Parallel reaction steps

The reaction mechanism depends on the chemistry of the active oxidant and chemical contaminants. Multiple sequential and parallel reaction steps occur frequently. Partial oxidation produces noxious byproducts. 

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

As discussed herein, it can be concluded that the mechanistic pathways of the per-oxyoxalate system in the presence of imidazole are well described and that there is a consensus at least in regard to the steps prior to the formation of the HEI. Conventional kinetic studies on this complex transformation, with various consecutive and parallel reaction steps, permit the formulation of a mechanistic scheme, containing the important steps in the Tight-producing pathways, for which rate constants have been determined. Even so, the smdies concerning the EIEI strucmre are still controversial. Many authors favor 3 and/or 48 as the most probable candidates for the HEI, while others have suggested... 

These observations are all consistent with the proposed tandem-catalysis mechanism shown in Fig. 8. Central to this proposal is that the reaction proceeds via several parallel reaction steps as might be expected for formal eight-electron coupling of two methane molecules to acetic acid. The observation that the reaction is effectively stopped by high concentrations of added CO but proceeds at low levels of CO can be explained if the rate-limiting step is oxidation of Pd(0) by sulfuric acid. From independent experiments we have found that under high CO pressure... 

A discrete library is a set of compounds that are obtained as individuals at the end of the library synthesis. Parallel synthesis leads to a discrete library by simultaneous addition of reactants in different reaction vessels and parallel handling of each library sample (see the example in Fig. 4.2, where 15 discretes are prepared from the common intermediate A with two parallel reaction steps). Conversely, a pool library is a set of compounds that are obtained as mixtures, or library pools, at the end of the synthesis. Mix and split (or divide and recombine) is the process leading to an SP pool library... 

Here we discuss the oxidation of SO2 by ozone. 802(g) dissolves in water, depending on pH (see Figure 5.7), as SO2 H2O, HSOj", and SO3 . These species are oxidized (nucleophilic attack by ozone) with different reaction rates. The oxidation reactions occur as parallel reaction steps ... 

Obviously, the assumption of non-dissociative adsorption as well as that of the reaction being first order in both reactants are gross simplifications, since both methane and oxygen will dissociate prior to the reaction, and the further surface reaction mechanism will involve several consecutive and parallel reaction steps. Nevertheless, the model should allow for a qualitative test of the assumption that site competition between methane and oxygen account for the observed trend in ignition temperatures. 

RD may also be a favorable alternative if complex reactions comprising consecutive and parallel reaction steps are considered. A prototypical reaction system for further study is the synthesis of ethylene glycol (EG) from ethylene oxide (EO) and water (W) according to... 

Electrochemical impedance spectroscopy Reaction mechanisms, kinetic parameters of surface reactions, identification of sequential/parallel reaction steps, evaluation of post-CMP formulations Zheng and Roy (2009), Shi et al. (2012), Sulyma and Roy (2010a), Tsai et al. (2003)... 

The isocyano-isocyanates 1752a-d can easily be prepared in yields of 50-78% by parallel reaction steps in a one-pot synthesis from monoformyldiamines 1751a-d... 

As an example supporting the above statements regarding parallel reaction steps, parallel reactions of isocyano-isocyanate 1752b at both the isocyano and iso-cyanato groups can be accomplished in a one-pot procedure, whereby methanol is added to the isocyanato group and a Passerini three-component reaction (P-3CR) with benzaldehyde and acetic acid occurs at the isocyano group, simultaneously affording both the urethane and Passerini product moieties in l-N-(0-methyl-carbamato)-10-N -[(2-acetoxy)-3-phenylacetamido]decane 1753 in 92% yield [1289]. 

It is necessary to underline that mixing a carbonaceous support with a precursor containing carbon, oxygen, nitrogen, and hydrogen with appropriate coordination to the transition metal, two parallel reaction steps coexist during the pyrolysis treatment. The first one is the graphitization of the support until the structure collapses... 

Sulphate and chloride ions have a substantial inhibitory effect on the reactions. In the corresponding reaction with Sb parallel reaction steps have been identified with [HCrO ]- and [CraO,]-- as oxidizing species. For the monomeric ion a scheme identical to that above is suggested, although at low [Cr T there may be the initial formation of a 1 1 complex. At higher [Cr " ], the rate-determining reaction is postulated as... 

Almost all the micellar-mediated bimolecular reactions so far reported involve parallel reaction steps that constitute reactions occurring simultaneously in both aqueous phase and micellar pseudophase as expressed in Scheme 3.8. As described earlier, there are only a few reports that show an apparent failure of bimolecular reactions to obey the reaction in Scheme 3.8. However, this failure is attributed to various factors, including the possibility of cross-interface reaction. ... 

