number of chemical species which enter in the troposphere and which may have an impact upon the environment. It is necessary to emphasize that all species are in a dynamic equilibrium for the meteorological turbulence so that in some cases redox and acid-base reactions occur with the formation of stable compounds with high formation constants but nevertheless the reagent species are detectable in the atmosphere. A typical example is the evaluation of the atmospheric aerosol where besides the presence of ammonium sulphates, free ammonia and sulphuric acid may coexist. 

The vast majority of catalysts used in modern chemical industry are oxides. Because of their ability to take part in the exchange of electrons, as well as in the exchange of protons or oxide ions, oxides are used as catalysts in both redox and acid-base reactions. They constitute the active phase not only in oxide catalysts but also in the case of many metal catalysts, which in the conditions of catalytic reaction are covered by a surface layer of a reactive oxide. Properties of oxides are also important in the case of preparation of many metal and sulphide catalysts, which are... 

Let us consider, as examples for redox and acid-base reactions, water molecules, H20, in the chemical reactions with four different chemical species of metallic iron, Fe, chloride ions, Cl-, ferrous ions, Fe2+, and fluorine, F2, in aqueous solution. In the cases of Fe and F2, the uppermost frontier orbital level is sufficiently far away from that of H20, and hence the complete electron transfer takes place for these two species out of or into the water molecules. Water, H20, is an oxidant against metallic iron, Fe, and is a reductant against fluorine, F2 ... 

Ammonium halides have been used as versatile reagents in low-temperature solid-state redox and acid-base reactions. For example, direct reaction with the appropriate metal at 270-300° yields the ammonium salts of ZnClq , LaCls ", YCl6 , YBre, CuCb , etc., whereas Y2O3 yields either (NH4)3YBr6 or YOBr depending on the stoichiometric ratio of the reagents. Solid-state reactions of ammonium sulfate, nitrate, phosphates and carbonate have also been studied. 

Life can use both redox and acid-base reactions to accomplish its purposes, but it appears to prefer redox reactions for its most important functions, like gaining energy from food and building new structures. In water, redox reactions are easier to control than acid-base reactions. In acid-base reactions, the protons can easily hop on a passing water molecule, while free electrons cannot. 

Show how redox and acid-base reactions parallel each other—how they are similar, but also what makes them different. 

In earlier sections of this chapter, we showed how to write and balance equations for precipitation reactions (Section 4.2) and acid-base reactions (Section 4.3). In this section we will concentrate on balancing redox equations, given the identity of reactants and products. To do that, it is convenient to introduce a new concept, oxidation number. 

Two very important classes of chemical reactions are oxidation-reduction (redox) reactions and acid-base reactions, which are defined by molecules or ions accepting and donating electrons or protons, respectively. 

In general, titrations governed by precipitation reactions do not really constitute an appreciable number in volumetric determinations in comparison to either redox or acid-base reactions. The interaction between silver-nitrate and sodium chloride in solutions result into the precipitation of silver chloride as shown below ... 

A reaction is said to be enthalpy-driven if it involves a large, negative AH" with a smaller and usually unfavourable TAS at all accessible temperatures. In a thermochemical analysis of such a reaction, and in comparing several such related reactions, only the enthalpy terms need normally be considered. Most redox reactions and acid-base reactions come into this category. The latter term can be interpreted liberally to include many instances of complex formation, e.g. ... 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

Finally, it is interesting to compare redox reactions and acid-base reactions. They are analogous in that acid-base reactions involve the transfer of protons while redox reactions involve the transfer of electrons. However, while acid-base reactions are quite easy to recognize (since they always involve an acid and a base), there is no simple procedure for identifying a redox process. The only sure way is to compare the oxi-... 

The key properties of oxide surface are the chemical bonding feature, coordination environment, oxidation state and acidic or redox properties of surface cations, and the basicity of surface anions. The longer term challenge to oxide surface chemistry is to address important issues in selectivity in catalytic oxidation and acid-base reactions, in particular the principle of tuning of metal reactivity by oxide ligands. In the development of new catalysts, new chemical concepts regarding structure or composition are conceived. The requirements... 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

HPA have become interesting materials for catalysis because of their tunable redox and acid-base properties and their stability in the solid state. In fact it is not only possible to change the heteroatom and the framework metal atom, drastically affecting the chemistry of the compound, but it is also possible to exchange the counter cations, thus modifying the overall acid site strength of the catalyst and ultimately the activity and selectivity of the catalyst towards a particular reaction. 

While the above mentioned reactions had caught the attention of radiation chemists in the sixties in order to contribute to some fundamental aspects in the redox and acid/base chemistry of nitro compounds, focus turned again on this class of chemicals in the late seventies and early eighties in connection with their potential as radiosensitisers and chemotherapeutics. Chemically, the parameter of interest in this context was the one-electron reduction potential of RNO2 / RN (02) couples 14 which, in the majority of cases, afforded accurate determination of the radical concentrations. We shall not elaborate on further details and general considerations because that will be done in other chapters of this book. It should just be mentioned that the underlying radiation chemical studies were particularly concerned with nitrobenzenes, nitrofurans, 2-, 4- and 5-nitroimidazoles, nitroazaindoles, nitroacridines, and quite a number of other nitroaryl compounds. 

In this section we will first present the development of a simple diagram (the so-called pe-pC or E -pC diagram) for a reaction that involves only electron transfer in a homogeneous system. Next we will consider the predominance area diagrams for presenting combined redox and acid-base equilibria. Finally, we will add a third dimension and consider systems with multiple phases using the pe-pH predominance area diagram. 

---