Environmentally important reactions

Demonstrations of Environmentally-Important Reactions. Classroom demonstrations make an important contribution to students understanding. "NOx and SOx" is a series of demonstrations illustrating the preparation and properties of... 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

Although dehydrohalogenation of polyhalogenated hydrocarbons is not hydrolysis by the strictest definition, it is an environmentally important reaction in water because many haloge-nated compounds are environmentally important. Equation (25) shows dehydrohalogenation ... 

Colloidal semiconductor particles have been found to act as heterogeneous photocatalysts in a number of environmentally important reactions. 

Semiconductor particles and films have been found to act as heterogeneous photocatalysts in a number of environmentally important reactions. Materials such as Ti02 and CdS have been found to be efficient in laboratory-scale pollution abatement systems, reducing both organic and inorganic pollutants/impurities to harmless species (see [1-35] and references therein). Photocatalysts have been shown to be useful for decomposition of O3 [36], destruction of bacteria [37-39] and viruses [40], purification of air [41], photosplitting of water [42,43] and clean-up of oil spills [44,45]. Photocat-... 

Temperature dependence of environmentally important reaction rate constants. (A) CO2 reaction with H2O or OH in seawater. The data are plotted so that the sfope of the line is the Arrhenius activation energy, divided by the gas constant, R (8.315 J mor deg ) (Johnson, 1982). (B) HS oxidation (Millero et a/., 1987a). (C) Fe(ll) oxidation in seawater (Millero et o/., 1987b). (D) The dissolution rate constants for opal as a function of temperature. The values plotted here are normalized to a constant area volume ratio, A /V = I0cm . The high dissolution rate constants (circles) are for acid-cleaned siliceous sediments (Hurd, 1972) the squares are data for peroxide and acid-washed plankton (Lawson et a/., 1978) and the other results (triangles) are for two untreated single species of diatom (Kamatani, 1982). 

The rate law and experimentally determined surface characteristics can be combined to determine the mechanism of this environmentally important reaction. The equations describing the amphoteric (acid/base) properties of the solid surfaces have been previously presented in Eqs. (3.47) and (3.48). One interpretation of the second-order dependency of manganese exchange with respect to hydrogen ion is that two surface sites are required to adsorb one dissolved ion, which is known as a bidentate complex of Mn +, on the surface. Thus, two previously adsorbed hydrogen ions exchange places with Mn + in solution, which is denoted by the equilibrium constant /3 ... 

Balomenou S, Tsiplakides D, Katsaounis A, et al.. Novel monohthic electrochemically promoted catalytic reactor for environmentally important reactions, Appl. Catal. B 2004 52 181-196. 

The periplasmic dimethylsulfoxide reductases (DMSORs) of the photosynthetic bacteria R. capsulatus and R. sphaeroides function in a respiratory chain with DMSO as the terminal electron acceptor and catalyse the environmentally important Reaction (2). " ... 

Sparks DL, Fendorf SE, Zhang PC, Tang L. (1993). Kinetics and mechanisms of environmentally important reactions on soil colloidal surface. In Migration and Fate of Pollutants in Soils and Subsoils, Vol. 32, NATO ASI Series G (eds. D Petruzzeli, F Helfferich). Berlin, Germany Springer-Verlag, pp. 141-168. 

Factors controlling regioselectivity in the reduction of polynitroaromatics in aqueous solution were examined with the use of SM2/AM1 model. The analysis supported the reliability of aqueous geometries predicted by SM2/AM1. Moreover, successful predictions for the regioselectivity of reduction further supported the validity of the modeling approach. The authors stated that "the ability of the SMk series of solvation models to predict the electronic structure of reagents in solution should open the door for the study of many environmentally important reactions taking place in aqueous or other liquid media" [123]. 

R.M. Lambert, M. Tikhov, A. Palermo, I.V. Yentekakis, and C.G. Vayenas, Electrochemical Promotion of Environmentally Important Catalytic Reactions, Ionics 1, 366-376 (1995). 

Reactions involving the catalytic reduction of nitrogen oxides are of major environmental importance for the removal of toxic emissions from both stationary and automotive sources. As shown in this section electrochemical promotion can affect dramatically the performance of Rh, Pd and Pt catalysts (commonly used as exhaust catalysts) interfaced with YSZ, an O2 ion conductor. The main feature is strong electrophilic behaviour, i.e. enhanced rate and N2 selectivity behaviour with decreasing Uwr and , due to enhanced NO dissociation. 

