Indirect reaction

The indirect reaction pathway involves radicals. The first step is the decay of ozone, accelerated by initiators, e. g. OH , to form secondary oxidants such as hydroxyl radicals (OH°). They react nonselectively and immediately (k = 108 - I010 IVT1 s-1) with solutes (Hoigne and Bader, 1983 a, b). The radical pathway is very complex and is influenced by many substances. The major reactions and reaction products of the radical pathway based on the two most important models are discussed below (Staehelin and Hoigne, 1983 a, b Tomiyasu et al., 1985). 

The reaction between hydroxide ions and ozone leads to the formation of one superoxide anion radical 02° and one hydroperoxyl radical H02°. 

The ozonide anion radical (030-) formed by the reaction between ozone and the superoxide 

This OH° can react with ozone in the following way (Hoigne, 1982)  

With the decay of H04° into oxygen and hydroperoxide radical the chain reaction can start anew (see equation 2-1), Substances which convert OH0 into superoxide radicals 02°7H02° promote the chain reaction they act as chain carriers, the so-called promoters. 

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A multistep synthesis is strategically designed such that the labeled species is introduced as close to the last synthetic step as possible in order to minimize yield losses and cost. Use of indirect reaction sequences frequently maximizes the yield of the radioactive species at the expense of time and labor. 

Carboxylate soaps are most commonly formed through either direct or indirect reaction of aqueous caustic soda, ie, alkaH earth metal hydroxides such as NaOH, with fats and oils from natural sources, ie, triglycerides. Fats and oils are typically composed of both saturated and unsaturated fatty acid molecules containing between 8 and 20 carbons randomly linked through ester bonds to a glycerol [56-81-5] backbone. Overall, the reaction of caustic with triglyceride yields glycerol (qv) and soap in a reaction known as saponification. The reaction is shown in equation 1. 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

The two mechanisms proposed to account for the partial oxidation of methane to syngas may be dedgnated as the IPO (Indirect Partial Oxidation) mechanism and the DPO (Direct Partial Oxidation) mechanism. The IPO mechanism was proposed by Prette et al [16] and Lunsford et al [12]. They think CO and H2 are the products of indirect reaction, the overall reaction of the POM reaction is composed of three different reactions... 

In terms of scope and chemoselectivity, hydrozirconation takes its place between hydroboration and hydroaiumination. However, the synthetic applications of organozirconocene complexes have been considerably expanded over these last few decades, and it can be expected that they will become more and more attractive in the future. Beside the direct substitution sequences, indirect reaction pathways involving transmetalation or activation by ligand abstraction have been successfully applied in a number of cross-coupling and C-C bond-forming reactions. 

Mechanism for Gluconeogenesis. Since the glycolysis involves three energetically irreversible steps at the pyruvate kinase, phosphofructokinase, and hexokinase levels, the production of glucose from simple noncarbohydrate materials, for example, pyruvate or lactate, by a reversal of glycolysis ( from bottom upwards ) is impossible. Therefore, indirect reaction routes are to be sought for. 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

CL reactions are commonly divided into two classes. In the type I (direct) reaction the oxidant and reductant interact with rate constant kr to directly form the excited product whose excited singlet state decays with the first (or pseudofirst)-order rate constant ks = kf+ kd. In the type II (indirect) reaction the oxidant and reactant interact with the formation of an initially excited product (kr) followed by the formation of an excited secondary product, either by subsequent chemical reaction or by energy transfer, with rate constant kA. The secondary product then decays from the lowest excited singlet state with rate constant kt. Type II reactions are generally denoted as complex or sensitized chemiluminescence. 

An indirect method for the determination of lead by coupling reactions was developed based on the replacement of Fe(II) by Pb(II) from the Fe(II)-EDTA complex. The subsequent CL reaction was based on the Fe(II)-luminol-02 system. The method was used to determine lead in polluted water samples [75], Such methods may be extended to other ions with proper complex constants as compared to the Fe(II)-EDTA complex, after HPLC separation. Analysis of elements based on indirect reactions is summarized in Table 4. 

At these temperatures, singlet oxygen atoms could also react with hydrogen or methene to form OH. The OH reacts with 03 to produce hydroperoxy radicals H02. Both HO and H02 destroy ozone by an indirect reaction that sometimes involves O atoms ... 

Degradation of organics could be performed by (a) direct reactions with highly reactive species (OH radicals), (b) indirect reactions over radicals formed from stabile molecules (H2O2), and (c) direct reactions with stabile molecules. 

