Overall Mechanistic Scheme

Based on the data presented and discussed so far, the reactivity of NO/NO2 -I- NH3 over Fe- and Cu-promoted zeolites at low temperatures appears consistent with what previously reported over vanadium-based catalysts [7, 17, 19, 24] as well as with the mechanistic proposals for the Fast SCR chemistry over BaNa-Y [22, 23]. 

In such a chemistry, the reactivity demonstrated in the previous paragraphs attributes the following roles to the three main SCR reactants (1) NO2 forms surface nitrates and nitrites via a disproportionation route (2) NO reduces the nitrates to nitrites (3) NH3 decomposes/reduces the nitrites to N2. The related basic reaction steps, originally identified by transient reaction analysis and recently confirmed also by in situ FT-IR [10] over Fe-ZSM-5, are summarized in Table 9.1. 

This chemistry explains the optimal 1 1 molar ratio of NO and NO2 in the Fast SCR reaction. It also explains the full range of selectivities observed upon varying the NO2/NOX feed ratio [25-28]. In the presence of NO2 excess (NO2/NOX V2), in fact, incomplete reduction of nitrates by NO, the critical step R7 in Table 9.1, is responsible for the undesired formation of NH4NO3 at very low temperatures (step R6 in Table 9.1), and of N2O at intermediate temperatures (step R8 in Table 9.1). 

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The range of species produced reflects the overall mechanistic scheme shown in Figure 1. The detail however gives insights into the different processes and the effectiveness of the catalyst components in catalysing a specific transformation. 

This rearrangement, however, is complex and further studies show it to be dependent on temperature, solvent, pH and, most importantly, the absence or presence of oxygen. A similar overall mechanistic scheme operates for the acid-catalyzed rearrangement of A/-nitroso-5//-dibenz[6,/]azepine to acridine derivatives e.g. acridine-9-aldehyde) (74CRV101). 

In light of the previous comments it is possible to construct an overall mechanistic scheme for the decarbonylation of benzaldehyde (see Scheme II). Recall that various reagents retarded the decarbonyla-... 

Now MetH is one of the best understood B12 enzymes [11,32,169-173], The overall mechanistic scheme (Figure 12) was suggested by early work in... 

Many experimental observations on xanthine oxidase activity are correlated by this scheme, and at present, there appear to be no major inconsistencies. The coupled proton-electron transfer scheme (66, 67, 68) has been successfully incorporated into an overall mechanistic scheme (69) which explains, with great economy, a large amount of rather demanding data, from both kinetic and electron uptake experiments. 

In summary, these data support the proposed overall mechanistic scheme for the a-Umpolung reaction (Scheme 8), which invokes rapid deprotonation of the enol radical cations 86Should the other mechanisms (Scheme 3, mech. 4 and 5) be valid for the a-Umpolung reaction of 86 one would expect the yields to go up with less electron rich enols, since both the benzyl radical ArCH -C(OH)(OMe)CH3 and the a-hydroxy radical ArCH(OMe)-C-(OH)CH3 should be readily oxidized even for a p-chloroaryl system. In competition experiments... 

The analysis and interpretation of kinetic isotope effects (KIE) from the carbonyl carbon has been used to postulate SET processes for the first step in the reactions between ketones and MeLi or Mc2CuLi however, the ratedetermining steps within the overall mechanistic scheme in Eq. (5) depend on the steric and electronic properties of the substrate [63]. 

In light of the above arguments, an overall mechanistic scheme may be postulated. Such a scheme is shown in Figure 11.3. Note that in this scheme the steps of both Equations 21 and 23 are incorporated. For Equation 21, ks is rate-limiting whereas for Equation 23, ki and k2 are the slow steps. The 5-coordinate CO and aldehyde adducts are only observed in the dppp system, while all species with monodentate diphosphine ligands are unobservable. However, there are many examples in the literature where metal complexes having monodentate diphosphine ligands have been isolated and characterized. This scheme provides an hypothesis for future experiments. 

The MeCbl-based enzymes (methyltransferases) catalyze the transfer of methyl groups, and the overall mechanistic scheme requires a reversible heterolytic cleavage of the Co-Me bond. The process catalyzed by AdoCbl-based enzymes (isomerase and eliminase) proceeds through a stepwise process initiated by the homolytic cleavage of the Co-C bond. ... 