The most important progress in the last decade has been in the design and synthesis of [RuCl2(diphosphine)(l,2-diamine)] catalysts exploiting the metal-ligand bifunctional concept developed by Noyori and co-workers.29-31 The Noyori catalysts seem to possess all of the desired properties, such as high turnover number (TON), high turnover frequency (TOF), and operationally simple, safe, and environmentally friendly reaction conditions. 

An HPLC-CL determination of environmentally important chlorophenols was reported by using 10-methyl-9-acridinium carboxylate as a CL label. A two-step derivatization was used to produce the CL derivatives (Fig. 10). Following the separation under reversed-phase conditions, the CL reaction was performed by the base-catalyzed postcolumn oxidation. The quantum efficiency was dependent on the species of analytes. The detection limit of chlorophenols (S/N = 3) ranged from 300 amol to 1.25 fmol per injection (Fig. 11) [52],... 

Phosphorus is one of the most important elements in soil chemistry because it is involved in numerous reactions with many different components. In addition to the species described earlier, phosphate will also form species with uranium, arsenic, and zinc. It also reacts with organic matter and with humic and fulvic acids to form environmentally important species [33-37],... 

Phthalate esters undergo a step-wise alkaline hydrolysis to monoesters and then to dicarboxylic acids. As a result of the relatively slow rates of this reaction at pH 6-9 and the low water solubility of di-n-octylphthalate, chemical hydrolysis of the compound is not an environmentally important transformation process (EPA 1992a). Hydrolytic half-lives at 25 C and pH 7 and 9 have been estimated to be 107 and 7 years, respectively (Howard et al. 1991). 

Lin, S. and Carlson, R.M. Susceptibility of environmentally important heterocycles to chemical disinfection reactions with aqueous chlorine, chlorine dioxide, and chloramine. Environ. Sci. Technol, 18(10) 743-748, 1984. 

Oxidation of alcohols to carbonyl compounds is an important reaction. Stoichiometric oxidants such as chromates, permanganates and MO4 (M = Ru, Os) are the commonly used reagents [19a,59,60]. However, they are going out of favour increasingly because they create heavy metal wastes . In view of this, development of environmentally friendly heterogeneous catalysts for alcohol oxidation is very important. In the use of catalytic amounts of transition metal salts or complexes as homogeneous catalysts for the oxidation of alcohols [61-64], separation of the catalyst from the reaction mixture and its subsequent recovery in active form is cumbersome. Heterogeneous catalysts for this kind of reaction are therefore necessary [65]. Clearly, encapsulation and/or immobilization of known... 

Stereoselective addition of allyl metal reagents to various functionalities is an important reaction in organic synthesis [32, 33]. The allylation of epoxides and aziridines with allyltin reagent is catalyzed by Lewis acids. Even though many Lewis acids have been reported to catalyze this reaction, Bi(OTf)3 is distinct because it avoids the formation of byproducts and is also environmentally more compatible. It catalyzes the reaction of aryl epoxides with tetraallyltin to afford the corresponding homoallyllic alcohol [34]. 

We begin with a discussion of the most common minerals present in Earth s crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is (1) a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH (2) the nature of the electrical double layer and how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid and (3) dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. We finish with a discussion of some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces. 

Thiols can also be converted to disulfides, as in the CdS-photocatalyzed conversion of cysteine to cystine In the latter reaction, the uptake of oxygen was pH dependent. Since the reaction rate was not increased in deuterium oxide and was not decreased by added azide, the authors conclude that singlet oxygen is not involved. Since superoxide dismutase inhibited the conversion, a photoinduced electron transfer is probably responsible for the observed transformations. Such organosulfide oxidations may be environmentally important since naturally occurring hematite suffers a photoassisted dissolution in the presence of thiols... 

The photogenerated hydroxy radical is probably the significant intermediate in the complete mineralization of alkyP °, vinyl , and arylhalides, e.g., Eq. (33). Such reactions are of great environmental importance, for they allow for an interesting means of water purification. 

We should point out, however, that depending on the relative importance of the various reactions, kohs may not be a simple function of pH and temperature, and that product formation may strongly depend on these two variables. Furthermore, we note that many environmentally important organic compounds exhibit halogen atoms bound to a carbon-carbon double bond, be it an olefinic (e.g., chlorinated ethenes) or an aromatic (e.g., chlorinated benzenes, PCBs) system. In many cases, under environmental conditions, these carbon-halogen bonds undergo SN or E reactions at extremely slow rates, and we therefore may consider these reactions to be unimportant. 

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