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

In the case of parallel reactions, the fastest reaction will set or control the overall change. In all rate determining cases, the relative speed of the reactions will change with the temperature. This is caused by different energies of activation among the steps in the sequence. This is just one more reason for limiting rate predictions from measurements within the studied domain to avoid extrapolation. 

Similarly, for a parallel reaction, in which all the steps are of the same order... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

Fig. 4. Dependence of relative concentrationa nj/nt of reaction components A, B, and C on time variable r (arbitrary units) in the case of consecutive (— — ) reactions according to scheme (Ha) or parallel (C ) reactions according to scheme (lib). Ads X, Ads A, Des Y denotes that the rate determining step in the overall transformation is adsorption or desorption of the respective substance Des (B + C) denotes that the overall rate is determined by simultaneous desorption of the substance B and C. Ki/Ki = 0.5 for consecutive, and Ki /Ki — 0.5 for parallel reactions, b nxVn. 0 = 2.5 for consecutive reactions Kt = 0.5, and for parallel reactions Ki/Ki — 0.5. c nxVnA0 = 2.5 fcdesBKi Ky/fcdesoXj Kx = 10 [cf. (53)]. d Ki = 1.75 for consecutive, and Ki/Ki = 1.75 for parallel reactions.

As a third and final example of a chain reaction, we shall consider a net reaction that produces sulfate and hydrogen phosphate ions.7 The scheme is more intricate than the earlier ones. It starts with the homolytic dissociation of S2Ojj- as one of two parallel initiation steps, and utilizes SO -, HO, and HPO - as intermediates. The scheme suggested is shown here, and one can easily allow for the products that are identical save for protonation ... 

Complex reactions can be broken into a number of series and parallel elementary steps, possibly involving short-lived intermediates such as free radicals. These individual reactions collectively constitute the mechanism of the complex reaction. The individual reactions are usually second order, and the number of reactions needed to explain an observed, complex reaction can be surprisingly large. For example, a good model for... 

This reaction cannot be elementary. We can hardly expect three nitric acid molecules to react at all three toluene sites (these are the ortho and para sites meta substitution is not favored) in a glorious, four-body collision. Thus, the fourth-order rate expression 01 = kab is implausible. Instead, the mechanism of the TNT reaction involves at least seven steps (two reactions leading to ortho- or /mra-nitrotoluene, three reactions leading to 2,4- or 2,6-dinitrotoluene, and two reactions leading to 2,4,6-trinitrotoluene). Each step would require only a two-body collision, could be elementary, and could be governed by a second-order rate equation. Chapter 2 shows how the component balance equations can be solved for multiple reactions so that an assumed mechanism can be tested experimentally. For the toluene nitration, even the set of seven series and parallel reactions may not constitute an adequate mechanism since an experimental study found the reaction to be 1.3 order in toluene and 1.2 order in nitric acid for an overall order of 2.5 rather than the expected value of 2. 

In the simple case, B grows at the expense of A. However, in the second case involving parallel reactions, two embryos form from A. In the third case, at least two steps are involved in the two consecutive reactions where "A" trcinsforms to "B" which transforms to "C". 

Beneficial micro reactor properties mainly refer to improving heat management as a key for obtaining a partial reaction which is part of a consecutive and parallel sequence, when large heats are released by reaction steps other than the partial one (see also Section 3.3.1). 

The electron transfer step is the only reaction step, which means that other parallel or consecutive steps are absent. 

Around the world chemical professionals continually commercialize new products and processes. Much of this activity results in batch processing. Fine and custom chemicals can involve as many as ten to twenty batch reactions in series, sometimes with multi-step parallel paths, with various separation technologies between reaction steps. This paper is an attempt to reflect the experience of many individuals as seen through the author s eyes over almost four decades, with several very typical situations. 

Thus mechanism B, which consists solely of bimolecular and unimolecular steps, is also consistent with the information that we have been given. This mechanism is somewhat simpler than the first in that it does not requite a ter-molecular step. This illustration points out that the fact that a mechanism gives rise to the experimentally observed rate expression is by no means an indication that the mechanism is a unique solution to the problem being studied. We may disqualify a mechanism from further consideration on the grounds that it is inconsistent with the observed kinetics, but consistency merely implies that we continue our search for other mechanisms that are consistent and attempt to use some of the techniques discussed in Section 4.1.5 to discriminate between the consistent mechanisms. It is also entirely possible that more than one mechanism may be applicable to a single overall reaction and that parallel paths for the reaction exist. Indeed, many catalysts are believed to function by opening up alternative routes for a reaction. In the case of parallel reaction paths each mechanism proceeds independently, but the vast majority of the reaction will occur via the fastest path. 

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