Reactions Involving Organisms

The biological contribution to metal cycling has been most studied for those metals for which biological 

In the case of mercury (Fig. 15-8), Wood suggests that reduction of Hg to Hg and alkylation to form methyl- or dimethylmercury can both be viewed as detoxification reactions, because all of the products are volatile and can be lost from the aqueous phase. Organisms can also convert the methylated forms to Hg , which is more volatile and less toxic. However, both the methylated and the reduced forms are more toxic to humans and other mammals than is Hg . 

The arsenic cycle in ocean waters and sediments also has important biological steps (Andreae, 1979). Arsenate, As(V), can be biologically converted into arsenite, As(III), and at least eight different organo-arsenic compounds, all presumably representing detoxification processes mediated by bacteria in 

F g- 15-8 The mercury cycle, demonstrating the bioaccumulation of mercury in fish and shellfish. Adapted from the National Academy of Sciences (1978) with permission. 

The range of processes that must be considered in the cycle of metals is described in Fig. 15-10 (Nelson et al., 1977). Both the complexity of metal cycle analysis in a real system and the importance of speciation are well-stated by Andreae (1979) in his overview of the arsenic cycle in seawater  

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Reactions involving organic substances have some special features. Many of these substances are poorly soluble in aqueous solutions. Sometimes their solubilities can be raised by adding to the solution the salts of aromatic sulfonic acids with cations of the type [NHJ or alkali metal ions. These salts have a salting-in effect on poorly soluble organic substances. In many cases solutions in mixed or nonaque-ous solvents (e.g., methanol) are used. Suspensions of the organic substances in aqueous solutions are also useful for electrosynthesis. 

The degree to which an electrode will influence the reaction rates is different for different electrochemical reactions, hi complex electrochemical reactions having parallel pathways, such as a reaction involving organic substances, the electrode material might selectively influence the rates of certain individual steps and thus influence the selectivity of the reaction (i.e., the overall direction of the reaction and the relative yields of primary and secondary reaction products). 

The nature of dangerous reactions involving organic chemicals depends on the saturated, unsaturated or aromatic structures of a particular compound. Saturated hydrocarbons are hardly reactive, especially when they are linear. Branched or cyclic hydrocarbons (especially polycyclic condensed ones) are more reactive, in particular as with oxidation reactions. With ethylenic or acetylenic unsaturated compounds, the products are endothermic . 

Many interesting biocatalytic reactions involve organic components that are poorly water-soluble. When using organic-aqueous biphasic bioreactor, availability of poorly water-soluble reactants to cells and enzymes is improved, and product extraction can be coupled to the bioreaction. Many applications in two-phase media can use the existing standard-type bioreactors, such as stirred-tank, fluidized-bed, and column reactors with minor adjustments. 

This polyazide (82.3% N) explodes on impact, shock or rapid heating to 170-180°C [1]. When preparing 2,4-diazido-6-dimethylamino-l,3,5-triazine from the 2,4-dichloro compound and sodium azide, there is the possibility of forming the triazido derivative, which detonates violently when touched. Reactions involving organic chlorides and excess sodium azide are extremely dangerous [2]. 

The fact that diffusion models describe a number of chemical processes in solid particles is not surprising since in most cases, mass transfer and chemical kinetics phenomena occur simultaneously and it is difficult to separate them [133-135]. Therefore, the overall kinetics of many chemical reactions in soils may often be better described by mass transfer and diffusion-based models than with simple models such as first-order kinetics. This is particularly true for slower chemical reactions in soils where a fast reaction is followed by a much slower reaction (biphasic kinetics), and is often observed in soils for many reactions involving organic and inorganic compounds. 

Because of their relatively fixed geometry, steroids provide excellent material for the study of the geometrical requirements of reactions involving organic structures. 

Adsorption and Electrode Reactions Involving Organic Compounds. 971... 

Each system considered in this section has a space of overall reactions whose dimension exceeds one. In many industrial reactions involving organic substances a major product is formed, but a side reaction contributes to loss in selectivity or yield of the desired product. Such cases may be said to exhibit a multiple overall reaction, unless the ratio of desired product to by-product remains constant over a range of operating conditions, so that a simple chemical equation might be employed to express the stoichiometry. 

PAH are formed by every high temperature reaction involving organic materials (10). There are a number of industrial processes where PAH can be identified in the workplace atmosphere. Well known examples are coke plants, ferroalloy plants, aluminum plants, secondary lead smelters and others (11,12). In some cases the harmful effect of these compounds has been indicated by epidemiological studies (12). 

Why is it so difficult to derive generally applicable quantitative structure-reactivity relationships (QSARs) for redox reactions involving organic compounds What is particularly problematic when dealing with reductive dehalogenation reactions ... 

The goal of this chapter is to give you a stronger handle on the basics of chemical reactions, which were introduced in Chapter 2. Then in the following chapters we ll look at specific classes of chemical reactions, such as acid-base reactions, oxidation-reduction reactions, and reactions involving organic chemicals. 

Thermochemical data for the solvation of ions as used in the preceding calculations are difficult to measure and even to estimate. Therefore this kind of calculation of AH° for ionic reactions involving organic molecules in solution usually cannot be made. As a result, we have considerably fewer possibilities to assess the thermodynamic feasibility of the individual steps of polar reactions in solution than we do of vapor-phase radical processes. Bond energies are not of much use in predicting or explaining reactivity in ionic reactions unless we have information that can be used to translate gas-phase AH°. values to solution AH° values. Exercise 8-3 will give you a chance to see how this is done. 

Redox Reactions Involving Organic Contaminants 16.2.1.1Assigning Oxidation States... 

In true fermentation, the free energy drop between substrate (say glucose) and anaerobic end products is always modest by comparison with respiration, because fermentation is never based on electron transfer chains coupled to phosphorylation. Rather, true fermentations depend upon a variety of oxidation-reduction reactions involving organic compounds, C02, molecular hydrogen, or sulfur compounds. All these reactions are inefficient in terms of energy yield (moles ATP per mole substrate fermented), and, therefore, the mass of cells obtainable per mole of substrate is much smaller than with respiratory-dependent species. 

Thus, the reactions involving ions, such as precipitation reactions, are almost instantaneous. This is because in such reactions no bond are to be broken. The reactions involving organic molecule proceed slowly. This is because in such reactions a large number of bonds have to be broken in reactant molecules and a large number of bonds have to be formed in product molecules. 

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