Some systematic studies on the different reaction schemes and how they are realized in organic reactions were performed some time ago [18]. Reactions used in organic synthesis were analyzed thoroughly in order to identify which reaction schemes occur. The analysis was restricted to reactions that shift electrons in pairs, as either a bonding or a free electron pair. Thus, only polar or heteiolytic and concerted reactions were considered. However, it must be emphasized that the reaction schemes list only the overall change in the distribution of bonds and ftee electron pairs, and make no specific statements on a reaction mechanism. Thus, reactions that proceed mechanistically through homolysis might be included in the overall reaction scheme. 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

Correa and Waters50 also proposed a mechanistic scheme where the key step of the overall reaction involves the recombination of sulfonyl radicals to form an intermediate with an O—S bond, the decomposition of which yields a sulfinyl radical (ArSO ) and an oxygen-centered radical ArS020 Later this suggestion was further strengthened on the basis of ESR studies51 and thanks to the elucidation of the electronic structure of sulfonyl radicals. However, it seems to the author that the available literature data point to an overall mechanism for equation 17 more complex that the one suggested50 (cf., for... 

In microkinetics, overall rate expressions are deduced from the rates of elementary rate constants within a molecular mechanistic scheme of the reaction. We will use the methanation reaction as an example to illustrate the... 

Therefore, it has been considered that the formation of the dimer involves a mechanism different to the simple head-to-head radical coupling of the parent monomer. As suggested by the authors, it is likely that the overall mechanistic sequence is initiated by the radical-anion 472 of compound 469 formed by a single electron transfer (SET) process, which is the first stage of the bromine-lithium exchange (Scheme 68) [128],... 

Even relatively complex reactions can behave very simply, and 99 percent of the time, understanding simple first-, second-, and zero-order kinetics is more than good enough. With very complicated mechanistic schemes with multiple intermediates and multiple pathways to the products, the kinetic behavior can get very complicated. But more often than not, even complex mechanisms show simple kinetic behavior. In complex mechanisms, one step (called the rate-determining step) is often much slower than all the rest. The kinetics of the slow step then dictates the kinetics of the overall reaction. If the slow step is simple, the overall reaction appears simple. 

We report here about the investigation of the low temperature watergas shift reaction on an industrial catalyst (GIRDLER G 66-B and E with copper and zinc oxides as main components) under transient conditions by means of wavefront analysis. After a qualitative analysis to obtain information about the relevant mechanistic scheme the main effort has been concentrated on the dependence of the microkinetics on different oxidation states of the catalyst. The watergas shift reaction in its overall formulation... 

In order to illustrate the application of LSV in mechanistic analysis we can look at the redox behavior of the formazan-tetrazolium salt system which we studied some years ago [17], 1,3,5-Triphenyl formazane was oxidized at controlled potential in CH3CN-Et4NC104 solution to 2,3,5-triphenyl tetrazolium perchlorate which was then isolated in quantitative yield. Coulometry showed that the overall electrode reaction was a two-electron oxidation. It has been shown that the rate of variation of Ep with log v was 30 mV per decade of sweep rate and that there was no variation of the peak potential with the concentration of 1,3,5-triphenylformazan. According to Saveant s diagnostic criteria (Table 1), four mechanistic schemes were possible e-C-e-p-p, e-C-d-p-p, e-c-P-e-p and e-c-P-d-p. If cyclization is the rate-determining step, then the resulting e-C-e-p-p and e-C-d-p-p mechanisms would not imply variation of Ep with the concentration of base. However, we have observed the 35 mV shift of Ep cathodically in the presence of 4-cyanopyridine as a b e. These observations ruled out the first two mechanisms. The remaining possibilities were then e-c-P-e and e-c-P-d, as shown in Scheme 3. 

The overall mechanistic picture of these reactions is poorly understood, and it is conceivable that more than one pathway may be involved. It is generally considered that cycloheptatrienes are generated from an initially formed norcaradiene, as shown in Scheme 30. Equilibration between the cycloheptatriene and norcaradiene is quite facile and under acidic conditions the cycloheptatriene may readily rearrange to give a substitution product, presumably via a norcaradiene intermediate (Schemes 32 and 34). When alkylated products are directly formed from the intermolecular reaction of carbenoids with benzenes (Scheme 33 and equation 36) a norcaradiene considered as an intermediate alternatively, a mechanism may be related to an electrophilic substitution may be involved leading to a zwitterionic intermediate. A similar intermediate has been proposed143 in the intramolecular reactions of carbenoids with benzenes, which result in substitution products (equations 37-40). It has been reported,144 however, that a considerable kinetic deuterium isotope effect was observed in some of these systems. Unless the electrophilic attack is reversible, this would indicate that a C—H insertion mechanism is involved in the rate-determining step. 

An efficient and economical new approach toward the synthesis of 7-aminoaziridinomitosenes, as represented by compound 96, has been developed in less than 15 total operations from commercially available chemicals. Overall our scheme represents the second total synthesis of a fully functionalized aziridinomitosene. At one stage, new insights into the mechanistic details involving the reaction of l,2-epoxypyrrolo[l,2-c]indoles with nucleophiles are provided. Notably, the route presented has the advantage of accessing both the aziridine and 7-amino substituents in deprotected form. Further application of this synthetic approach to additional C-9-substituted mitosenes can be anticipated. 

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

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