It is assumed that under the conditions mentioned, nitrate itself is not the electroactive species. Another species is involved in the electron transfer reaction and the product of this reaction reacts with HNO3 to reproduce the electroactive species. The following schematic overview (Sch. 1) of the indirect reaction was given in [51] ... 

Oxidative cleavage by means of electrochemically generated cation-radicals is also possible thus benzyl ethers may be cleaved and carboxylates decarboxylated using cation-radicals of brominated triphenylamines in acetonitrile containing a weak base.34 35 Such as indirect reaction makes it... 

A second consequence follows from rewriting the original transformation A —> B in terms of alternative chemical reaction pathways that involve the same initial and final states. As depicted in (3.100a), the indirect reaction sequence A —> X —>Y —>Z—>B must have exactly the same overall AH as the direct reaction A —> B,... 

For the determination of kD the indirect reactions have to be suppressed. This is generally done by inhibiting any reactions between the hydroxyl radicals and the target substances... 

To inhibit the indirect reaction, Beltran et al. (1994) found that it is sometimes not sufficient to use a low pH-value, even as low as pH = 2. Comparing the reaction rate of atrazine with ozone at pH = 2 with and without teri-butanol, they observed a decrease in the reaction rate in the presence of TBA. This means that there were radical reactions even at this extremely low pH. 

This method can be applied universally for direct and indirect reactions. 

In general the reaction rate constants of the direct reaction are between 1 and 103 L mol-1 s , and the indirect reaction rate constants are between 108 and 1010 L moL1 s-1 (Hoigne and Bader, 1983 a, b). 

Table 4-3 Examples of reaction rate constants for direct and indirect reaction of well-known drinking water contaminants (micropollutants) (Yao and Haag (1991) Haag and Yao (1992).

Since the ozonation of a compound M involves both the direct and indirect reaction pathways, the general rate equation 4-2 has to be modified to include both reactions ... 

Knowing the reaction rate constants of the direct and indirect reactions and the concentrations, the total reaction rate can be calculated. Unfortunately some data continue to be generated that fail to distinguish between the direct ozone reaction and hydroxyl radical chain reaction. Knowledge of independent rate constants for each pathway is useful to predict competition effects. In drinking water the direct oxidation kinetic is often negligible compared with the indirect, in waste water there is often no clear preference and both pathways can develop simultaneously. This was found for example in the ozonation of 4-nitroaniline at pH = 2, 7 and 11 (T = 20 °C) (Saupe, 1997 Saupe and Wiesmann, 1998). 

Every ozonation process where gaseous ozone is transferred into the liquid phase and where it subsequently reacts, involves physical and chemical processes which need to be considered in modeling. Physical processes include mass transfer and hydrodynamic properties of the reaction system, e. g. gas- and liquid-phase mixing. Chemical processes include, ideally, all direct and/or indirect reactions of ozone with water constituents. Of course these processes cannot be seen independently. For example, fast reactions can enhance mass transfer. 

The oxidation process consists of direct and indirect reactions which can occur at the same time. A second order rate equation is commonly accepted for both reaction pathways. So that the oxidation of the model compound M can be described as the sum of the two reaction pathways as follows ... 

Modeling in drinking water applications is largely confined to describing chemical processes. The mathematical models used in this area are based on the reaction rate equation to describe the oxidation of the pollutants, combined with material balances on the reaction system to calculate the concentrations of the oxidants as a function of the water matrix. As noted above, the reaction rate equation is usually simplified to pseudo-first order. This is based on the assumption of steady-state concentrations for ozone and the radicals involved in the indirect reaction. 

The assumption of a steady-state ozone concentration for the direct reaction is based on the relatively large concentration of ozone compared to the micropollutants, which means the change in the ozone concentration over time is negligible. Several authors have shown that the indirect reaction of OH° with organic compounds is pseudo-first order due to the steady-state concentration of the hydroxyl radicals (e. g. Yao and Haag, 1992 von Gunten et al., 1995). Further assumptions are that the concentrations of the intermediates, e. g. 02°, 0,°-, H0,° and organic radicals, are also at steady-state (Peyton, 1992). 

An equation for the steady-state concentration of OH° can be developed from the indirect reaction mechanisms and mass balances on the liquid phase for the system 03. The same... 

Table 5-1 Hydroxyl Radical (Indirect) Reactions initiated by OH or H202. ...

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